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Sub Panel - Sizing Questions

Posted by kevingw (My Page) on
Sun, Nov 30, 08 at 23:12

I am putting in a sub panel in a new section of my basement to run some power tools. My questions are :

1. What size breaker do I need in the main panel. The panel is full but because I am moving my saw I can take out a double 30 amp breaker and replace it with what I need. The largest load at a single time would consist of table saw and cyclone. The table saw is rated at 18 amps and the manufacture says you should use a 30 amp breaker. The cyclone has max amp draw of 14 amps. Both are 220 volts.

2. What size wire to feed sub panel. Can I use copper. The distance is between 35 and 40 feet from the panels.

3. In the new basement I want to run one 220 romax cable for 3 separate tools and just branch off that run a very short secondary run for the box with the receptacles. Thus there would be two boxes where a tool was. One for the splice and a second for the receptacle. Is that a problem? Only one machine would ever run at a time and the cable would exceed the needs of the saw which use the most power.
Thanks


Follow-Up Postings:

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RE: Sub Panel - Sizing Questions

Although the two tools mentioned in #1 add up to a full load of 32 amps (18 + 14), they both feed motors with higher start-up surges, so it sounds to me like a 50- or 60-amp breaker in the main panel, with #6AWG wire to the subpanel would be the best choice. (#6-3 copper would be appropriate whether 50 or 60 amps.)

[A 40-amp subpanel (#8AWG wire) might also work OK, but it would be near capacity with those two tools alone.]

I'm confused about your requirements for receptacles. In #1, you note that largest single load would be the saw and the "cyclone" (dust collector, I presume) and, in #3, you say "[o]nly one machine would ever run at a time."

I may not be fully understanding this, but I wouldn't put the saw and the dust collector on the same circuit. IMO, they each need independent circuits--30A for the saw and 20A for the "cyclone".

Would the other proposed circuit serve three tools, only one of which would run at the same time, in addition to the dust collector?


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RE: Sub Panel - Sizing Questions

Tom covered this very well with breaker sizes & Wire to be used. I would also recommend not putting the saw & cyclone on the same circuit. You would'nt save any money because the wire would have to sized to both of them & the same with the breaker, so you would not be protecting them properly either, Hope this helps as to why they should be on seperate circuits..


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RE: Sub Panel - Sizing Questions

Sorry for the confusion. I didnt consider the cyclone a tool. It would always run along one with one other tool and would have its own circuit. The three "tools" would share a circuit but never run at the same time.

I already purchased sub panel. I think it is 100 amps, definitely over 40. There was virtually no cost difference between it and the smallest one. Can I still use the #6-3 copper with that? I assume #6-3 is a romax style cable?

Thanks


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RE: Sub Panel - Sizing Questions

6-3 is available as Romex. Note the E.

If you have to ask questions like what kind of wire to use, it would probably serve well to remind you that, in a subpanel, your neutrals and grounds are to be kept seperate from one another, which may require the purchase of a seperate grounding bar for your subpanel.

Check it out - if there's only one ground/neutral bar, then you need to go buy a second one that fits the panel. Also, whatever method of bonding is employed (often a green screw or a jumper) needs to be removed.

Please forgive me if I'm telling you things you already know.


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RE: Sub Panel - Sizing Questions

I already purchased sub panel. I think it is 100 amps, definitely over 40.

Not a problem. A 100A box is exactly what you want, even though it'll only be supplied by 40, 50 or 60 amps of available power. Where there's a problem is if you use a device--any device, including the load center--that's rated for less than the overcurrent protection device (fuse or breaker) upstream.

Can I still use the #6-3 copper with that? I assume #6-3 is a romax [sic] style cable?

Yes (nodding at Pharkus about the spelling). "Romex" is actually a brand name of long standing and the term is often used for any brand of NM-B (non-metalic) cable.

Perhaps I should mention that NM-B shouldn't be used where it's exposed to damage--such as in a run along a masonry wall in a basement.

You don't mention where the feeder will run. If it'll be up in the ceiling--along a joist, for example, NM-B should be fine.

As for the circuits from subpanel to the tools, how will the walls be finished? If you'll be running your receptacles circuit through studs that'll be covered with drywall or pegboard, etc., then NM-B is fine. (The wire size for these runs will, of course be smaller--#12 or #10.) If the run(s) will remain exposed, then type THHN individual wires in conduit (either PVC or EMT) is the better choice.

While most 240V outlets are installed to serve a single device (e.g., water heater, dryer, etc.), it's OK to have multiple receptacles, as you propose, for conveniently plugging in tools which will be used on a one-at-a-time basis. Where you can get into trouble is by mix-and-matching receptacle sizes.

