Another NM-B amp rating question.

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Mgarf33

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I am supplying an air handler in a residential garage with a 15kW aux heater on a #6 NM-B circuit. Circuit one requires 58A min amp conductor, and 60A MOCP. Circuit 2 is well below the 25A rating using #8 NM-B. I wish I could move the blower circuit to circuit 2 which would drop my min amp of circuit 1 to 50, but that would void the UL listing of the appliance.

So, this circuit passed on the original home construction, and the Min Amp of the existing equipment was 59A with a 60A MOCP. Correct me if I'm wrong, but isn't #6 NM-B only rated for 55A as a cable set? The run is ~35'-40' routed through a crawl space.

Yes, I realize the individual conductors are 90°C rated and can be used with 90° lug connections, but the cable set is only rated at 60°C. Is this different for residential applications?
 
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You are correct. The #6 NM is rated 60C and what is there is a violation. I wish the manufacturers would do something like lower the strip element size.
 
My guess is that a standard 10 kw units is what is used and then a 5 kw strip is added to the mix. The elements may be sequenced but I am not sure of that.
 
Yeah, the issue is the blower amp rating on the larger equipment. This particular unit has an 8A blower. So, even a 10kW would have been the same, because circuit 1 of the 2 circuit 15kW heater is 10kW, and circuit 2 is only 5kW.

Some mfgs. have lowered the rating of the 10kW to 9.8kW, but that only helps if my blower is 4A or less.
 
Look at the model number and locate the tech/service pdf. I know on many units you can lower the strip heater KW. May or may not be noticed by the HO.


In all honesty I would not be concerned- in Canada this would be compliant.
 
mbrooke said:
Look at the model number and locate the tech/service pdf. I know on many units you can lower the strip heater KW. May or may not be noticed by the HO.


In all honesty I would not be concerned- in Canada this would be compliant.
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Yeah, I don't feel it is a concern when it comes to operation, but as for accountability, I would be responsible should an issue occur involving this cable.
As for reducing the rating, the 15kW strip heaters are 4.9kW each. So, circuit 1 has 2 which is 9.8kW @ 40A + the blower at 8A. So, (40x1.25) + 8 = 58A. I would love to rewire the UL listed air handler and switch the 8A circuit to circuit #2 which is only 4.9kW @ 20A, but would this not void the UL listing? Back in the day, doing things like this wouldn't be a question, but today, insurance companies want to hold others liable any way possible. A simple action like this could leave my company accountable.
 
Mgarf33 said:
Yeah, I don't feel it is a concern when it comes to operation, but as for accountability, I would be responsible should an issue occur involving this cable.
As for reducing the rating, the 15kW strip heaters are 4.9kW each. So, circuit 1 has 2 which is 9.8kW @ 40A + the blower at 8A. So, (40x1.25) + 8 = 58A. I would love to rewire the UL listed air handler and switch the 8A circuit to circuit #2 which is only 4.9kW @ 20A, but would this not void the UL listing? Back in the day, doing things like this wouldn't be a question, but today, insurance companies want to hold others liable any way possible. A simple action like this could leave my company accountable.
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What does the manual say? If you can disconnect heaters then its fine in that regard.
 
Most units I ever run into usually have 4.8 kW elements, in groups of two so 9.6 per circuit max. They are often referred to as 5 kw each though.

With that you still only have room for a 4 amp max blower before your #6 @ 60C is maxed out, though most are usually 4 amp or under.
 
The only option is to either rewire or get a heating guy to see if a smaller heater strip would be sufficent-- something like a 7.5 kw in place of the 10kw.

BTW, most 10 kw units are not 10kw but rather 9.6 or, as the op mentioned 9.8 kw. Still doesn't work...
 
Dennis Alwon said:
The only option is to either rewire or get a heating guy to see if a smaller heater strip would be sufficent-- something like a 7.5 kw in place of the 10kw.

BTW, most 10 kw units are not 10kw but rather 9.6 or, as the op mentioned 9.8 kw. Still doesn't work...
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Yeah, I am the heating guy. The alternative option is a more expensive air handler. The cost to route a new cable would exceed the expense of a new air handler rated at 4amps for the blower which leaves me right at 55A.
I realize most HVAC contractors see #6 and think 60A, we're golden, but unfortunately I have electrical schooling, so I should know better.
Fortunately, the new unit in question has not been installed yet.

kwired said:
But your MCA is still 58 according to your OP.
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Yes, that's correct. I guess that still leaves me in a pickle.
 
I'm not clear on the internals of the air handler, but is rewiring the heat strips easier/more legitimate than rewiring the blower? If you could swap the two heat strips, then you'd have the 5 kw strip on the same circuit as the blower.

Cheers, Wayne
 
The thing that really gets me is, this is the original wiring and the breaker is 60A. I would think this should not have passed inspection with equipment requiring 59A MCA. This is not a very old home. It was built in 1990.
 
Mgarf33 said:
Yeah, I am the heating guy. The alternative option is a more expensive air handler. The cost to route a new cable would exceed the expense of a new air handler rated at 4amps for the blower which leaves me right at 55A.
I realize most HVAC contractors see #6 and think 60A, we're golden, but unfortunately I have electrical schooling, so I should know better.
Fortunately, the new unit in question has not been installed yet.


Yes, that's correct. I guess that still leaves me in a pickle.
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increase to 4 AWG or use different wiring method.

Raceway method you can pull both circuits in same raceway and with less then 6 current carrying conductors shouldn't need to increase conductor size in nearly all installations, may not be as expensive as some think it is. Need to fish portions of the circuit - flexible raceways or even ENT fish just as easy if not easier then #4 NM cable would.
 
wwhitney said:
I'm not clear on the internals of the air handler, but is rewiring the heat strips easier/more legitimate than rewiring the blower? If you could swap the two heat strips, then you'd have the 5 kw strip on the same circuit as the blower.

Cheers, Wayne
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Essentially we are doing the same thing. There are internal breakers located inside the air handler. One breaker (circuit 1) has the blower/control circuit and 10kW heater. The other breaker (circuit 2) has only a 5kW heater. Circuit 2 has a MCA of 20, and MOCP of 25, but we have a 40A 8awg NM-B wire going to it. So, swapping the blower/control circuit to the 5kW breaker would be fine, I could just drop the breaker to 30A or 35A fuse, but I was told my local Trane tech support, that this could void the UL rating on the equipment.
 
Mgarf33 said:
Essentially we are doing the same thing. There are internal breakers located inside the air handler. One breaker (circuit 1) has the blower/control circuit and 10kW heater. The other breaker (circuit 2) has only a 5kW heater. Circuit 2 has a MCA of 20, and MOCP of 25, but we have a 40A 8awg NM-B wire going to it. So, swapping the blower/control circuit to the 5kW breaker would be fine, I could just drop the breaker to 30A or 35A fuse, but I was told my local Trane tech support, that this could void the UL rating on the equipment.
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Which for the most part may void any warranty that Trane would support.

Brand new equipment, often nobody wants to make unauthorized modifications because of warranty issues.
No longer under warranty - maybe not so much of a problem.
 
Well, that's why I'm on here talking to the experts. So, if you're hvac guy was installing a piece of equipment with the options I have, would you all say, "suck up the ~$370 for the better unit and keep it legit"?
 
Mgarf33 said:
If I disconnect heaters, that doesn't change the rating on the equipment, because someone can go back and reconnect them, and still leave me liable. Also, my heating capability significantly reduced.
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I agree with Dennis, have an HVAC tech chime in. There could be an alternative by chance.
 
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