Another NM-B amp rating question.

[kwired]

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"?

If we are still in planning stages I might make suggestions on what to get. Usually they already have the unit and is mostly if not completely installed when I would find out exact details of what is needed for branch circuit, by then I have to do what I have to do for a correct install.

Was one time I got lucky enough to request they install three phase heat strip kit before they went ahead and got single phase, which did cost more then same kw single phase, but I wanted three phase mostly to help keep load on the service/feeders balanced.

 

[Jamesco]

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"?

"suck up the ~$370 for the better unit and keep it legit"?

Have you talked with the homeowner you are doing the job for about what you discovered with the existing wiring?
Good chance at the very least he/she will split the $370 added cost for you looking out for them and wanting to do the job right.

 

[Mgarf33]

If we are still in planning stages I might make suggestions on what to get. Usually they already have the unit and is mostly if not completely installed when I would find out exact details of what is needed for branch circuit, by then I have to do what I have to do for a correct install.

Yes, we are installing tomorrow, but still have time to travel to the supply house and get the higher grade unit that would put us right at 55A MCA. I was just looking for an alternate solution that would be legitimate. I didn't know for sure if there were any leeway with the NM-B ratings for residential use, as I know the individual conductors are rated at 90° and can be connected to lugs higher than the cable set's 60° rating.
I originally saw the 60A main panel breaker on the estimate. On the final preview after signing the contract I looked more closely and saw they improperly used NM-B. Being an hvac estimator, you learn not to touch anything, or as little as possible until you have the job, because homeowners will call in a heartbeat and say you messed up their unit and need to come out and fix it for free.

 

[kwired]

Is replacing the 6 AWG with 4 AWG or a different method that would allow 6 AWG @ 75C going to add more than $370?

Take your pick if it is going to be close.

 

[Mgarf33]

Thanks for all your help fellas! I gotta do the right thing for my clients. So, I'm gonna eat the $370 and give them an upgraded air handler. I try to carry the highest level of service by following the rules. If there's only a "jury rig" solution, then I'm going to pass on that.
Just have to look more closely at the MCA every time, typically the blower is smaller, and not an issue. We don't have very many larger units in our area. Typically, there are several smaller units with lower blower MCA ratings that allow for 9.8kW heaters on NM-B.

 

[Jamesco]

Thanks for all your help fellas! I gotta do the right thing for my clients. So, I'm gonna eat the $370 and give them an upgraded air handler. I try to carry the highest level of service by following the rules. If there's only a "jury rig" solution, then I'm going to pass on that.
Just have to look more closely at the MCA every time, typically the blower is smaller, and not an issue. We don't have very many larger units in our area. Typically, there are several smaller units with lower blower MCA ratings that allow for 9.8kW heaters on NM-B.

Good for you! I would still talk to the customer. You may find he/she appreciates your honesty and looking out for them.

 

[JFletcher]

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.

10kw heater @ 240V is 41.7A... why is #6 insufficient for that and the blower? Where is the 58A MCA coming from, the nameplate, or somewhere else?

 

[kwired]

10kw heater @ 240V is 41.7A... why is #6 insufficient for that and the blower? Where is the 58A MCA coming from, the nameplate, or somewhere else?

I am having some difficulty finding where it says you must use 125% of the rated current, but I am pretty certain that is the requirement. Most MCA I ever see on a nameplate is high enough that they must be using 125%. So if 10KVA @ 240 volts you need 52.1 conductor ampacity for the heater. This only leaves you with 2.9 amps for a blower, which I think must also be taken at 125% of it's rating, before you get to 55 amps rating of a 6 AWG at 60C.

But like I said earlier most of the units I see are only rated 4.8 kW per element, with two on one circuit that is 9.6kW which is 40 amps. 125% is 50 amps - leaves you more room for the blower, most are under 5 amps. It is when you have a full 10kW heating element or a rather large blower when you may get into trouble with using NM cable.

 

[JFletcher]

I am having some difficulty finding where it says you must use 125% of the rated current, but I am pretty certain that is the requirement. Most MCA I ever see on a nameplate is high enough that they must be using 125%. So if 10KVA @ 240 volts you need 52.1 conductor ampacity for the heater. This only leaves you with 2.9 amps for a blower, which I think must also be taken at 125% of it's rating, before you get to 55 amps rating of a 6 AWG at 60C.