If, for example, you put one 40A/240V receptacle on the circuit (say, for a large thickness planer or a welder) and then a 20A/240V on the same 40A circuit with a 40A breaker, then any 20A device (including the receptacle itself) would be subjected to inadequate overcurrent protection. When you say 240V (or 220V) plugs, never assume "one size fits all". Is this making sense?

What types of plugs do the three 240V tools have? If they're all a mix of 15-amp and/or 20-amp plugs, you shouldn't have a problem with a single "240V tools" circuit assuming, of course, that the dust collector's on it's own 20A circuit.

Thus there would be two boxes where a tool was. One for the splice and a second for the receptacle. Is that a problem?

No problem per se. But with #12 or #10 wire on the "tools" circuit, you could also do "one stop shopping" with single boxes. Just pay attention to the size of the boxes in cubic inches versus the number and size of the wires, receptacle and internal clamps. (There's a code specified formula for determining minimum box size.)


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RE: Sub Panel - Sizing Questions

Let me provide some more detail:

Cut Off Box
The feed is going to go from the main panel into a "General Duty Safety Switch" (Cutler Hammer) which is rated at 60 amps than go to the sub-panel. The reason for this is to be able to lock out power to the tools with a lock which can go onto the switch on this box. It looks like it takes fuses, that is, fuses that are shaped like cylinders. I dont need more than 60 amps so hope this is not a problem. I assume the same cable that goes into this would go out to the sub panel.

Running of Feed
It will run up in ceiling of basement next to the feed the electrician installed for our second floor addition until it gets to the common wall for the new/old basement. It will then go into a pvc pipe that goes through the wall that was put in for this purpose. On the other side I put up some 1x4 which are screwed into the wall. I would attach the cable to these boards for the 3 feet until it reaches the sub panel.

Running of Circuits.
They are all in metal conduits attached to 1x4s anchored to the wall. For efficiency purposes one stretch has two Romex cables in one conduit.

Mix and Match Problem
The tools are either 2 hp or 3 hp 220 motors. That said, the table saw manual said it should have a 30 amp breaker. The jointer does not have that requirement and I think it draws no more than 18 amps. They all have different plugs which I have to buy separately. If this is a problem I could narrow the difference by running a separate line for the table saw. I thought they were all in the same ball park and did not see this as an issue.

Wiring Panel Ground Bar.
I am going to hire a licensed electrician to wire the panels. I am going to just run all the de-energized lines myself. But I want to make sure I am doing it right so it does not have to be done over. When it comes to the main cable between the panels I will measure the distance, but how much extra should I figure just to avoid the possibility I have a piece that is too short.

I greatly appreciate all of your helpful advice.


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RE: Sub Panel - Sizing Questions

There is a problem here. You must size your wire & feed breaker to the appliance you are protecting including a load center ! A 100amp subpanel should be fed with #2 awg cable & no smaller & the breaker should be 100a in your main panel. You cannot assume that you will never use more than 60 amps of power. This is absurd! You NEVER undersize wire for any appliance or load center! The house may be sold down the road & the new owner may add stuff to the subpanel, thinking it is good for 100a. Also, it is not a good idea to put these multiple 240v receptacles on one circuit.. What is the reasoning? That your not gonna use two at once?? Not enough room in the new panel. Assuming again & you know what that means! Too much money to run seperate wires? What about the new owner?? PLEASE DO IT RIGHT.


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RE: Sub Panel - Sizing Questions

"A 100amp subpanel should be fed with #2 awg cable & no smaller & the breaker should be 100a in your main panel."

That's nonsense. You can feed a 100A sub with any feed breaker you want, as long as it's not MORE than 100A. Of course the cable feed to the sub must be sized according to the feed breaker's amp rating.


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RE: Sub Panel - Sizing Questions

I don't see any need here for a 100A feeder to the subpanel, the OP is not running a 24/7 commercial wood shop. If the later "new owner" lacks the skill set needed to properly identify the sub-panel feed wiring and breaker, he/she should call someone who does. I do recommend running each tool on a separate circuit to simplify future troubleshooting and allow appropriate overcurrent protection.


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RE: Sub Panel - Sizing Questions

What's the purpose of having fuses in the "cutoff box". We ran mixers and electric ovens with simple knife switch cutoff boxes, no fuses, because the breakers were there anyway. These boxes were lockable also, two 40a and 0ne 20a.


