Furnace install

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NegPF

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Milford PA
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Mechanical Engineer
I have a furnace that calls for 15 AMPS MCA, but a minimum and max overcurrent protection device of 20 amps. The builder installed a 15 amp breaker with 14/2 wiring. I asked them to swap it to a 20 amp breaker with 12/2 as the manufacturer states it wants a minimum of a 20 amp breaker, but builder is pushing back that MCA is 15 amps, so what is installed in fine. He states that electrical inspector passed it, so its fine. He also did something similar with condensing unit, unit has MCA of 26 but minimum breaker of 35 and max of 45, but has it with 10/3 wiring on 30 amp breaker, i asked them to atleast change the breaker to 35 or 40. Does this sound right?

It is a 1001521SA furnace.

 
As the manufacturer specifies a "minimum overload protection device" you have described two violations of 110.3(B). You are correct that the breakers should be upsized to the minimum size specified.

Cheers, Wayne
 
As others stated the install is wrong. But at least the conductor sizes are correct (even if using NM cable) so this is an easy fix. The furnace breaker should be changed to a 20 amp and the condensing unit breaker should be changed to a 35, 40 or 45 amp.
It is clear that the parties involved here do not understand how conductors and OCPDs are properly determined for quipment such as this. Also note that the conductors do not have an ampacity equal to the OCPD in this type of application-they only need ampacity of the MCA on the data plate.
 
Does the Furnace fall under 240.4G as AC equipment/Motor device? I would agree that the outdoor unit condensing unit. I am on the fence, but I would argue that the furnace would not meet the parameters.
 
Welcome to the forum.

NegPF said:
Does this sound right?
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No and yes. HVAC equipment comes with specs for minimum wire size (MCA) and maximum breaker (MOCP).

The wire must be sized at least large enough for the unit to run, and the breaker large enough for it to start.

Actual over-current protection is built into the unit, so the breaker is there mainly for short-circuit protection.
 
If it is a new build, the walls are all open, bumping to the next wire size goes a long way for future proofing.
 
If it's installed, approved and it works, I think I'd let it go.

Looking at the cutsheet, none of the electrical information makes sense. I think the manufacturer just puts numbers in without really knowing what it means.
 
texie said:
As a side note to this discussion, I don't recall ever seeing a manufacturer listing a minimum OCPD as part of their specs. Nice thing in my view. I wonder if it is also on the data plate.
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Several manufacturers list both in the specs and on the nameplate.
 
Ken_S said:
Several manufacturers list both in the specs and on the nameplate.
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This manufacturer list minimum and maximum OVERLOAD protection devices. Which, of course, is nonsensical. You can't protect and 8.3A motor against overload at 20A.

Even better from this manufacturer, however, is the unit that calls for a Minimum device of 25A and a Maximum device of 20A. I'd love to see the proper protection for that one.
 
david luchini said:
This manufacturer list minimum and maximum OVERLOAD protection devices. Which, of course, is nonsensical. You can't protect and 8.3A motor against overload at 20A.

Even better from this manufacturer, however, is the unit that calls for a Minimum device of 25A and a Maximum device of 20A. I'd love to see the proper protection for that one.
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Yep, I missed this before. They are using the wrong term. It should say max. OCPD. not overload. These are the kind of issues that really should be reported to the NRTL. I've done it in the past and usually they are pretty good about investigating. I would want to know what the actual data plate says.
 
Rock86 said:
Does the Furnace fall under 240.4G as AC equipment/Motor device? I would agree that the outdoor unit condensing unit. I am on the fence, but I would argue that the furnace would not meet the parameters.
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That was my thinking. Art 424 does not fall under 240.4(G). Were I inspecting I would be hard pressed to approve it.
 
augie47 said:
That was my thinking. Art 424 does not fall under 240.4(G). Were I inspecting I would be hard pressed to approve it.
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The OP is about a gas furnace, so it has both a heat source (gas) and an air handler. The air handler uses a motor, so the unit as a whole falls under Article 430. The branch circuit conductors would be covered by 430.25, and the branch circuit OCPD would be covered by 430.54, both of which refer to "Multimotor and Combination-Load Equipment."

Similarly electric baseboard heat would be governed by Article 424 only. But if you add an air handler to make an electric furnace, the same considerations would apply. In this case, as the load from the electric heating elements would dominate, I would expect MCA and MOCP to be fairly close to each other.

Cheers, Wayne
 
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