But like I said earlier most of the units I see are only rated 4.8 kW per element, with two on one circuit that is 9.6kW which is 40 amps. 125% is 50 amps - leaves you more room for the blower, most are under 5 amps. It is when you have a full 10kW heating element or a rather large blower when you may get into trouble with using NM cable.

the breaker must be 125%, but I dont know where it says the conductors must be sized 125%...they need to be sized for the actual ampacity of the heating load.

If you had a baseboard heater that draws 20A, you could wire it with #12 IF you could use a 20A breaker. Like if you had a 100% rated 20A breaker, you could use #12 NM with a 20A heating load, yes?

Small conductor rules of 240.4 dont apply to #6 NM tho, as long as the actual ampacity doesnt exceed 55A, it's fine to use, amiright?

The blower , yeah, a few amps, and at 125%, but again a 10kw heat strip draws 41.7A @240V. The blower is not likely to draw more than 4A, x 1.25%, 5A... 47A MCA. #6 NM is fine for this imo and interpretation of the code.

If it were wired with 6-6-8 SEU copper wire, one could use the 75* column, and 65A ampacity. If...

 

[kwired]

the breaker must be 125%, but I dont know where it says the conductors must be sized 125%...they need to be sized for the actual ampacity of the heating load.

If you had a baseboard heater that draws 20A, you could wire it with #12 IF you could use a 20A breaker. Like if you had a 100% rated 20A breaker, you could use #12 NM with a 20A heating load, yes?

Small conductor rules of 240.4 dont apply to #6 NM tho, as long as the actual ampacity doesnt exceed 55A, it's fine to use, amiright?

The blower , yeah, a few amps, and at 125%, but again a 10kw heat strip draws 41.7A @240V. The blower is not likely to draw more than 4A, x 1.25%, 5A... 47A MCA. #6 NM is fine for this imo and interpretation of the code.

If it were wired with 6-6-8 SEU copper wire, one could use the 75* column, and 65A ampacity. If...

You could be right from NEC perspective - what I was having trouble finding was anything that says the conductor must be 125% or the load, I found more then one place that mentions 125% on overcurrent protection.

That said most nameplates on this kind of equipment have a MCA on them, and if you calculate it out they used 125% to come up with what is on the nameplate. OP apparently has 58 amps MCA on his unit.

 

[Dennis Alwon]

Fixed electric space heating

424.3 Branch Circuits.
(A) Branch-Circuit Requirements. Individual branch circuits
shall be permitted to supply any volt-ampere or wattage rating
of fixed electric space-heating equipment for which they are
rated.
Branch circuits supplying two or more outlets for fixed electric
space-heating equipment shall be rated not over
30 amperes. In other than a dwelling unit, fixed infrared heating
equipment shall be permitted to be supplied from branch
circuits rated not over 50 amperes.


(B) Branch-Circuit Sizing. Fixed electric space-heating equipment
and motors shall be considered continuous load.

 

[kwired]

Fixed electric space heating

TY.
I was pretty sure they were supposed to be treated as continuous loads, just wasn't finding where it said that.

 

[Mgarf33]

Yes, this type of equipment has MCA on the label for each individual heat kit which is capable of installing in the air handler.
In this case, they MCA is rated @125% full continuous load of the heating elements + 100% of blower RLA.

It's like an AC unit. The MCA is rated at the highest motor RLA @125% + all other smaller motors at 100% of RLA.

For instance. If we have a condenser section that has 2 motors, a compressor and fan motor. In regards to MCA, the compressor would be rated @125%, + the fan motor at 100%.

We must use the MCA as the conductor size, and the MOCP as the breaker/fuse size.

As it turns out, I began the installation and found another abandoned disconnect box concealed behind some duct work. Apparently, this was not the original equipment and the last HVAC contractor must have had the two cables installed, but not certain. 6awg & 8awg NM-B. There was up-sized equipment added when they had a large addition built on to the first floor. The original disconnect looks to be 6awg SE aluminum.