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RE: Sub Panel - Sizing Questions

Feed Cable Matching Box Amp
While I want to do this right, I am going to be in my house for the next 15 years and I dont see the need to wire it to the possible needs of some future owner in 2023 unless it is required by code. No one I have spoken to has hinted that this a code requirement so I would not be inclined to use a 100 amp feed just because I happen to have a 100 amp box, although I do understand the point being made.

Circuit For Each Tool
A separate circuit for each tool seems overkill for a simple basement shop. Currently I have only two tools (table saw and jointer). At most I would have three. Also, these tools will have a run time of under an hour a month. If I can get away with one circuit I would prefer it unless it is simply an unsafe practice.

Cutoff Box
The only cut off box I could find is this one which appears to take fuses. If it does not make a difference I would rather just use it since I have it.

Wire Length
How much should I add to the measured wire length to account for hook up and bends?

Thanks again.


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RE: Sub Panel - Sizing Questions

Phew, a lot of stuff happening here. Starting with a footnote to this statement:

You cannot assume that you will never use more than 60 amps of power.

You are, of course, making no such assumption. If you or anyone else uses more than 60 amps of power, the 60A breaker in the main panel will trip. And if someone in the future is stupid enough to swap-in a larger breaker without evaluating the wiring, the same can be said for every circuit in your house: There is no known protection against an idiot putting in a larger breaker where it doesn't belong.

Also, it is not a good idea to put these multiple 240v receptacles on one circuit.. What is the reasoning? That your not gonna use two at once??

Yes, that's exactly the reasoning. The principle is called diversity, and it is the reason that the sum of the circuit breakers in a panel can add up to more than the ampacity of the service. It's also the reason why you can put a dozen ordinary 120V receptacles on a single 20-amp 120V circuit. Try putting two women people in the same bathroom with two 1800-watt hair dryers and see what happens. The breaker will trip.

Why? Because the design of the circuitry assumed that two big hair dryers would not be used at once in the same bathroom. If you've had teenage daughters, you'll probably find the two hair dryer scenario at least as probable as the jointer plus bandsaw (or whatever) scenario. :-)

Running of Circuits.
They are all in metal conduits attached to 1x4s anchored to the wall. For efficiency purposes one stretch has two Romex cables in one conduit.

Yikes! While it is permissible to run Romex in conduit for short legs where it's necessary to protect a portion of the cable, that's the exception, not the rule. As a general principle, Romex is not designed to run in conduit.

Since you'll be using conduit for the branch circuits, the wire type you want is THHN individual conductors, not NM cable! Among other things, individual wires take up less space in the conduit which, in turn, is important for proper heat dissipation. Once conduit is in place, THHN is usually cheaper than cables as well. (I can't guarantee that'll always be much cheaper when bought in custom lengths at a big box store, but it certainly shouldn't cost more.)

Mix and Match Problem
The tools are either 2 hp or 3 hp 220 motors. That said, the table saw manual said it should have a 30 amp breaker. The jointer does not have that requirement and I think it draws no more than 18 amps. They all have different plugs which I have to buy separately. If this is a problem I could narrow the difference by running a separate line for the table saw. I thought they were all in the same ball park and did not see this as an issue.

Based on what we know so far, I'd recommend putting the table saw on its own circuit (30amp breaker, #10 wire). As for the other two, it would really help to confirm that they are 15- and/or 20-amp plugs.

If so, I believe you can mix and match them on a single 20-amp circuit. I am certain that it is legal to put a 120V 15-amp receptacle and another 120V 20-amp receptacle on the same 120V 20-amp circuit. I am pretty sure (but not 100% certain) that the same mix-and-match exception applies for 240V 15/20 amp receptacles on 240V circuits.

Nevertheless, I agree with Wayne in that it'd be better to run three separate circuits. The cost difference should be modest (mostly a two-pole breaker and a bit more THHN wire).

Oh, BTW, what size EMT conduit do you plan on using?


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RE: Sub Panel - Sizing Questions

First of all there are some very good points here, but I would never install a 100a sub-panel for anyone using #6 awg wire with a 60a at the Main, and I wouldn't recommend it either. There is absolutely no point to it. The 100a breaker is to protect the ampacity rating of the cable size which should be sized to the appliance or panel. The panel is made to support 100a, so why would you wire it up for 60a? Yes, the breaker would trip,but this is not what you want to happen,with more amps going thru the cable than it is made to support.. What if you want to add on to the sub panel in the future? Just does'nt make sense to go smaller!.
The same can be said for multiple circuits-that breaker protects the wire that feeds the cicuits. It is there to protect the wire from overloading and catching fire. THe principle of "diversity" is an excellent point for duplex receptacles, & yes it could be used here for the 220v circuits but again I would never install it this way for anyone,as the wire would not support it if both were used at once. Breakers do not always trip..........


