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How to determine sub-panel amps?

Posted by redowns (My Page) on
Sun, Feb 9, 14 at 19:49

On more basic wiring question(s).. I'm trying to determine just how many separate circuits I can hang off my 100amp sub-panel. I'm wiring for the following 4 rooms;

- Office/Media room
- Rec room
- Small electronics hobby shop & equipment room
- Craft and general workshop room

I plan on being very liberal with the number of wall-sockets, ceiling lights, etc. which implies more circuits than would otherwise be 'needed' for a basement build-out. So, even if I max out most of the general wall socket circuits, which I understand the allowable is, 12/15amp circuit and 16/20amp circuit. Please correct me if that assumption/calculation is wrong. I'm trying to map my circuits to rooms, logical space, and load demands for my own sanity and convenience of control. With that methodology in mind my current electric map consists of the following;

- 6, 15amp circuits
- 1, 20amp circuit
- 1, 15amp for 12 can lights (6-Office and 6-Rec rooms)
- 1, 20amp circuit for Split-Mini Heat-pump

This map adds up to 145 maximum amps. Clearly too much for sub-panel to handle. Now I recognize that not all of these circuits will ever be at their maximum load. So in order for me to have the desired amount of wall sockets (and hence more circuits), and even though they all won't max out their load at the same time, how do I determine the maximum allowable 15 and 20 amp circuits my sub-panel can handle (including the feeder current requirements) and that will pass typical local code restrictions?


Follow-Up Postings:

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RE: How to determine sub-panel amps?

You do a load calculation which is based on square footage for the general lighting plus heating/cooling load and any required appliance circuits. It's all in the NEC with examples located in Annex D.

Unless you plan on some major equipment like kilns, ovens or big compressors, or plan to have live bands playing down there, I can tell you what you have listed is peanuts for a 100 amp sub panel.


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RE: How to determine sub-panel amps?

Thx for the reply. My total sq ft is < 500, but that is irrelevant based on what I want to do. The problem is I don't want the # of circuits based on typical square footage. That provides for only the minimum # of sockets. I simply want to place more sockets, more often along a wall say - not necessarily more load. But more sockets potentially translates into more load.

For sake of discussion, let's just say I want to double the # of general wall sockets in my < 500 sf space. (Please don't ask why.) How do I calculate what total current (and consequently # of circuits) is permissible for a 100-amp sub-panel?

I agree, that having 45% more calculated total load, doesn't translate into reality - even my more liberal socket reality. No appliances, no base-board heating, e ct., etc.


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RE: How to determine sub-panel amps?

You don't calculate anything based on the number of convenience receptacle outlets in a residential dwelling, like I said it is calculated on square footage + dedicated loads - demand factor + HVAC load. Convenience receptacles and their circuits are not dedicated you can have 10 or you can have 100 of them, you can put each one on it's own circuit, it doesn't change the calculated load, the square footage hasn't changed. All you are doing is adding convenience and eliminating extension cords. Now if you decide to add dedicated circuits and receptacles for say window AC units or kitchen/laundry appliances then you have to add those to your calculations.


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RE: How to determine sub-panel amps?

Also, adding up the amperage like you did is a meaningless number. You are forgetting that most of those circuits are 120V, and your panel is 120/240V. So if you have even 145A worth of 120V breakers that is equivalent to 72.5A @ 240V. Again, these numbers mean nothing sicne they are just the number on the breaker. NOT any kind of indication of actual load.
Probably the only thing that is 240V is the mini-split. Even then, it is likely 10-12A draw on a 20A circuit.


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RE: How to determine sub-panel amps?

Ok. Without knowing my exact 'demand load' (inspectors aren't going to know that either!), what is a typical 'demand load' to use for calculating a single circuit in a residential building/room?

I'm way beyond calculating based on sf. Like I said, that only provides for a minimum of outlet to pass code.

