On my electrical service panel, the "MAIN" breaker is a double pole breaker and each side is marked "100 Amps." Does that mean that when the total amperage exceed 100 one side it will trip, or if the total of both sides exceed 100 amps it will trip? Also, I recently bought a new electric range. I need a double pole breaker because it uses 240 volts, and the total consumption of the range should not exceed 40 amps. Should I get a double pole breaker with 20 amps marked on each side (the whole thing is marked 20 amp double pole breaker) or should I get one with 40 amps marked on each side (the whole thing is marked 40 amp double pole breaker)?

240 V is really two 120 volt sources 180 degrees out of phase with one end connected to ground. In order to pro

Tect the load, both sources of 120 need to be removed. The breaker trips when either side exeeds 100 A. If the load was a water heater, both breakers would see the same current. If on side shorted to ground, then one side would be exceeded. In 4-wire 240 volt installations where a neutral is involved, the currents would be different.

You have a 100 amp main breaker.

Current consumption and required breaker may be two different things. You need a 40 A double-pole breaker (40 A on both sides).

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Originally Posted by KeepItSimpleStupid

240 V is really two 120 volt sources 180 degrees out of phase with one end connected to ground. In order to pro

tect the load, both sources of 120 need to be removed. The breaker trips when either side exeeds 100 A. If the load was a water heater, both breakers would see the same current. If on side shorted to ground, then one side would be exceeded. In 4-wire 240 volt installations where a neutral is involved, the currents would be different.

You have a 100 amp main breaker.

Current consumption and required breaker may be two different things. You need a 40 A double-pole breaker (40 A on both sides).

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So if both sides of my main breaker were allowing 99 amps to pass through (total of 198 amps), then it will not trip, and will trip if one side has 101 A passing through it and the other has 50 A passing through?

In theory, yes. The tripping time would be different depending on the amount of overload. Meaning a 100 A breaker would trip faster at 150 A than 101 Amps.

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Originally Posted by shadowx360

So if both sides of my main breaker were allowing 99 amps to pass thru (total of 198 amps), then it will not trip, and will trip if one side has 101 A passing thru it and the other has 50 A passing thru?

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Just to clarify. We do NOT add up amperage in a 120/240v service to get the equivalent 120v current.
For instance, it is not accurate to say a 100A service is the same as 200A at 120v. This is incorrect, even for comparison reasons. It is 100A @ 240v, or 100A on each of two legs of the service.
This is NOT the typical case of 1+1=2. 100A + 100A does not equal 200A.


Also, KISS makes a very good point about trip curves. A 100A breaker will likely never trip at 101A.
It can sit at 110A for several minutes and longer. It can be at 150A for quite a few seconds. It will trip instantly at a 20,000A fault.

KISS: I like that. Wow you guys must have a very close relationship> :D

You've never heard that acronym before? "The KISS method"?

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Originally Posted by stanfortyman

You've never heard that acronym before? "The KISS method"?

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No, never heard of it. Yet I have seen the name: KeepItSimpleStupid

How about the SWAG method?

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Originally Posted by KeepItSimpleStupid

How about the SWAG method?

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Yes, I heard of it! Sop. Wild---Guess