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RE: Sub Panel - Sizing Questions

"First of all there are some very good points here, but I would never install a 100a sub-panel for anyone using #6 awg wire with a 60a at the Main, and I wouldn't recommend it either. There is absolutely no point to it."

Doesn't sound like you've done too much actual wiring. It is very common to wire a 100A sub with only a 60A feeder. The reason being is that if you need many 120V cc'ts, such as in a lab, computer area, shop, you can have up to 32 of them with a 100A sub. In Canada you can only have 12 devices max on each cc't. So, if you need lots of cc'ts, in which there are lot of low amp devices, you can accomplish this easily. Often 100A main lug subs are priced cheaper at the big box stores than 60A or 40A subs. Of course if you're running high amp double pole devices, like machines, heaters, and motors, it wouldn't make any sense to use a 60A feed.


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RE: Sub Panel - Sizing Questions

"Doesn't sound like you've done too much wiring" Answer: I was an certified Industrial Electrician for 25 yrs & an electrical Planner for a shop of 20 guys for 5 yrs and did residential work on the side all those years until I retired & now I am in business for myself.
Just because it may be common pratice with some contractors & homeowners, does not make it right. Thats what causes trouble down the road ! That is my point-do it right the first time.
Your reasoning does not make sense with the 32 slot panel. Why wouldn't you have it correctly wired with a 100amp breaker and #2 awg wire & not have to worry if you might use over 60 at one time? What if the homeowner decides to install a 50 amp range, powder coating machine, etc? He will think he can because he has that 100a sub panel with all those spaces. He will probably not even think about the cable feeding it. It is always better to be safe than sorry, and to be sized right for future add -ons.


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RE: Sub Panel - Sizing Questions

What is important is that the breaker in the main panel protect the wire feeding the subpanel. The rating of the subpanel itself, so long as it is at least as large as that breaker, is irrelevant. There is nothing incorrect about using a 100A subpanel fed from a 60A breaker. What am I missing, mrmarv?


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RE: Sub Panel - Sizing Questions

"Why wouldn't you have it correctly wired with a 100amp breaker and #2 awg wire"

Because there is no such thing as "correctly" wired. There is only code compliant and non-code compliant wiring. Again, there is no code related reason why a 100A sub needs to be wired to its maximum ampacity.
If a homeowner decides to put a 50A DP breaker in a sub without even looking at the feed breaker, then he has no business doing that wiring himself.

"Your reasoning does not make sense with the 32 slot panel."

What part of my reasoning doesn't make sense? That it may be cheaper to buy a 100A main lug only sub, or that a 32 slot 100A sub can't be wired with 6/3 and a 60A feed, or that you may need many low amp circuits that won't exceed 60A total at any one time?


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RE: Sub Panel - Sizing Questions

"What if the homeowner decides to install a 50 amp range, powder coating machine, etc?"

If we had to plan for all the "what ifs" where do you stop?? Should we put in a 2" water main in case they decide to open a laundromat or a car wash? We had better put in 6 bedrooms, they might decide to have more kids. While we're at it, we better put in a 400 amp service in case they decide on a guest house.

Running 100 amps to a 1 man shop is a waste of copper and money. There is absolutly no code or safety reason to do it. It's like buying a two ton truck for grocery shopping.


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RE: Sub Panel - Sizing Questions

Mea culpa if I've seeded a $h1tstorm, folks, but I can't resist commenting on this enunciation:

You must size your wire & feed breaker to the appliance you are protecting including a load center !

Please, mrmarv, just sit back for a moment and put on your thinking cap. Assume the Lotus position. Light some candles and/or incense. Pray for guidance. Ponder long and hard and I think you'll understand why your "DO IT RIGHT!!!!" mantra isn't getting much traction in this alternative reality.

According to your encyclical, increasing the feed from 60 to 100 amps is necessary to protect the load center.

Hmmmmmmmm. Really?


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RE: Sub Panel - Sizing Questions

Man, there are some hardheaded woodchucks here. Can't even read or comprehend.! I suggest you take up another profession. Also, Please do NOT be advising others here. What am I missing here? You all missed the point. What is your reasoning for saving a little money on it?? Save now- Pay later! You probably frame your walls 24" oc too to save a buck. I could'nt imagine a Company's plant with 100a sub-panels being wired up with #6awg wire & 60a breaker "because we are only gonna put 2 things on it. Thats absurd! DO IT RIGHT THE FIRST TIME I'm outta here-no more dealing with woodchucks!


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RE: Sub Panel - Sizing Questions

"Man, there are some hardheaded woodchucks here. Can't even read or comprehend.."