Would you agree, that if 10 or 12 sockets is the maximum physically allowed on a single 15A circuit - regardless of load - because I believe it's code where I live, then I were to double that amount in a room for convenience, wouldn't I need two 15A circuits just to meet code restrictions? You can't put 30 receptacles on a single circuit just for convenience - even if they only have a clock radio plugged in (.05A)

Let's forget about the single dedicated 20-amp circuit for a minute and use the 80% rule of 1440w for receptacles only, for a max of 12amps/circuit. Let's also assume I want to halve the distance between sockets to 3' instead of the minimum 6' as required by code - hence doubling the # of receptacles in the room. So I've met and exceed the sf requirements. 2 circuits per room. So with only general consumer electronics plugged in (radios, chargers, desk lamp, clocks, stereos, TVs, computers, computer monitors, vintage amps, phones, etc.), how many of these 2-circuit rooms could I run from a 100A sub-panel, and still meet code?

Aren't there some absolutes that I must work and plan to? Sq ft only provides for a minimum. What if I want to put 100 sockets in a closet? How do I calculate that?


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RE: How to determine sub-panel amps?

For some reason you seem to want to overcomplicate the calculations. Estimating the load and complying with code for number of receptacles on a circuit are are just two requirements to consider. I can't figure out if you think you will have a high load, or just want put lots of receptacles.

However, there are only two estimate factors that are typically used.

1. If you have dedicated loads (like the heat pump) then add that factor. In your case you might also have a high output sound system or unique appliance you want to add as dedicated loads.

2. Since it is hard to predict how many items will be plugged into receptacles, you use square footage as a minimum basis of estimate. 100 receptacles in a closet is the same as 2 receptacles unless you know there will be dedicated loads.
If you don't feel square footage estimates are adequate, then you are back up to step one where you identify specific loads.

You also have to comply with other code requirements, such as the number of receptacles on a circuit, size of wire, etc.

Once you get the load estimates you can then use it to estimate the panel requirements. Its not that complicated if you have a draft plan of the area you want to wire and what appliances will be there.

Bruce


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RE: How to determine sub-panel amps?

"Ok. Without knowing my exact 'demand load' (inspectors aren't going to know that either!), what is a typical 'demand load' to use for calculating a single circuit in a residential building/room?

I'm way beyond calculating based on sf. Like I said, that only provides for a minimum of outlet to pass code.

Would you agree, that if 10 or 12 sockets is the maximum physically allowed on a single 15A circuit - regardless of load - because I believe it's code where I live, then I were to double that amount in a room for convenience, wouldn't I need two 15A circuits just to meet code restrictions? You can't put 30 receptacles on a single circuit just for convenience - even if they only have a clock radio plugged in (.05A)

Let's forget about the single dedicated 20-amp circuit for a minute and use the 80% rule of 1440w for receptacles only, for a max of 12amps/circuit. Let's also assume I want to halve the distance between sockets to 3' instead of the minimum 6' as required by code - hence doubling the # of receptacles in the room. So I've met and exceed the sf requirements. 2 circuits per room. So with only general consumer electronics plugged in (radios, chargers, desk lamp, clocks, stereos, TVs, computers, computer monitors, vintage amps, phones, etc.), how many of these 2-circuit rooms could I run from a 100A sub-panel, and still meet code?

Aren't there some absolutes that I must work and plan to? Sq ft only provides for a minimum. What if I want to put 100 sockets in a closet? How do I calculate that?"

You're making a LOT of assumptions and mistakes about a code you clearly don't know very well.

First off, where are you??? You're stating "you think" certain things are code in your area. What area is this?
In general:
There is NO 12 receptacle limit. You CAN put 30 receptacles on a circuit.
There is NO blanket "80% rule".
You need a receptacle every 12', NOT every 6'.
You can have 100 receptacles in a room, but what if the room is so small that you physically can't plug in 100 things? What if it's for location convenience of plugging in ONE thing? Then SO WHAT if you have 100 receptacles.
Receptacles and outlets do not draw any power. What's attached to them does.


