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USA NEC 2014.

Is there any way to dodge the requirement for a dedicated circuit for a gas furnace? I need to hang one more small hardwired load right near the furnace, and would rather not run a new circuit.

The other load is not HVAC related, and this is an office outbuilding not a domicile.

This is on the side of the furnace and supplies the furnace.

enter image description here

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  • Could you pull a feeder to a small subpanel? The branch circuit would just be from the subpanel to the furnace then, correct?
    – Hari
    Commented Apr 5, 2018 at 0:30
  • What is providing the service disconnecting means for the furnace at this moment? Commented Apr 5, 2018 at 0:30
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    @Harper -- I can't find anything in articles 312 or 408 that would prohibit slapping a surface-mount loadcenter (panelboard+enclosure combo) on the furnace itself, provided it can be attached securely to the furnace Commented Apr 5, 2018 at 3:46
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    I would consider mounting the panel on the unit, I have seen panels mounted on equipment many times in industrial equipment, not so much in residential because there is room in most cases. Three phase make that an answer I will up vote
    – Ed Beal
    Commented Jun 13, 2018 at 15:39
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    @Mazura Yeah, I gave up and dedicated it. Commented Jan 9, 2020 at 4:10

4 Answers 4

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Put a subpanel here

Provided there's enough clear working space out from that side of the furnace, you can replace that fusible mini-disconnect (switched fuse cover) with a small subpanel (a "spa panel" type of disconnect is large enough for this job, although GE makes? or at least made the TL510RT that is basically perfect for a 120V only application if you can get it, and SqD has the QO24L50TTS that will do the trick here as well), that has 2 15A breakers on the same leg, one for the furnace and one for the other circuit. (You can also put the service receptacle for the space on this panel, if one is not already present.)

Or, if this is all in conduit...

If this circuit is run entirely in conduit, then there should be enough space for another 15A circuit (hot+neutral) in there for the remaining loads, allowing the furnace to be left alone on the existing circuit, or vice-versa, with the furnace moved to the new circuit and the other loads left on the existing circuit.

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  • It's in EMT.... I'd have another circuit run before you could get back from the store, having spent not even $5 in wire, while you're spending +$50. - How to dodge and not have to run a new circuit? Spend fifty bucks.
    – Mazura
    Commented Jan 9, 2020 at 2:02
  • @Mazura -- if it's in conduit all the way, pulling another circuit would be an option, but we don't know that from just the photo given Commented Jan 9, 2020 at 3:53
  • True, or if there's a whole bunch of junction boxes. They ended up running a new circuit, +1
    – Mazura
    Commented Jan 9, 2020 at 4:15
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I don't think you can dodge the requirement with Gas Furnace; the code is quite clear about this so dodging it is not possible unless an inspector is blinded by a vision of Ben Franklin (not an endorsement of using such a method, just saying something not right would have to be going on to 'dodge' the code) .

The potential for a "potential" for a serious problem due to overheated wiring and overloaded wiring - not too mention the spikes from other items on the same circuit might be hazardous (even if YOU do know what you are doing - some other wingnut might not). I think this was the reasoning behind the rule and it does make a whole lot of sense. I remember working with industrial size gas roasters and some jobs I just could not do with out a Gas Certification - and that was for good reason too.

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Absolutely not, and for good reason too. Who's to say the next person won't assume what you've done and add-on to your add-on.

And probably the main reaaon why: people can be seriously hurt, even die w/out heat.

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  • Code is about electrical safety not if you have heat or not. There are ways to do this such as creating a sub and then the additional load is on its own circuit from the sub. In addition to this it is not a living space commercial/ industrial requirements are less restrictive.
    – Ed Beal
    Commented Jun 13, 2018 at 15:42
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In addition to the code making it a requirement, there's a considerable amount of line noise that may come from even a small residential furnace. I'm not sure why there's so much noise, but the noise is likely to affect some what you might connect, including lighting. This is from personal experience. A good understanding of electronics enabled accurate diagnosis and correction of the problem - accomplished by restoring it to compliance. I won't make the same mistake again.

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  • Realize as well that noise potential IS a two way street. You don't want something adversely affecting your furnace. It's too risky, expensive, and could be dangerous. I know the people who created the codes know more than I do.
    – Jeff
    Commented Nov 29, 2021 at 6:31

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