And now there is one less.


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RE: Sub Panel - Sizing Questions

well, this is not a COMPANY'S PLANT, this is a homeowner's workshop.

jsut for your info, my workshop at my house has a 100A panel. i have 6 20A 120V circuits for the 17x17 building. the panel is fed with #4 copper and protected with a 60A breaker. Why you aks, well the panel was the smallest any one here carries with enough circuits, teh #4 THWN wire i already had, and a buddy that owed me for some work gave me everything else i needed. even with several power tools runnign at once in my shop, i have not come close to tripping the 60A main. if i ever need more power, i can go up a little on teh breaker size no problem. if i need a lot more power, i would need a bigger shop anyway.

my stepdad's shop at his house has 2 20A circuits.

several buildings we have here are fed with 100A circuits yet have 200A panels.


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RE: Sub Panel - Sizing Questions

"Man, there are some hardheaded woodchucks here. Can't even read or comprehend.! I suggest you take up another profession."

I think mrmarv is the only hhw here (hardheaded woodchuck). I asked him a question about what part of my reasoning he didn't understand so I could explain it to him again, and again, and again.... He couldn't/didn't answer, so I'll assume he's the one who can't read or comprehend.


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RE: Sub Panel - Sizing Questions

What is your reasoning for saving a little money on it?? Save now- Pay later!

IMHO, what makes this hillarious is that building in a bit of room for expansion with another circuit or two was behind my reasoning when I recommended going with 50-60 amps rather than 40 amps in the first place.

Oh well....


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RE: Sub Panel - Sizing Questions

The load to be served is well defined at this point.

If the OP adds more large equipment that requires power and needs to run more than one piece of equipment at a time (beyond a dust collector and a single tool) he can always upgrade the feed.

There are significant economies of scale in purchasing things like 100 A panels instead of 60 A.
They make a lot more 100 A panels, and while it is overkill for a 60 A feed it is not in violation of any code in feeding it with less.

I have even fed 200 amp panels with 100 amp feeds for large workshops.
The ability to have plenty of circuits each on separate breakers has advantages.

I would not install a main lug panel unless it was adjacent to the panel feeding it.
It is worth the few $$ for a main breaker in the sub.
The only time you need to wander over to the main is if you are working on the feed to the sub.
Work in the sub can be done with the main at the sub turned off, leaving opnly the input to the main breaker hot.


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RE: Sub Panel - Sizing Questions

I would not install a main lug panel unless it was adjacent to the panel feeding it.

In this case, though, the OP states that s/he plans to install an additional lockable disconnect. Hence, it seems to me that a subpanel main amounts to serious overkill. Hey, if the panel already has a main, fine. But I don't see this as being a useful recommendation.

If it's convenient enough to lock/unlock with each use of the tools, it's certainly convenient enough to for work on the panel.


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RE: Sub Panel - Sizing Questions

hello just want to toss my 2 cents inn here from a consumers view and i have to admit thati agree with mmarv above and i like his thoughts, to me even though it might be a bit overkill why not possibly make your life easier down the road for the sake of a few pennies more. to me most people always are shortsighted whether it comes to widths of front sidewalks,,,"oh well i wished i made it wider"....garden sheeds,, 'oh well i wish i made it wider and bigger"....building decks,,"oh well i wished i made it with better lumber and bit bigger"....on and on......

perroanlly i agree 100% above withthe mrmarv. unfortunately its a personal choice but maybe the buyer should be provided all the facts and potential options with them making the final decision. most will opt for the cheaper way out, but thats fine. thanks fred.


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RE: Sub Panel - Sizing Questions

Why not wire the subpanel for 100A? Because the wire costs more and the capacity isn't needed. The 60A breaker and wiring being discussed are already an upgrade from the minimum to get the job done. Wiring for 100A is overkill.

The 100A panel itself was a good idea because it didn't cost more and provided more flexibility in setting up the circuits.

I'm not telling the OP that he shouldn't wire for 100A if he wants to. Most of the argument with mrmarv was that he seemed to think it was NECESSARY and most of us thought it was highly optional.


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RE: Sub Panel - Sizing Questions

If mrmarv had ever wired a 100A sub, he'd know that using 3/3 copper is WAY tougher to work with than 6/3, and is a lot more expensive, not "pennies" more.


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RE: Sub Panel - Sizing Questions

I like a main cutoff in the panel.
It would probably be cheaper to lock the panel and not use a lockable disconnect.
100 A panels with main breakers are common (and cheap).
A 100 A main lug is not as common, and you still have to add in the disconnect.


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