110.14 (C) - 75C Terminations

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Sean.Day72

Member
Location
Florida
I have a scenario where we are replacing VAV with heaters. The existing wiring is #12AWG. The kw load of the new heaters went up to 5KW @ 277 = 18Amp FLA for the new heaters. We are replacing existing 20Amp breakers with 25Amp breakers to accommodate this upsized load.
To save cost, are we able to utilize the existing #12AWG wire? I understand for loads under 100Amp to assume using the 60C ampacity table unless terminations are marked for 75C. The VAV has a label that says "Use wire suitable for at least 75C" that is not really stating the termination if rated for 75C. Has anyone ran into this? It is a lot of VAV units and could save a decent bit if existing #12AWG wire can be used vs installing new #10Awg to each new unit. Check the O&M and it does not state electrical termination temp ratings.

The Sqaure D circuit breaker is rated for 75C so good on that end of the circuit.
 
Since the documentation says to use 75C wire I would say that's what they are saying the terminations are. I would use the #12 and not look back.
 
Do VAV heaters fall under article 424? My guess is they do, then your branch circuit conductors would need to be 125% of 5kw or ~23 amps @ 277
 
In my opinion, you have 18 * 1.25 = 22.5A of load. You should use 25A OCPD. (424.4)

240.6(D) says, unless you meet one of the exceptions, #12 must be protected at 20A. That is regardless of termination temp.

I think you need #10 and 25A OCPD.
 
If a VAV has a motor (either for a fan or a damper or both), then Article 430 would apply. In which case #12 Cu is not limited to a 20A OCPD.

If further 424 applies to the heater component, then the MCA for the unit would be 18A * 125% plus the motor FLA. If that's no more than 25A, and no derating applies, then #12 on a 25A breaker would work.

Cheers, Wayne
 
wwhitney said:
If a VAV has a motor (either for a fan or a damper or both), then Article 430 would apply. In which case #12 Cu is not limited to a 20A OCPD.

If further 424 applies to the heater component, then the MCA for the unit would be 18A * 125% plus the motor FLA. If that's no more than 25A, and no derating applies, then #12 on a 25A breaker would work.

Cheers, Wayne
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So a space heater with a small fan can use #12 conductors protected at 25 amps?
 
Manufactured published MCA is 22.5 Amps. Published MOCP is 25 Amp. The VAV has a very tiny servo motor and electric heater. We are changing breakers from 20 Amp to 25Amp. The whole thing is trying to justify the 75C ampacity table for the #12AWG conductors so they can be used on 25Amp breaker. I just need code backing and I'm not really finding it. The "Use only 75C wire" I don't take at the termination is 75C. I thought that was just so the heating of the wire does not cause nuisance tripping of breakers.
 
Sean.Day72 said:
Manufactured published MCA is 22.5 Amps. Published MOCP is 25 Amp. The VAV has a very tiny servo motor and electric heater. We are changing breakers from 20 Amp to 25Amp. The whole thing is trying to justify the 75C ampacity table for the #12AWG conductors so they can be used on 25Amp breaker. I just need code backing and I'm not really finding it. The "Use only 75C wire" I don't take at the termination is 75C. I thought that was just so the heating of the wire does not cause nuisance tripping of breakers.
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For this application the #12 is not large enough for the 22.5 amp MCA.
 
infinity said:
So a space heater with a small fan can use #12 conductors protected at 25 amps?
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Yes, do you see any code language that differentiates between whether the motor is the majority of the load or just a small fraction? (*)

Since the OP has subsequently stated that the unit comes with MCA and MOCP markings, it sounds to me like it falls under 240.4(G). Checking how it is listed may confirm that.

That would mean the only obstacle to using #12 Cu at 25A would be if the terminations are rated as "size using 60C, but provide minimum 75C rated insulation." Which is possible. Does the unit manual not provide any guidance?

Cheers, Wayne

(*) I agree it seems odd that for a load consisting of only a 17A heater (and subject to a 125% factor), #12 Cu is limited to 20A and hence too small, while if you add a 3A FLA motor to the load, now #12 Cu is not limited to 20A and may be sufficient.
 
augie47 said:
Seems to be a real stretch to call it a motor or motor operated equipment.
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So if this were a bathroom exhaust fan (with heater), Article 430 would apply, but because the motor operates a damper rather than a fan (if I understand the OP's information correctly), it does not? I'm not finding that distinction in Article 430.

