Need help understanding amperage ratings for panels and sub panels

Question

Im trying to understand the concept of amperage ratings for panels, and sub panels, but I'm not sure I have it figured out, sorry for any novice terminology.

I'm in America. Grounds omitted for clarity. Thanks!

Part 1:

Let's say I have a 100amp main service panel (two hots at 120v each and a neutral coming into it).

I install a 60amp double-pole breaker in my main. (Lets say for ease of example that this is the only breaker in the main panel, aka nothing else is using electricity in the main panel.)

I run 6 gauge feeder wires (two hots, one neutral) from this 60amp double-pole breaker to a smaller 60amp panel, a few feet away.

This sub-panel has a 60amp main breaker.

Now I install four 20amp single pole breakers, all in line, so two on each leg.

Now lets say I install four total 20amp single outlet receptacles, 1 outlet per circuit (using 12 gauge wire).

I have four theoretical high powered devices and i plug one into each outlet and turn it on. Each device continuously draws about 1,920 watts. So in total i am drawing about 7,680 watts (four x 1,920 watts).

The 60amp sub-panel can handle this because it is less than 14,400 watts (60amp x 240v). Is this the correct way to calculate this?

Now lets say i double up and install eight 20amp single pole breakers, and eight total 20amp single outlet receptacles, one on each circuit -- but now i also have eight total high-powered devices, and i plug one into each receptacles and turn them all on.

Each devices continuously draws about 1,920 watts. So in total i am drawing about 15,360 watts (eight x 1,920 watts).

This would trip the breaker in the sub panel, because it is over the 14,400 watts (60amp x 240v) it can handle, correct?

If this is all correct, then I am wondering this:

Part 2:

In the first example, I installed four 20amp single-pole breakers, two on each leg. But what if i install them all on one leg -- for instance, I leave a space between each breaker I install.

Now I run the same experiment -- one 20amp outlet on each circuit, four devices total, each plugged into one outlet. Each device draws 1,920 watts, for a total of 7,680 watts (four x 1,920 watts).

Would the 60amp breaker still be fine, or would it trip because all the circuits, and therefore devices, are on one leg?

Essentially I'm asking, would i still calculate the amount the breaker can handle by multiplying 60amp by 240v, or would i now have to multiply 60amp by 120v, which would only be 7,200 watts (less than the 7,680 watts my devices are drawing).

Thank you so much for your time!

Answer 1

Part 1:

100% correct.

Part 2:

The way I think about it is this:

• A 60A two-pole breaker - e.g., like you have in a main panel for the feeder to a subpanel - is not exactly 60A @ 240V. Rather, it is 60A @ 120V x 2. Each pole can, in theory, trip independently, but with 240V loads they would always trip together. With unbalanced 120V loads - e.g., 4 x 20A all on one leg - you can indeed end up with one leg overloaded and trip even though the other side is OK.

Remember, you are not limiting "how much power can go through the cable", you are limiting "how much current can go through each individual wire". Each wire is limited to 60A of current, with the total power, in this example, either 60A x 120V per leg or 60A x 120V x 2 for the two 120V circuits or 60A x 240V for a 240V circuit. All of which in the end are exactly the same.

Your calculations and interpretations are basically correct, it just gets a little confusing along the way.

This all relates, of course, to two key things:

• As much as reasonably possible, 120V loads should be split between the two legs so that you can make the best use of the available circuits/breakers/etc. There is another small advantage, though it doesn't make much real difference - the neutral current is the difference between the hots. If you have 50A x 120V then your neutral carries 50A as well. If you have 25A x 120V x 2 then your neutral does nothing at all!

• Panels and subpanels can be oversubscribed. In your example, a 60A panel could easily have 8 x 20A - 4 x 20A on each leg (and actually quite a bit more than that). That means that in theory if you plugged full power devices into everything - heaters on every circuit all on at the same time - then you could easily trip that subpanel breaker (and the same goes for the 100A main). But sizing is based on realistic expectations - and there are formulas and guidelines to follow. But some simple examples:

  • If you have air conditioning and electric resistance heat then it is a pretty safe bet that you won't run them at the same time, so only one (the larger) really counts.
  • You may have a bunch of 15A or 20A receptacle circuits in various rooms. Except for the kitchen (where you could have toaster, coffee maker, etc. all at once) most of the time only a few receptacles are in use at a time and most of those will typically be lighting (very low power with LEDs, but even incandescent might be only a few hundred watts on a 15A circuit), cell phone chargers, TVs, etc. - which might use anywhere from a few watts to a few hundred watts each. The big exception is someone with a workshop full of power tools, but most people don't have that. So a 15A or 20A circuit might have 6 or more receptacles - and that's OK as long as you don't use them all for high-power devices at the same time.
  • Lighting circuits are typically 15A or 20A. But even in the past (incandescent), the amount of light needed was far less than the circuit capacity. Today with fluorescent or LED a lighting circuit might use 100W or less for a large room.

So adding up the total capacity of all the circuits is important but does not directly determine the subpanel or main panel size.

In addition, as I have learned from the pros here, while the breaker in the main panel for the feeder to your subpanel is sized to match the wire (and the wire is therefore sized to fit your actual needs since larger costs more), the main breaker in your subpanel functions as a convenience shutoff switch and therefore does not need to be the same size. Going with your original example, the subpanel could have a 60A breaker in the main panel feeding it but in the subpanel itself the main breaker could be 100A (or more!) and that's OK because the ultimate protection will be provided by the 60A breaker in the main panel. If you use 60A in both then there is no guarantee which breaker will trip first or if they will both trip at the same time.

Answer 2

You don't need a main breaker in a subpanel. If it's in an outbuilding, you need a main shutoff. The cheapest way to get it is use a panel with a main breaker, but nobody cares the breaker value if you're only using it as a shutoff switch. In fact, small breaker value is bad, because small panels come with few spaces, which means running out of breaker spaces is a real risk. It's pretty dumb to ever run out of breaker spaces given how cheap they are.

So making your subpanel "main" breaker match your supply breaker is misguided. (no, it does not help the nearer breaker trip first, coordination is not that easy).

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Part 1: Breakers in a panel totalling up to near breaker rating

Not quite. You need to calculate load separately on each pole.

So you were incorrect in the first place to be looking at total watts served as compared to total watts of the supply breaker. You just can't do that; you must compute each pole unless you know the loads are 100% balanced.

Wow, we got to part 2 quickly!

Part 2: Poles matter

Let's do this in amps. You have a 60A 2-pole breaker, which you can use as 2 120V legs of 60A each. You have 4 breakers in there feeding 16A loads each, or 64A. All of them are punched down onto pole L1. Did we overload the breaker? Yes, yes we did. We're pulling 64A on a leg only rated for 60A.

Now suppose we balance the panel with two 16A loads on each pole. Now we have 32A per pole - that's fine.

You could also do 3-and-1 (48A and 16A) but it's best practice to balance panels as best you can.