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RE: How to determine sub-panel amps?

"You also have to comply with other code requirements, such as the number of receptacles on a circuit, size of wire, etc."

Thanks for replying bcarlson. Your statement above is what I'm trying to determine. 'The number of receptacles on a circuit.' There are guidelines that electricians use, but according to what I've read, there is no hard and fast code that controls the number of receptacles on a circuit. That has been my basic question from the get-go. I understand the minimum, but that's not my question. My load demand or diversity for the wall-sockets - which is what's in question here will not adversely change except for the simple fact that the more sockets you have the more likely you may plug more things in! :) I want to add general wall receptacles for convenience and convenience only. This seems very difficult for everyone to understand. GENERAL use, I went thru the list in an earlier post.

So back to my question...you seem to imply that there is NEC code that controls the # of receptacles on a single circuit, but yet others in this very thread have said, not so. So which is it? Is 10 receptacles the code, or just good judgement based on typical diversity for a given sq ft room? I've actually got a call in to my local electrical inspector to see what he will approve - regardless what the code says.

I'm not trying to purposely overload a circuit just because I want more outlets. Once I know this I can work backwards to the sub-panel to hopefully determine # of circuits, what size cable I need and panel breaker to handle the extra load because of my extra circuits. I've got plenty of wire, boxes, and cost is not an issue. I want my media room and office to be done right with plenty of power to spare. Green is not in my vocabulary for this room. - redjr


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RE: How to determine sub-panel amps?

... except for the simple fact that the more sockets you have the more likely you may plug more things in!

That's not true. Just because I have 20 outlets in my bedroom doesn't mean I'll plug 10 bedside lamps in and use them instead of just one.
More outlets just means more convenience not more load.
That's why they use sqft instead of number of outlets.

There is no upper limit on general use circuits. Unless it's a local code addition.

If in your specific case, you are going to have more load, then take that into consideration. But if it's just for convenience, it doesn't matter.


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RE: How to determine sub-panel amps?

I believe I stated..

"...the more sockets you have the more likely you MAY plug more things in..." emphasis on may! I never said I would plug more things in. If there's no upper limit on # of receptacles per circuit can someone please quote chapter and verse of the US, NEC code where that is stated. I can't seem to find it.


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RE: How to determine sub-panel amps?

And adding up the amperage of the breakers to see what the sub panel feed should be is nonsensical. Go to your main panel right now and add up all the breakers. It'll be WAY more than your main breaker. And has your main breaker ever tripped?
Just because a circuit breaker says 15a, doesn't mean it's using 15a. For instance, your lighting circuit with 6 cans on it will be using very little of that. The 15a is the UPPER limit, not the actual use.

If you think you are going to be using more juice than typical calculations anticipate, then sit down and figure out what those usages are actually going to be. You'll probably be surprised that it still falls into the range of 'typical'.


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RE: How to determine sub-panel amps?

If there's no upper limit on # of receptacles per circuit can someone please quote chapter and verse of the US, NEC code where that is stated. I can't seem to find it.

An absence of a rule means that there is no rule. ie. No upper limit.

"...the more sockets you have the more likely you MAY plug more things in..." emphasis on may

If I only have one outlet on a circuit, I MAY plug a power strip in and plug 10 hair dryers into it. :)
The rules don't take into account what you MAY do. It's what the average person WILL LIKELY do.


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RE: How to determine sub-panel amps?

This gentleman on another forum said it more eloquently than I did...

