Sub panel load

wireday

Senior Member
Have a 240volt 6 space sub panel with a 2 pole 40 amp breaker feeding panel, how many 20 amp breakers can I install? All loads are non continuous, I would like to use four.
 

goldstar

Senior Member
Have a 240volt 6 space sub panel with a 2 pole 40 amp breaker feeding panel, how many 20 amp breakers can I install? All loads are non continuous, I would like to use four.
IMHO, you can install 6 if you want to. I'm sure you know the rule of thumb is not to load a breaker to more than 80% of its value. That being said, in your case 80% x 40 amps = 32 amps. If you don't estimate the draw to be more than that for any length of time you're OK. Just my opinion.:cool:
 

Little Bill

Moderator
Staff member
You can fill it up with breakers if you want. But if you know the load and it will be a simultaneous load, then you will want to only put in enough breakers/loads to not overload the 40A feeder.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Thank you gold star, that would be 32 per leg?
Don't think about "amps 'per leg' ". Think about amps at 120v or amps at 240v. With 120/240v, the more heavily-loaded leg will be the one that matters with a 2p breaker.

You can use as many 1p breakers as you want to. If you overload any one circuit, that breaker will trip. If you heavily load several of those circuits simultaneously, the 40a feeder breaker may trip. It's the total current "per leg" (actually meaning current on either (or both) leg at any given moment) that matters.

That's why the breakers in the main panel can add up to way higher than the main breaker's rating; you're not using many circuits near capacity at any given moment.
 

goldstar

Senior Member
Don't think about "amps 'per leg' ". Think about amps at 120v or amps at 240v. With 120/240v, the more heavily-loaded leg will be the one that matters with a 2p breaker.

You can use as many 1p breakers as you want to. If you overload any one circuit, that breaker will trip. If you heavily load several of those circuits simultaneously, the 40a feeder breaker may trip. It's the total current "per leg" (actually meaning current on either (or both) leg at any given moment) that matters.

That's why the breakers in the main panel can add up to way higher than the main breaker's rating; you're not using many circuits near capacity at any given moment.
Sorry I wasn't clear in my post but I think Larry gives the best explanation here.:thumbsup:
 

kwired

Electron manager
Don't think about "amps 'per leg' ". Think about amps at 120v or amps at 240v. With 120/240v, the more heavily-loaded leg will be the one that matters with a 2p breaker.

You can use as many 1p breakers as you want to. If you overload any one circuit, that breaker will trip. If you heavily load several of those circuits simultaneously, the 40a feeder breaker may trip. It's the total current "per leg" (actually meaning current on either (or both) leg at any given moment) that matters.

That's why the breakers in the main panel can add up to way higher than the main breaker's rating; you're not using many circuits near capacity at any given moment.
That is correct. could supply 60 space panel with this same feeder, as long as you never exceed 40 amps on the feeder there is nothing wrong with it.

Is possible to have many loads that only draw an amp or so yet some desire to put them on individual circuits.
 

powerpete69

Senior Member
Have a 240volt 6 space sub panel with a 2 pole 40 amp breaker feeding panel, how many 20 amp breakers can I install? All loads are non continuous, I would like to use four.
I look at it like this, 240 X 40= 9,600 Watts. You have 9,600 watts available. IN a nutshell, you can install a whole bunch of 20 amp single pole breakers. You say it has full 6 spaces, which means you likely can use mini breakers which could double the amount of breakers you could use, up to12 breakers if that is your set up. Let's say one load is a space heater-1500 watts, another load is a air compressor-1500 watts, another load is a general receptacle that occasionally gets a vacuum, iron or hair dryer-1500 watts. Another breaker is a microwave -1500 watts. Another breaker powers your coffee maker and toaster at the same time, 2000 watts. You could literally have all these devices on and your 40A breaker would not trip and your #8 wire feeding the panel would be fine! However, what are the chances of you having all these on at the same time? Rare. I just listed 5 high powered breaker situations.

Now let's list more realistic situations in a home. Let's add another breaker for a bedroom for lights and receptacles. Another breaker for bedroom 2 and another breaker for bedroom 3. Let's add your Washing machine and gas powered dryer on another breaker. You maybe just added 1000 watts and occasionally 2500 watts if someone uses a circular saw or space heater or whatever. You are now up to roughly 10 single pole 20A breakers....no problem.

Forget about thinking about amps, think more in terms of watts. Less intimidating (And think VA if you are more advanced)

NEC code actually list outlets at 180 watts or more accurately 180VA.
(220.14 (I))​

If you hooked up your panel with the loads prescribed, I doubt it would ever trip. If you had a meter tracking it I doubt it would ever go over 4800 watts. Think of it this way too, you would need a whole bunch of people in the house for all these loads to be connected.

Worst case, let's say that main breaker trips one day. Ok, no big deal. Reset it. Don't use 4 space heaters, a vacuum, an air compressor, a microwave, a circular saw and brew coffee at the same time~!!~

Also, try to balance your large load single pole breakers between phase A and phase B for obvious reasons or not so obvious reasons as the case may pertain.

If you want to look in the NEC code book, look under section 220.44. Again, a single outlet is just 180 VA. (220.14 (I)). If you are HARD WIRING a space heater or dishwasher, it then counts for 1500 Watts or whatever it's load is. Table 220.56 shows kitchen equipment which pertains to electric cooking equipment, dishwashers, water heaters, etc. Again, your coffee maker and toaster just count for 180 if plugged into single duplex outlet. If separate outlets, jump up to 360 VA, not a big deal.

Table 220.54 shows the dryer derating, but that has nothing to do with you since you are just interested in single pole 20 amp breakers.

With LED lights these days, lighting hardly pulls any power assuming you are using LED's.

Load that panel up, you will love it!!!! I need more POWER Scotty!!!!
 

infinity

Moderator
Staff member
Now let's list more realistic situations in a home. Let's add another breaker for a bedroom for lights and receptacles. Another breaker for bedroom 2 and another breaker for bedroom 3. Let's add your Washing machine and gas powered dryer on another breaker. You maybe just added 1000 watts and occasionally 2500 watts if someone uses a circular saw or space heater or whatever. You are now up to roughly 10 single pole 20A breakers....no problem.

Forget about thinking about amps, think more in terms of watts. Less intimidating (And think VA if you are more advanced)

NEC code actually list outlets at 180 watts or more accurately 180VA.
(220.14 (I))​

If you hooked up your panel with the loads prescribed, I doubt it would ever trip. If you had a meter tracking it I doubt it would ever go over 4800 watts. Think of it this way too, you would need a whole bunch of people in the house for all these loads to be connected.
I think that you were just trying to make a point about using VA instead of watts but if you're referring to a house there is no 180VA receptacle value.
 

powerpete69

Senior Member
I think that you were just trying to make a point about using VA instead of watts but if you're referring to a house there is no 180VA receptacle value.
Fascinating. I see it in section J right after section I in 220.14. As you know, we always use 180 VA in commercial and industrial panels for receptacles...hence the incidental overlap of knowledge if you will. Thanks for the tip.
 

infinity

Moderator
Staff member
Fascinating. I see it in section J right after section I in 220.14. As you know, we always use 180 VA in commercial and industrial panels for receptacles...hence the incidental overlap of knowledge if you will. Thanks for the tip.
Coming from the commercial side that one always threw me too. I believe that it was someone on this forum that straightened me out a while ago. :)
 
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