Posted by Bryan Wright on March 31, 2001

Some power monkey of questionable repute once told me that if I have a dual 50A breaker in a single-phase, 4-wire 220V breaker panel (like at a club or in my house or something) that each hot leg coming off the breaker gets knocked down to 110V, 50A for a total of 100A. I know Ohm's Law and stuff, but that makes no sense to me. He implied that somehow between the bus bars in the panel and the two hot leg lugs on the dual breaker it gets broken down to 110V, 50A per leg. This sounds like horseshit to me... am I stoned? Breakers are just a means of applying safety and (basically) a switch to anything downline, not stepdown transformers... RIGHT?!?!?

If I AM stoned and its NOT horseshit, please explain why this happens.

Humbly waiting to be schooled for such a stupid post...


Reply posted by Doug Matthews on March 31, 2001

Your "power monkey" is correct. No horse droppings involved.

In the case of single-phase power (US residential, some commercial - 110/220V, 120/240V, etc. - let's use 110/220V since you already started down that path) what you have coming into the building is two hot legs, a neutral and a ground.

Each of these hot legs is 110V with respect to neutral (and hopefully safety ground ;-)). The neutral and safety ground are typically bonded at the building service entrance (and should NEVER be bonded downstream from the service entrance - this includes inside your distro).

The hot legs (110V), additionally, are of opposite POLARITY to each other (NOT out of phase by 180 degrees). This is accomplished by the distribution transformer that knocked the voltage from distribution level (i.e. 12,470V hot-neutral or whatever the distribution level is in your area) down to the 110V per hot leg. The two secondary (low voltage) windings of the transformer are wound around the transformer core in opposite direction to each other so that when one voltage waveform is going negative (wrt neutral) the other is going positive.

So, if you meter across hot-neutral, you get 110V. When you meter across the two hots, the opposite polarity waveforms add and you get 220V.

Now, the circuit breaker does not provide current - it limits it. In the case of, for example, a two-pole 50A breaker, the breaker is limiting the amount of current that can be pulled (by the load) in each of the hot wires to 50A (let's ignore transients for now - short peaks in current when a motor starts, or your amps capacitors start to charge from empty when the amp first is turned on). Whether you load with 110V loads balanced on each side of the two-pole 50A breaker (to a maximum total of 100A 110V - approximately 10KW), 220V loads across both sides of the two-pole 50A breaker (to a maximum of 50A 220V - approximately 10KW), or some combination of both, each side of the two-pole 50A breaker will allow only 50 amperes to flow.

Circuit breakers are sized according to the wire used, to keep the wire from overheating. A 15 amp breaker is paired with 14-gauge wire, a 20-amp breaker is paired with 12-gauge wire, etc. This breaker to wire gauge pairing is irrespective of voltage level (however the wire insulation must be rated appropriately for the voltage level).

When current flows away from the breaker in one of the hot legs, an equal amount of current needs to flow back somewhere. If no load is present in the other hot leg, the current returns in the neutral. Current flow (assuming each leg is loaded) in each hot leg, at any instant, is in the opposite direction to the other. Thus, if the hot legs are exactly equally loaded, each hot leg provides the return path for the other and there is no current in the neutral. If one hot leg is drawing 5 amps more than the other, the neutral will be carrying 5 amps back to the panel. It is desirable to have the two hot legs reasonably equally loaded.

[Note that if you somehow (incorrectly!) hook your disto up so the hot wires both come from the same busbar ("roll your own" two pole breaker from two open single pole breakers on the same 110V side, or use a cheater to power the distro from two edisons on the same 110V side) your neutral wire would be providing double duty. This would mean that if you loaded both of the hot legs to 50A each the neutral would see 100A - not a good thing!]

Well, I've rambled on enough for now. Hopefully someone will have learned something. As always, respect electricity. If you don't know FOR SURE what you are doing, find someone who does (and I guess, officially, that person should be licensed)!

Doug "degreed power monkey" Matthews


Further reply posted by Doug Matthews on March 31, 2001

I re-read this more carefully. Maybe your power monkey was off-base in the details, or maybe you misunderstood him:

“He implied that somehow between the bus bars in the panel and the two hot leg lugs on the dual breaker it gets broken down to 110V, 50A per leg. This sounds like horseshit to me... am I stoned? Breakers are just a means of applying safety and (basically) a switch to anything downline, not stepdown transformers... RIGHT?!?!?”

There is NO transformation happening in the circuit box. The breakers are simply current limiters. The transformation to 110V (per leg) happens outside the building. In the two-pole breaker, each pole is connected to one 110V hot leg.

Next:

“(i.e. 12,470V hot-neutral or whatever the distribution level is in your area)”

Make that 12,470V line to line - 7,200V line to neutral.

Doug