"@Craig You are correct. As far as I know, there is no limit on the number of receptacles on a circuit in a residential dwelling. However, common sense says the more receptacles that are on the circuit, the more likely it is the circuit will be overloaded. 10 is just a reference point, I'm not saying if you have 11 the world will end. ��" Tester101♦ Dec 14 '11 at 12:51

I'm not worried about 11 either, but I may have some concern with, say, 24! :) Seems to me there's a happy, safe middle ground somewhere in between. Of all codes where you might expect an upper (reasonable) limit for some degree of safety margin, you'd think it would be the electrical industry. Especially the crazy things homeowners do with extension cords! Seems odd to me. But hey, who am I to argue with the NEC. I'm waiting to hear what my local inspector has to say about it.


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RE: How to determine sub-panel amps?

If you have 100 outlets in a closet, will you have all of them fully loaded at one specific moment?
If so, how?


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RE: How to determine sub-panel amps?

Common sense does not say that more receptacles means more load. At least my common sense doesn't say that. If those additional receptacles are spread further throughout the house, then I would agree, and hence the sqft calculation that is used. More outlets usually means more square footage that those outlets are covering, which is why you'll hear the 10 outlet general rule of thumb. In your case, you aren't talking about more square footage.

I've never walked into a room and counted the number of outlets to figure out what I'm going to plug into them. I've looked at a room, figured out what I want to use it for (eg. a home theatre) and then figured out how I was going to plug everything in that I wanted to power and how many power strips and/or extension cords I'd need to achieve that.

If you look at an outlet and get an irresistible urge to plug something into it, then I think you have OCD or something. :)

This post was edited by greg_2010 on Tue, Feb 11, 14 at 10:57


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RE: How to determine sub-panel amps?

And the safety is built into the system by the breaker. If you attempt to pull too much power, it trips.
That could happen with just one duplex outlet or it might not happen with 100 outlets. Which is why the number of outlets doesn't matter.
In fact, it could be argued that more convenience outlets leads to a safer design since that means fewer extension cords are used. So having one circuit powering the only 3 outlets in a room is less safe than having one circuit powering 10 outlets in a room.


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RE: How to determine sub-panel amps?

The number of receptacles is primarily driven by the fact that the code ABHORS extension cords (especially when crossing traffic areas such as doors, etc...). As pointed out, the load is not determined by counting the receptacles but by how much space is there.


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RE: How to determine sub-panel amps?

I spoke to my electrical inspector at length today and he confirmed that there is no upper limit on # of sockets on a circuit other than common sense. He personally recommends 4-5 receptacles per 20-amp circuit, but cannot obviously enforce that. Therefore, I plan to use 12AWG, and wire all my rooms on 20-amp circuits and place a liberal amount of sockets where I want them. My large sub-panel will accommodate numerous breakers, so I can group most circuits physically and/or logically as I see fit. Overhead lighting will be on separate 15-amp circuits.


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RE: How to determine sub-panel amps?

There's nothing wrong with doing more than necessary. There's just the law of diminishing returns to take into consideration. Having way bigger of a sub panel than you would possibly use just means that it'll cost more and the thicker wires are harder to work with and it will achieve little to no benefit over a smaller, more appropriate size.

Are you still planning on running a 100a sub panel? If not, post as much info about the load that you think you'll be using (not the number of circuits, because that doesn't matter) and I'm sure the electricians here will give you a better idea of the size that you require.

Just to put things in perspective, a lot of houses have 100a main panels to power the whole house and they never (okay ... rarely, if ever) use enough power to trip the main breaker.

This post was edited by greg_2010 on Tue, Feb 11, 14 at 13:28


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RE: How to determine sub-panel amps?

All I was trying to do guys was get an answer to the upper limit on receptacles for a given circuit, and the upper limit of circuits for a sub-panel. I did speak with my friendly and very helpful inspector and I now have my answers. I can proceed with circuit layout/map. I know all about load, current, fire hazard, what causes a breaker to trip and especially extension cords which I loathe and what I'm trying to eliminate. Without creating a long list, just think of my space as a; home theater, listening room, office and vintage audio showcase room all rolled into one! No, I don't think that's a typical basement room. :)


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RE: How to determine sub-panel amps?

