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I live in an older home I bought 2 years ago. It was built in 1924. While becoming familiar with the electrical system I discovered the circuit powering the heating furnace, from a 20 amp CB in the main box. As I traced the conduit to the gas furnace I found "power switch" box that contained 2 olde-fashion fuses.

Circuit Diagram

When I opened the fuse/switchbox I found 2 fuses. A 30amp & a 15amp. Is this correct? Seems to me the fuses should be the same rating, right? Is 2 30amp fuses correct considering that the CB Box breaker is rated at 20 amps? Thanks! Dave

Fuse/Switchbox

Here's the CB in the box. 20amp 120v. 20amp CB in Main Box

Here is the fusebox with door closed. Not sure what the text means. Is it possible that prior to the currently installed gas furnace there was a 240v electric furnace that was protected by this fusebox? And then when it was changed to gas they used the previous 240v wire for the 120v line needed? Fusebox Closed

Here's the furnace name plate. Furnace Name Plate

Here's the Gas Furnace Manual title page with the model number, etc. Manual Title Page

I found this wiring diagram. Wiring Diagram #1

Here's another diagram. Another Diagram

Here's the 3rd diagram. 3rd Wiring Diagram

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  • What make and model is this furnace? Can you post a photo of its nameplate and/or wiring diagram please? Nov 14, 2020 at 5:19
  • Welcome to the site Dave, Please take a look at the tour and How to Answer for more information about why your posts were deleted here.
    – BMitch
    Nov 14, 2020 at 23:55

3 Answers 3

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Background: The 20A fuse is a Type S, meaning it is a "time delay" type fuse that allows things with motors to get to speed without making the fuse blow. The 30A fuse is a Type W with is not time delay, it is a "fast acting" type of fuse.

Best guess... The furnace originally had Type S 20A fuses, but something was wrong so fuses kept blowing and after getting frustrated (but not finding out why), someone just put a larger Type W fuse in there so that it would not blow. Totally the wrong thing to do and is one of the biggest problems with fuses, which is why they are not used in houses any more.

Some time later your house was upgraded to using circuit breakers (circuit breakers would not have been used in 1924) and the installer wisely put a 20A breaker on that circuit, rendering the fuses irrelevant (assuming the wire is 12ga or larger).

But... here's another potential problem. Your drawing seems to imply that there is a 1 pole 20A breaker on the one wire feeding this, then the photo seems to show that there is a fuse (the 30A) on the white wire, which would be typically indicating the neutral circuit. It is illegal and DANGEROUS to have a fuse on the neutral circuit! If the breaker is 2 pole and this is a 240V circuit going to the furnace, then it's fine, other than the use of a white wire (white is only to be used for neutral circuits).

If none of this makes sense to you, I suggest getting a qualified electrician to look at it and, if possible, just remove those fuses.

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  • If it is a 240v circuit to the furnace then there is still no purpose for different size fuses considering there is no additional wire to carry differential current in the disconnect. Nov 14, 2020 at 19:20
  • @Dave H, Re: your last question, "Is 2 30amp fuses correct considering that the CB Box breaker is rated at 20 amps?" No, A 15A might be required if the circuit for a 20A circuit was supplied, but the (replacement) furnace label requires 15A. Nov 14, 2020 at 19:46
  • @JRaef .. There is only 1 20amp CB .... not a double & not 240v. I checked with a meter. It is a gas-furnace. I'm new to this forum. I will try to post more pictures I took.
    – Dave H
    Nov 14, 2020 at 20:53
  • @JRaef .. and I am assuming that this should be a dedicated circuit. Powering nothing else (i.e. outlets, lights, etc.) right?
    – Dave H
    Nov 14, 2020 at 21:45
  • @JRaef .. You said " and the installer wisely put a 20A breaker on that circuit, rendering the fuses irrelevant (assuming the wire is 12ga or larger)." Does this mean I could rewire this directly to a 20amp CB with #12 wire & bypass this old fuse box completely?
    – Dave H
    Nov 14, 2020 at 21:49
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The problem: some prior installer kludged things inappropriately because the existing switch lacked a neutral block

For some (historical?) reason, a prior installer used a plug fused safety switch for the furnace. However, the switch they used lacks a solid neutral block, so they ran it through the second pole of the switch, using a 30A plug fuse because dummy plug fuses aren't a thing. That was indeed wrong of them as fusing the neutral is unsafe (imagine if the neutral fuse blew but the hot fuse didn't for some reason: now you have something that's not working, but is still live inside, and can zap you as a result) and against Code (NEC 240.22).

Thankfully, it's easy to fix. Simply turn the breaker off, open up the fused switch, unfasten the two white wires from the terminal screws they are attached to, join them with an appropriate wirenut, button things back up, flip the breaker on, and enjoy!

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What you have is correct. Your output from the breaker panel is 20 Amp one phase, in other words 120Volt. Now the switch box has 15 Amp through the black wire which is (the power) correct because the control and the motor won't need more. The neutral line could even be shorted together and no need for the fuse (whatever the value is) but the fuse box mostly sold this way, with 2 fuses to be able to use it mainly for 240 volts. Whoever said that it is dangerous to have a fuse on the neutral is mistaken. Conclusion, what is on your photos is correct. Only one more point to add, the 20 Amp breaker could even be replaced to 15 Amp but no problem to keep it as is if no other circuits other than the furnace taking current from it.

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    See NEC 240.22 for why fusing the neutral here is a no-go. Mar 12, 2021 at 3:20

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