Cheers, Wayne
 
There are fan powered VAVs and VAVs without a fan. In any case if the MCA is 22.5 amps I don't think you can justify #12 wire.

Since the heater load is way above the motor or servo or other loads I don't feel like you can call this a motor load.

Also In my view though others may not agree I consider all HVAC loads as a continuous load.

When HVAC heating and air conditioning is sized the equipment size is based off the heat load or heat gain of the space at the warmest or coldest temperature outside. But the selection is made at about 98% of the warmest or coldest temp so even that temp will be exceed a few times a year.

We all know that some fudge factor is included so the equipment may not run 100% of the time but still is a continuous load to me
 
infinity said:
In this case it's Article 440 which would not override 240.4(D).
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If it's Article 440, then the first entry of Table 240.4(G) is "Air-conditioning and refrigeration equipment circuit conductors: 440, Parts III,
VI." So 240.4(D) would not apply.

But the second sentence of the scope of Article 440 is "It provides for the special considerations necessary for circuits supplying hermetic refrigerant motor-compressors and for any air-conditioning or refrigerating equipment that is supplied from a branch circuit that supplies a hermetic refrigerant motor-compressor." I don't believe a VAV has a hermetic refrigerant motor-compressor, and so Article 440 would only apply if the branch circuit for the VAV also supplies a hermetic refrigerant motor-compressor. Whereas it sounds like the VAV will be on individual branch circuit.

Cheers, Wayne
 
As a thought experiment, suppose we change the OP's configuration to two separate pieces of equipment, a damper with an FLA <= 2A and a heater with a load of 18A. Then if we accept that the damper is a motor load (Article 430) or motor-operated appliance (also subject to Article 430 per 422.3), then a feeder or branch circuit supplying just those two loads would be subject to Article 430 as well. Meaning that #12 Cu could be used at 25A if the terminations are 75C rated.

Now I don't see any particular reason to expect the rules to be substantially different when the two functions are instead in a single piece of equipment. The argument against the applicability of Article 430 is basically "the motor is such a small part of the load, let's ignore it." But I'm not aware of any section of the NEC that compares the motor portion of the load to the non-motor portion of the load.

Cheers, Wayne
 
Are each of these 12 AWG circuits in their own conduit or cable do they trunk together at some point into the panel?
Sean.Day72 said:
Manufactured published MCA is 22.5 Amps. Published MOCP is 25 Amp.
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Under article 424 conductors get sized as a continuous load, 125% of the load (5kw), technically cant use MCA if say MCA were a lower number, however its nice they calculate it for you and the they gave you number matches 125% of the load.

wwhitney said:
If a VAV has a motor
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I dont think the heater assembly contains a fan motor, but you bring up a interesting scenario, its basically a giant toaster oven with a 24V contactor, I think this is what the OP is installing:
 
As I mentioned before there are basically 3 types of VAV boxes. In all cases air is supplied from a roof top unit.

A standard VAV is a box with a damper in it to provide zoning control where a local thermostat or BMS system controls the space temp by modulating the damper. This would be used in a space that would require cooling year-round like an office interior space. As the space is cooled the damper would be modulated down to a mostly closed position. They do not close the damper completely it would go to a minimum position for ventilation. It also has controls to measure air flow.

The second type would be a VAV with a hot water coil connected to a boiler or a electric heat coil. This would be used in an office exterior zone where heat or cooling is needed. It also has controls to control the damper and measure air flow and control the heat. Air flow comes from a RTU

The third type is a fan powered VAV. This type may or may not have hot water or electric heat but has a fan to recirc room air and also takes air from the RTU and it also measures air flow and damper controls and heating controls.
 
wwhitney said:
As a thought experiment, suppose we change the OP's configuration to two separate pieces of equipment, a damper with an FLA <= 2A and a heater with a load of 18A.
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The NEC classifies more things as continuous loads for branch circuit sizing than feeder sizing.
Residential water heaters come to mind.
There is in theory more load diversity on a feeder.
If you look at the UL spec for calculating HVAC MCA they take a resistance heater like the OP's at 125%.
 
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