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sub panel question

Posted by clax66 (My Page) on
Wed, Jan 23, 08 at 7:56

What is the largest sub panel you can run off of a 100 amp main? My main is 24 circuit and I am out of space. Ideally i would have about 16 more circuits. The amperage is not a problem just the number of circuits. I would like to avoid changing the whole panel as this would require removing the meter and I don't want to do that.


Follow-Up Postings:

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RE: sub panel question

Not sure I understand your need for 40 circuits especially with 100 amp service. Another option would be to replace large breakers with double slims which give you two breakers in the space taken up by one. What brand of panel do you have? It sounds like you are really in need of 200 amp service. What's the problem with pulling a meter?


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RE: sub panel question

You can run any size sub you want. The limit would be the feeder, which is 100A, but you could even run a 200A sub if you want, as long as the feeder is 100A or less so that you don't trip the main disconnect.


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RE: sub panel question

The main panel is full as the original owner ran very few things on each circuit. Now I am redoing a kitchen and this requires many new circuits.

The wire coming into the main panel are very short and to replace the panel I would have to replavc the wire to the meter. Half of the meter is live correct? Just would like to avoid this.

The panel i have now is an older one and breakers are very expensive for it. Double slims are not even available for this. I think it is a commander


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RE: sub panel question

"Now I am redoing a kitchen and this requires many new circuits."

I assume those new cc'ts will be 20A for countertops, maybe a dishwasher, garbage disposal, microwave, etc. I'd suggest the same thing as Texas, bite the bullet, put in a 200A service, and a 40 slot panel. That way you can recombine many of those small individual cc'ts into single 15A or 20A cc'ts (depending on wire guage) and have lots of future kitchen options. If you're doing this yourself and are hesitant, you could hire a licensed electrical contractor to do the service entrance and panel upgrade, and you do the rest.


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RE: sub panel question

It seems to me that the issue here isn't a matter of what size subpanel can be run from a 100A main service; it is more a matter of the load demand on your overall electrical service.

If, as you note, there are a number of circuits with small loads and your house has relatively modest electrical needs (especially those heavy 240V circuits for hot water, dryer, range/oven, heat pump, baseboard heat, air conditioning, hot tub--that sort of thing), then it is entirely possible that your 100A service is fully adequate and you can add more circuits with one or more subpanel(s).

Many if not most moderately sized houses with primarily gas appliances (e.g., furnace, oven, dryer) can operate just fine with more than 20 circuits on a 100amp main service. Then again, some can not.

There are a number of free spreadsheet tools out there to assist you in doing a demand load calculation and many jurisdictions require that this calculation be done before adding to electrical service (e.g., room additions, outbuildings). The information you will need includes: the square footage of the house (to estimate general lighting demand), the face-plate draws (in watts or volt/amps) of major appliances and HVAC systems and the requirements of any additional large loads such as hot tubs, pool heaters, large power tool motors, etc.

The demand load calculation does not merely add up all the power draws; it also takes into account probable diversity of use. (For example, the heat and air conditioning do not usually run at the same time. Similarly, not all of the lights are on at the same time, nor is every receptacle used simulateously.)

If you're interested, I can probably help locate one of the online spread sheets or the written methodology for performing the calculation. Or, one of the sparkies at this site might assist.


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RE: sub panel question

Not sure I understand all I know about the subject, but Commander is related to Cutler-Hammer. CH breakers are available at Home Depot. CH1515 and CH2020 are slim double breakers. It could be that Commander is the Canadian version of Cutler-Hammer. I believe the large bird is the Canadian expert.


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RE: sub panel question

Good info from everyone.

Clax66: I had a 2650 sq ft house with full basement that had 100 amp main service, and we never tripped any breakers. It had all gas major appliances.

The number 1 thing you should do is do a proper load calculation as terribletom said. Base your decisions on data, not opinions. If with the kitchen appliance additional loads you are still under 100 amps calculated total load, fine. I'm not an electrician, but I've done a lot of electrical work to various houses, and my understanding is you can put a 40 circuit subpanel in if you want, as long as you don't exceed a total calculated 100 amp load.

So, you can have a subpanel fed from a 100 amp breaker, in your 100 amp old panel. The subpanel can be any size number of circuits, as long as it's rated for the breaker that's feeding it. Some small subpanels only have a 70 amp rating. Most seem to be 100-125 that I've seen.

Do the load calculations. Then if your main service is adequate, you can install a 40 circuit subpanel fed by a 100 amp breaker, as long as your TOTAL calculated load is under 100 amps.


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RE: sub panel question

Here you go:

Here is a link that might be useful: Commander---> Cutler-Hammer


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RE: sub panel question

I would be surprised if you have a 100A panel, look at the nameplate. If you main breaker is below several 220 breakers you have a much larger box. The rule as I understand it is it doesn't matter what size sub-panel you use. You can use a 200 amp sub panel and feed it with a 30 amp feed if you like. Just size the breaker appropriately for the wire size you use to feed the panel. I have over 50 breakers in my home and the major appliances are gas same as you. I think I have about 20 breakers feeding the benches in my basement. I ran 12X3 W/G and split it into two 20 amp circuits which I then ran around the bench. I used GFCI breakers as the initial duplexes and regular duplexes thereafter on each circuit. If the neutral breaks the GFCIs will open so no chance of 220V in anything. My total basement load is normally about 15 amps but I can plug in a compressor or whatever I like without worry. One person suggested using Tandon breakers (never herd of slims). You need to do some de-rating if you put more then four of them in a panel. No one could tell me how but I just fed 100A branch panels with 60 amp breakers and the inspector had no problems.


