Gas Fired Residential Furnace Power Cable

**Bob Knauff** · 03-01-2010, 08:50 PM

A fellow inspector in the Las Vegas area is adamant about calling out the common (here at least) use of a 16AWG, stranded copper, 2 conductor w/ground, appliance power cord attached to gas fired furnace and plugged into the dedicated outlet nearby for electrical power to the unit. Since most units are mounted in the attic here, he maintains the area gets so hot in this climate that the extreme heat can and has caused the "light weight" (his term) cord to fail and even catch fire in the past. He insists the furnaces need to be hard wired with at least the typical NM 12 Gauge, solid copper wiring, no plugs.

I am new to this extreme climate and although I can see the reasoning I am unable to find any requirements by mfg. or codes, regarding the issue.

He offers no written facts or references to back up his assertions.

Any thoughts, references or facts one way or the other?

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**Markus Keller** · 03-01-2010, 09:12 PM

Have to agree with your friend. I'd write that as D&H regardless of install conditions but especially in a hot attic. Besides the obvious, I don't see 16 awg being rated for the blower motor amp draw.

**Rick Cantrell** · 03-02-2010, 05:19 AM

If it is that hot in the attic, why would the furnace be on?

**Raymond Wand** · 03-02-2010, 05:23 AM

Extension cord w/plug is not acceptable, the furnace should be hardwired.

Don Burbach · 03-02-2010, 09:08 AM

Originally Posted by Rick Cantrell

If it is that hot in the attic, why would the furnace be on?

I think the OP means that the blower would be powered because the AC was running.

But, isn't the furnace listed by a proper authority?

Almost ALL of the furnaces that I see are powered by an appliance type extension cord with a molded 3-prong grounded plug. I've never been concerned about the wire size because I assumed it was delivered with the unit, therefore factory approved and listed.

**Raymond Wand** · 03-02-2010, 09:57 AM

Not with 16 awg.

**Rick Cantrell** · 03-02-2010, 10:44 AM

"I think the OP means that the blower would be powered because the AC was running. "

Oops, I wasn't thinking.

Joe Asta · 03-02-2010, 10:45 AM

Is this what it looks like?

cord.jpg

**Bob Knauff** · 03-02-2010, 11:52 AM

Good one Joe! Ha!

It is the exception to see hard wired air handlers (A/C - furnace) here. The plug is considered the service disconnect avoiding the need for a switch.

As I said, I can't find any specifics from either York or Ruud mfg. regarding the issue. I searched their websites for stats.

Thanks.

David Bell · 03-02-2010, 05:01 PM

If it is a fuel burning furnace in the attic space it has to be served by a thermal saftey switch that can also be considered the local disconnect as it is a pull out device. The wire size is determined by the total load of all on that circiut, whether it's just the furnace,or a condensate pump or heat tape that could be found on a condensing gas furnace in a very cold climate.

**Jerry Peck** · 03-02-2010, 06:05 PM

Originally Posted by Bob Knauff

It is the exception to see hard wired air handlers (A/C - furnace) here. The plug is considered the service disconnect avoiding the need for a switch.

It may be the exception to see permanently wired in AHU, but they should be as the cord and plug sets are not rated for that use - especially if you are talking about electric AHU/furnaces (electric strip heaters).

They burn the plugs/receptacles up on a rather alarming basis as they are not rated or approved for that use.

**Bob Harper** · 03-02-2010, 11:00 PM

I don't have it but the requirement would be found in the NEC.

**Bob Knauff** · 03-03-2010, 10:10 AM

Thanks guys. So it's the plug that is the major problem it seems. I'll keep researching the issue.

**H.G. Watson, Sr.** · 03-03-2010, 12:38 PM

Originally Posted by Bob Knauff

Thanks guys. So it's the plug that is the major problem it seems. I'll keep researching the issue.

Not just the cord cap, the cord itself, the ambient temperatures in the unconditioned attic space, and the wiring method (AHU listed to be wired thusly, and is the cord set listed & supplied by the AHU manufacturer?).

You describe a very hot environment - attic temperatures which rise to what I suspect would require extensive derating to use NM as well, to accomodate the fan motor draw for the air handler when called upon to circulate for AC when the attic has experienced its maximum heat gain, voltage drop/increased resistance would cause an increased current draw.

As you describe the environment it seems reasonable to expect that attic temperatures may exceed 140 degrees F (60 degrees C). The listed appliance cords I am familiar with are limited to 60 degrees C. 16 awg SPT-3s are usually limited to 13 amps and 60 degrees C and are usually classified as power supply cords, not major appliance cord sets.
Major appliance cord sets are usually 12 awg & 14 awg type SPT-3, and are rated for 20 and 15 amps respectively, usually limited to 60 degrees C and 300 volts. (such as the attached pdf document). They are usually listed and used to replace existing cord sets for portable dishwashers, microwaves, freezers, portable dehumidifiers and portable air conditioners and the like, not for permanently (attached) installed equipment (microhoods and range/cooktop, if permitted by local code, exhaust hoods excepted).

Since the gas fired appliance (furnace) and the Air Handler are not "attended" they operate in an "automatic" and unattended mode - and vibrations, environmental temperature swings influencing more rapid metal fatigue of the receptacle springs, even if a permissable wiring method would expect a locking type cord cap set and receptacle rated at least 20 amps, and dust, debris, pollen, etc. may freely blow through an attic via its ventillation, etc.

Non conditioned space and nightime cooler temperatures at certain times of the year - i.e. temperature swings and unconditioned ventillated (even if just "natural ventillation") space - dew points, condensation likely - damp location wiring methods, this effectively rules out NM cable for this location (but not necessarily NMC Cable if properly sized, rated and properly derated). I much prefer metalic conduit methods in attics as the metal conduit has the ability to "put off" its heat, unlike plastics, and are much more "critter resistant".

**Bob Knauff** · 03-04-2010, 01:21 PM

Yes, Mr. Watson, I had been mulling over the hot environment/derating consideration as well for the wiring but hadn't yet considered the temperature change/metal fatigue aspect of the receptacle and so on. Very insightful!

I have been researching the issue in the NEC but you know how convoluted that publication can be and so haven't had much luck there yet, except for reference and data on appliance cords and some on motors.

Thanks for the cord information.

I too prefer the metal conduit approach for the same reasons!

I am continuing my research in the hopes of pin pointing something official in print before I contact the AHJ and see what their opinion is. In the mean time, if I find anything substantial I'll report it here.