Okay. So you now understand that there is no upper limit on receptacles on a circuit and there is no upper limit on breakers in panel? It's all about load.

Do you understand the difference between 240v circuits and 120v circuits? Because if you had a sub panel that is fed by a 20a 240/120v circuit (not that you'd ever have a sub panel that small) then it can supply 2 20a 120v circuits that are fully loaded without tripping.
If you understand that concept, then you won't completely oversize the subpanel.

Note the fully loaded clarifier. A 20a subpanel could also supply ten (or a hundred) lightly loaded 15a circuits without tripping.

This post was edited by greg_2010 on Thu, Feb 13, 14 at 10:15


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RE: How to determine sub-panel amps?

Yes. I'm aware of the difference between 240 and 120VAC and how a sub-panel works. I may opt for a smaller sub-panel (60a) rather than my original plan of a 100a sub-panel. I just need a sub-panel large enough to hold enough breakers - which should not be a problem. Plus I want to have extra slots for future expansion.


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RE: How to determine sub-panel amps?

Best brand of sub-panel to buy at Home Depot or Lowes?


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RE: How to determine sub-panel amps?

"Best brand of sub-panel to buy at Home Depot or Lowes?"

Any, literally any. Regardless of what some will tell you, for a simple sub-panel, ANY brand will do FINE.
Personally I like to match the existing main panel if I can.


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RE: How to determine sub-panel amps?

You can install a large subpanel with a 100a 'main' breaker in it but feed it from a 60a breaker using wires that are sized for 60a, if all you are worried about is getting enough spaces for your numerous circuits.

The 100a breaker in the sub panel would essentially just be an on/off switch for your sub panel instead of limiting the load. The 60a breaker in your main panel would be protecting against an overload.


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RE: How to determine sub-panel amps?

Unless your main is some bogus thing like an ITE pushmatic, you may wish to get the same brand you already have. Most of the borgs have Square D and CH panels and then whatever others by local custom (GE, etc...) they can sell.


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RE: How to determine sub-panel amps?

I can't remember where I read it, but someone had a negative opinion of one brand, but I can't recall which. Are there some that are easier to work with than others? I have to believe electricians by trade learn which brands are better than others since they work with a variety all the time. There must be some preferences out there. No? My original is 26 years old and made by Westinghouse.


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RE: How to determine sub-panel amps?

Just my opinion but I hate GE


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RE: How to determine sub-panel amps?

GE isn't my "goto" panel, but I've got them in some houses and they're not a problem. I kind of like the "skinny" breakers as opposed to the square d tandems.


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RE: How to determine sub-panel amps?

I can't remember where I read it, but someone had a negative opinion of one brand, but I can't recall which. Are there some that are easier to work with than others? I have to believe electricians by trade learn which brands are better than others since they work with a variety all the time. There must be some preferences out there. No? My original is 26 years old and made by Westinghouse.


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RE: How to determine sub-panel amps?

we usually just use what our supply house carries, unless they carry GE then we find a new supply house.


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RE: How to determine sub-panel amps?

Many guys hate GE. I think they are fine. I like the layout of things and I think they are decent quality. Breakers are proprietary, but Culter CL Series "classifed" fit.

Sq D HOM Series, I like the layout of things and I think they are good quality. Breakers are proprietary, but Culter CL Series "classifed" fit.

Cutler BR series is my go to, but not nearly my favorite. Fit and finish is not top quality and I don't like the neutral bar layout.
They are simply what my supply house stock and there are more of them around here than others.

C-H CH Series is very good quality, but are much more expensive and I don't see too many of them. Breakers are proprietary.

Sq D QO Series is very good quality, but are much more expensive. I do see quite a few of them, mostly older. Breakers are proprietary. I absolutely HATE the neutral/ground bar layout. This is s last resort panel for me.

I really like Siemens, with a copper buss. More expensive, but great quality. I love the neutral bar layout. Hard to find around here though.


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