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RE: sub panel question

It is "tandem", NOT the name Tandon. Tandem as in side by side. Another name is twin breakers. Slim is common name for GE style breaker that are not connected, they are simply 1/2" wide as opposed to the typical 1".

NO, you DO NOT have to derate when using twins. You DO have to make sure your panel will accept them. THIS is the key.
Not all panels will accept twins, tandems, skinnys, etc.

The reason you can feed a panel with a larger breaker is because the panels are rated in large gaps of sizes. Wire and breakers are more definite. It is completely typical to feed a 125A "rated" sub-panel with a 50, 60, 70, or 90 amp breaker.
Besides, the code tells us we can do this. I can find it if you want.

Gil, are you an electrician?


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RE: sub panel question

Sorry my speller changed that one, you are correct it is tandem.

I got my journeyman card in about 1960 from the local IBEW, but I do not see that as any requirement to answer the questions posted here. I also have my electrical and electronic engineering as well.

What is your background?

I made the change to electronics in about 1972. Is my journeyman still valid probably not, don't matter to me.

You might look into the NEC or other code. The local code states no more then 4 tandem breakers unless the box is de-rated, I think I saw that in the NEC, not sure.

Consequently, as stated above, you were mechanically forced to limit the number tandem breakers in a panel, Currently I am not sure if that still holds true.

I asked the local inspector if feeding a 100A sub panel with a 60 amp breaker, he said that would suffice.


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RE: sub panel question

"I got my journeyman card in about 1960 from the local IBEW, but I do not see that as any requirement to answer the questions posted here."

No, but having a GOOD clue about CONSTRUCTION and BUILDING electric, and a strong knowledge of the NEC ARE mandatory.
This is not written anywhere but those of us who are versed in this do keep incorrect or erroneous replies in check.

"The local code states no more then 4 tandem breakers unless the box is de-rated, I think I saw that in the NEC, not sure."

That's fine for your local code, just don't post that information as fact. STATE that it is YOUR local code.
NO, it is NO in the NEC.


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RE: sub panel question

I'm feeding a 125amp sub panel from a 100amp main panel with 6g wire and a 50amp breaker is this ok? I have 3 wires going to the sub panel 2 hot and one ground. I have a neutral bar in the sub panel, but no ground bar, does the ground hook into the neutral bar or should I buy a ground bar and put it in the panel?


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RE: sub panel question

#6 wire being fed from a 50A double pole breaker is fine. You need a neutral (white) wire feeding your subpanel if any circuits will be 120V. You need to separate the grounds and neutrals in the subpanel, so yes, you need to buy a ground bar.


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RE: sub panel question

... and run a ground wire.


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RE: sub panel question

If there are a lot of small branch circuits you can do a load calculation and probably join some of them into a single branch circuit and free up space that way.

The slim/double/tandem breakers and the boxes they fit are designed to prevent more than 42 devices in a panel since that is the limit.

If the panel is old enough it may not be listed for use with the smaller breakers.


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RE: sub panel question

"... and run a ground wire."

OP already stated she had 2 hots and a ground.


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RE: sub panel question

I know this thread this old but here I go...If you only have 24 circuits in your panel you should swap your panel for one with 36 circuits homedepot has them with the main 100amp breaker and 6 free 20amp breakers for under $60..the line to the meter is called the kick which in not hard to replace and when the meter is pulled off the kick is no longer live..to run the sub-panel you can only use 25 percent of the breakes that are in your main panel so 25 percent of 24 would be the amount of circuits you can put in the sub-panel..and if you also do the math you can only use 80% of the amps on your 100 breaker..you can not take it to 100%..only in lighting can you you 100 percent of the breaker..and yes you must have the neutrals and grounds on separate bars only one neutral to a lug and up to 2 ground to a lug..to try and swap all that around it better just to swap the panel for one that has more circuits..if you feel uneasy about doing that then your better off paying a licensed electrician to do it for you...cause you don't want to try and save a buck and burn down your house or hurt yourself


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RE: sub panel question

"original owner ran very few things on each circuit"

Why not just map out the existing breakers and see how many can be pigtailed together onto a single breaker.

If the loads are low on the existing circuits you should be able to free up enough slots for your kitchen remodel.


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RE: sub panel question

As long as you don't have to do other things like install AFCI's which I've never seen tandems/skinnies to do.


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RE: sub panel question

"OP already stated she had 2 hots and a ground."

He needs two hots, a ground, AND a neutral if he wants 120/240 V.

Ground and neutral are NOT the same or interconnected except in a main panel or in older range and dryer circuits.


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RE: sub panel question

This thread is from 4 years ago.


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RE: sub panel question

"I know this thread this old but here I go...If you only have 24 circuits in your panel you should swap your panel for one with 36 circuits homedepot has them with the main 100amp breaker and 6 free 20amp breakers for under $60..the line to the meter is called the kick which in not hard to replace and when the meter is pulled off the kick is no longer live..to run the sub-panel you can only use 25 percent of the breakes that are in your main panel so 25 percent of 24 would be the amount of circuits you can put in the sub-panel..and if you also do the math you can only use 80% of the amps on your 100 breaker..you can not take it to 100%..only in lighting can you you 100 percent of the breaker..and yes you must have the neutrals and grounds on separate bars only one neutral to a lug and up to 2 ground to a lug..to try and swap all that around it better just to swap the panel for one that has more circuits..if you feel uneasy about doing that then your better off paying a licensed electrician to do it for you...cause you don't want to try and save a buck and burn down your house or hurt yourself"

WHY would you dig up a four year old thread to post a bunch of INCORRECT gibberish????


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