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10kw heater wire gauge?

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MackDaddy

Member
Location
Port St Lucie, Fl
Occupation
Air Conditioning contractor
I am dealing with a local plan reviewer that has denied me a permit on an air conditioner change-out that has a 6ga Romex type wire.
He states that nm wire must use the 60c scale which puts max. amps at 55.
The a/c manufacturer rates 10kw heater/air handler at 58.6a mca and 60a mop.
It also rates an 8kw heater @ 50.3 mca and 60a mop.
Using these two examples it would require a 4/0 wire for either heater.
I have tried to have him allow the 8% increase and he stated that NFPA 70 does not allow exceptions to the 60c scale.
Any thoughts?
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
He's correct. If using NM cable you need to have a conductor ampacity of 58.6 amps not less. You could use #6 MC cable.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Most other new conductors we use today are rated for 90C and the equipment terminals are rated for 75C - allowing us to use 75C conductor ampacity table.

NM and UF cables even if new must still use the 60C ampacity tables as NEC specifically mentions this for those cable types.

If your heater is 10kW you can squeak by with 60C ampacity, but if there is also a blower on same circuit that usually will put you over the 55 amps max for 60C. (keep in mind you must multiply heater amps by 1.25 to determine minimum ampacity needed)
With a lot of air handlers with heat strips the actual heater watts is only 9.6 kW, which leaves enough room for a blower most the time on 6 AWG @ 60C.
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
#6 is good for 55 amps so that would work for the 8kw unit but as stated by Larry #4 would be needed for the 10 kw unit. It used to be a 10 kw could be used with #6 but for some reason the manufacturers started to use larger fans or something because the amps varied somewhere between 56 to 59 amps.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Dennis Alwon said:
#6 is good for 55 amps so that would work for the 8kw unit but as stated by Larry #4 would be needed for the 10 kw unit. It used to be a 10 kw could be used with #6 but for some reason the manufacturers started to use larger fans or something because the amps varied somewhere between 56 to 59 amps.
Click to expand...
With fan coils that have heat strips in them often we call it 10kW when it actually is 9.6 kW
 
M

MackDaddy

Member
Location
Port St Lucie, Fl
Occupation
Air Conditioning contractor
Yes, the heater is actually 9.6kw
The mop is heater amps X 1.25 + blower amps X 2.25 to equal 58.6
Keep in mind that this is an existing situation, and we are not replacing the original wire.
I was told that in some situations [incorrectly?] that an of 8% correction is allowed.
If so, this would change the 55a 60c to 59.4a
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Well, the 310.15(B)(2)(a) temperature correction for 60C conductors at 21C - 25C is 1.08. So if you can argue that the heater will never run when the ambient temperature is above 77F (and that the NM cable will be at that ambient or lower), then you could argue for an 8% ampacity increase.

Cheers, Wayne
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
MackDaddy said:
Yes, the heater is actually 9.6kw
The mop is heater amps X 1.25 + blower amps X 2.25 to equal 58.6
Keep in mind that this is an existing situation, and we are not replacing the original wire.
I was told that in some situations [incorrectly?] that an of 8% correction is allowed.
If so, this would change the 55a 60c to 59.4a
Click to expand...
Where is that X 2.25 coming from?

It should be 125% of heater amps plus the blower.

A 9.6 kW heater should draw 40 amps x 1.25 = 50. That leaves room for up to 5 amps for blower (if you using 60C conductor) which most are less than that.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
MackDaddy said:
The 2.25 multiplier is for motor amps and is to be used when doing these calculations.
Click to expand...
For selecting short circuit and ground fault protection, maybe, depending on where you get that figure from. General rules for motors on thermal magnetic breakers is 250% max, can be less but also can have conditions where it can be higher if that won't allow the motor to start.

For selecting minimum conductor ampacity, is basically 125% of continuous loads plus 100% of non continuous loads. Motors, and space heating are both considered to be continuous loads.

Combination units like we are dealing with here they typically allow 125% of the largest motor or largest heating element plus all other load - for determining minimum conductor ampacity. Max OCPD depends on types of loads (to factor in if it will hold during starting) involved and the OCPD type used. A VFD driven motor or soft started might not need as high of an OCPD setting to allow starting as a motor that starts across the line will require.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Occupation
EC
Appears to have an 8.6 amp blower, rather large blower for only a 10kW heater, but I guess probably sized per the capacity of the refrigerant coil more so than the heat strip.
 
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