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Old 02-19-2012, 08:29 PM   #1
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Confused by furnace wiring


Hello,

I'm confused because my furnace is not wired directly to the 20 amp breaker. The line from the breaker runs through a series of two junction boxes to an old fuse box (see attached photos). From the fuse box, a line runs back up to the junction box directly above, where it is pigtailed to the lines going to the furnace. Itís been this way since I purchased the house 25 years ago. However, because of a remodel, I need to move the junction/fuse boxes. Here are my questions:

Is the fuse necessary? If so, what is the reason? Isnít the fuse redundant because of the 20amp breaker that feeds it?

If itís not necessary, can I simply run a line directly from the breaker and tie it into the furnace wires in a junction box and bypass the fuse altogether?

Thanks for your help!
Tom
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Confused by furnace wiring-1.jpg   Confused by furnace wiring-2.jpg   Confused by furnace wiring-3.jpg  

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Old 02-19-2012, 08:36 PM   #2
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Confused by furnace wiring


The fuse isn't necessary if both the fuse and breaker are the same value and the wire is sized properly. However, you do need a disconnect at the furnace. What you have now is a fused disconnect. How far is the breaker panel from the furnace?

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Old 02-19-2012, 08:38 PM   #3
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Confused by furnace wiring


edit out....slow typer

Last edited by zappa; 02-19-2012 at 08:41 PM.
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Old 02-19-2012, 08:39 PM   #4
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Confused by furnace wiring


Perhaps it was just easier to keep it the way it was and/or the disconnect of the original was easier to keep than installing a typical switch?
Shouldn't need two overload protection devices. It will need a disconnect (switch) though. Hard to tell from the picture; but is that 12g THW wire (new stuff)? If 15g, need to upgrade to 12g due to 20 amp breaker.
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Old 02-19-2012, 08:40 PM   #5
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Confused by furnace wiring


I think 3 of us were writing at the same time! At least the answers match!
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Old 02-19-2012, 08:40 PM   #6
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Confused by furnace wiring


The furnace is about 25' from the breaker panel, but add another 5'+ as the wires run.
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Old 02-19-2012, 08:42 PM   #7
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Confused by furnace wiring


The fuse is not required but the disconnect is. Run the supply to a single gang box, install a single pole switch as a disconnect and then feed to the furnace.


I am even a slower typist.
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Old 02-19-2012, 08:43 PM   #8
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Thanks for the responses! And yes, the new line 12g and not 15.
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Old 02-19-2012, 08:46 PM   #9
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...and there you have it....
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Old 02-19-2012, 08:47 PM   #10
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Okay, a gang box with a single pole switch it is. Thanks again for the help!

Tom
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Old 02-20-2012, 12:00 PM   #11
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Confused by furnace wiring


How many amps is your type S fuse?
"S and T fuses are heavy duty time delay fuses and used for circuits having critical or high motor loads or circuits having motors that cycle on and off often (e.g. a sump pump). "

I've seen fuses in series for the reason that the interrupting current rating of one fuse was much higher than the other, like in the RCA Voltohmyst.

When you compare the clearing time of the fuse to the trip time of the breaker you may get some surprises. I'd leave the fuse in; it's not doing any harm.
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Old 02-20-2012, 12:35 PM   #12
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Confused by furnace wiring


Yoyizit,
If you are suggesting the fuse won't blow before the breaker, the breaker will still have to be reset so what is the benefit of the fuse remaining?
He can't eliminate the breaker so the fuse and associated wire connections are nothing but a source of potential connection issues. IMO, there is no benefit to the fuse staying but perhaps I'm missing something?
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Old 02-21-2012, 11:30 AM   #13
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Confused by furnace wiring


Quote:
Originally Posted by bob22 View Post
Yoyizit,
If you are suggesting the fuse won't blow before the breaker, the breaker will still have to be reset so what is the benefit of the fuse remaining?
He can't eliminate the breaker so the fuse and associated wire connections are nothing but a source of potential connection issues. IMO, there is no benefit to the fuse staying but perhaps I'm missing something?
The clearing time vs. current of the type S fuse may be better tailored to protect the furnace than the trip curve of the panel breaker.

Which device clears first depends on the I squared T curve of each and the current drawn. Comparing time vs. current graphs may show this idea more clearly.

I'm reluctant to remove something unless I'm very clear as to its function in the circuit, but it's possible this fuse is redundant or useless or may cause unnecessary problems.

If it does do a better job of protecting the furnace and it's removed it may be a long time before the effects are seen.
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Old 02-21-2012, 12:37 PM   #14
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Confused by furnace wiring


Properly functioning and wired, the duplicative protection is not needed. Also if the fuse were to blow most are not going to think about it in the circuit and will just check the breaker.

I agree with the others, keep it simple and just feed a single pole switch near or on the unit.

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