circuit breakers

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larryadams

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I am an electrical apprentice and I have a question that came up at work the other day.

We were looking at an old 60 amp fuse box in an old house and the journeyman I was with said that a 60 amp fuse can actually handle a total of 120 amps at one time because the 2 buses are on opposite phases. But I thought a 2 pole fuse or breaker could only handle whatever amps was listed on it, regardless of the fact that it had two poles.

I know 2 pole breakers are used a lot, too and I'd like to have a clear understanding of how they work compared to 1 pole breakers.

Thanks in advance,

Larry
 
Re: circuit breakers

This is a fairly popular misconception about one of the fundamentals of electricity. It is not easily explained or easily understood unless you have had some theory background.

You are correct. A 60-ampere fuse will only permit 60-amperes to flow through it before it opens. When you have a single-phase, alternating current and a line-to-line load, the circuit is between the two ungrounded conductors. At a 60-hertz frequency, the current changes direction in these ungrounded conductors 60 times a second.

So, imagine slowing electricity down to 1-hertz. Evey second the current reverses its' course. If you have a 60A load connected between these two conductors, 60A will flow "out" "A" pole of the fuse while 60A is flowing "in" "B" pole. 1 second later, the current reverses and the 60A will flow "in" "A" pole, and "out" "B" pole.

Some like to think of a breaker or fuse as a water faucet that spills out electricity at each of its terminals, and that simply not the case. You need to remember that with electricty, everything is a circuit. Current will only flow if it has a return path to its source. Once that path is established, current will alternate between whatever conductors are connected to that source.

So, next time you lokk at a panelboard, imagine current racing in and out alternately between whatever conductors are connected to that circuit. If it is a line-to-neutral load, then the current races in the breaker and out the neutral, then reverses back in the neutral and out the breaker. With two pole devices, the alternation takes place at the terminals.

I would suggest that you obtain a good electrical theory book. Pictures would assist in this dicsussion dearly.
 
Re: circuit breakers

Perhaps he used the wrong wording.It is possible to have 60 amps line 1 to neutral at same time you have 60 amps line 2 to neutral.Your 240 volt load is limited to 60 amps.
 
Re: circuit breakers

Bryan?s description is a very good way to think about the way current flows. But let me add another way to think about it.

Half of what your journeyman said was correct. You can have 120 amps worth of load, all of it running at the same time, on a 60 amp panel. You can connect 60 amps of single phase, 120 volt loads between Phase A and the neutral. You can also connect 60 amps of single phase, 120 volt loads between Phase B and the neutral. You can then turn all of it on. I think this is what you were being told, even if your journeyman did not state it in this way.

However, the journeyman?s error, and as Bryan stated it is a common error, was to think that 60 plus 60 equals 120. That only works if the two ?60?s? are the same type of thing, like adding 60 apples to 60 apples. But the two currents you are adding are not the same as each other. There is no really simple way to say it, but I?ll try: You are adding ?60 amps of Phase A 120 volt load? to ?60 amps of Phase B 120 volt load.? The sum of these two is ?60 amps of Phase A to Phase B 240 volt load.?

Does this help?
 
Re: circuit breakers

Thanks for all the replies. I think I have a clearer understanding now.

If I understand you guys correctly, a 2 pole breaker or fuse will not trip as long as the current on each phase stays below the amp rating of the device. So if a multiwire circuit (2 circuits) with 15 amps of load on each circuit is protected with a 20 amp 2 pole breaker, the breaker wouldn't detect 15+15=30 and trip. I understand that there would be 240 volts between the two ungrounded conductors, but that shouldn't affect whether it trips. Am I right?

I don't get to decide what breakers to put in the panel or how to design the circuits yet, but I want to understand how things work. Someday I'll be the journeyman, and I sure don't want to be ignorant of stuff like this.
 
Re: circuit breakers

Originally posted by larryadams: . . . the breaker wouldn't detect 15+15 = 30 and trip. . . . Am I right?
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Not quite right. You are still missing a key concept: 15 + 15 does not equal 30. Rather, 15 + 15 equals 15. You are not adding two of the same type of thing. The two 15?s are different from each other, and the thing you get when you add the two 15's is different from either of the original 15's.
 
Re: circuit breakers

Originally posted by charlie b:
Originally posted by larryadams: . . . the breaker wouldn't detect 15+15 = 30 and trip. . . . Am I right?
Click to expand...
Not quite right. You are still missing a key concept: 15 + 15 does not equal 30. Rather, 15 + 15 equals 15. You are not adding two of the same type of thing. The two 15?s are different from each other, and the thing you get when you add the two 15's is different from either of the original 15's.
Click to expand...
Let me see if I get it now. The 15 amps on phase A plus the 15 amps on phase B would still be 15 amps as far as the 2 pole breaker is concerned, but it would be 15 amps at 240 volts instead of 120 volts. The 2 original 15's are different from each other because they are on opposite phases, which is why the currents are not added together but the voltages are. Or am I still missing something?
 
Re: circuit breakers

Now you've got it. Another way to look at it is if you convert to power:
Phase A: 30A x 120v = 3600VA
Phase B: 30A x 120v = 3600VA
Total power (Phase A=B): 7200VA
Total current Phase-to-Phase: 7200VA/240v=30A
 
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