20KW furnace

[wyboy]

I have a 20KW, 240V single phase furnace. The blower fan draws 7.6 amps. The furnace has two 60 amp breakers on it to land the two branch circuits that feed the unit.
The name plate states that the circuit that includes the blower fan minimum circuit ampacity is 61.59 amps. (10KW/240 + 7.6 for fan x 1.25 for continuous load) The other circuit’s listed minimum ampacity is 52 amps. However, the fields wiring requirements for the install require a 60 amp breaker for both branch circuits and NEC 424.22 requires heaters to have 60 amp overcurrent max. But NEC 210.20 requires overcurrent for continuous load to be no less than 125% of full load current. I am a bit confused. It is only 1.5 amps but still… Is this a legal install?

PART II
This furnace is for a dwelling unit. The name plate says use #6 wire with at least a 75 degree rating. The wire in MN is THHN and has a 90 degree rating. I know NM ampacity is from the 60 degree table. It is my contention that, despite what the name plate says, a 6/2 NM branch circuit may feed the 52 amps furnace load, but a 4/2 NM or a raceway with 75 degree rated #6 conductor must feed the 61.59 amp furnace load. Is this correct?

 

[Dennis Alwon]

My experience has been that a 10kw heater is not really 10 kw-- more likely 9.6kw. I think it works out that it is 10kw with the actual blower...

 

[augie47]

My experience has mirrored Dennis, but even at 9.6kw, your unit exceeds the 55 amp limit of #6 NM.
Locally, unless the unit has 60deg termination limitations, the installs usually change to #6 SE Cable or #4 NM.

 

[Dennis Alwon]

Just run 6/2 se cable and you are fine as se is now rated 75C depending on your code cycle

 

[kwired]

My experience has been that a 10kw heater is not really 10 kw-- more likely 9.6kw. I think it works out that it is 10kw with the actual blower...

Mine too but he says his nameplate says MCA is 61.59 Unless they calculated that value wrong he needs more than 60 amp conductor as well as more then 60 amp OCPD.

If he truly has 20 kW element and blower on one circuit - MCA should be 125% of heater = 52 amps plus blower 7.6 amps = 59.6.

60 amp conductor would be fine there - but #6 at 60C is only good for 55 amps.

 

[Dennis Alwon]

Yeah, I am thinking that nameplate just isn't correct but I guess he is stuck. I don't know why manufacturers do that. It used to be that a 10kw unit could be wired with #6 nm then the units were coming in with fans that would bring the loads up to 56 and 57 amps ...

 

[kwired]

Yeah, I am thinking that nameplate just isn't correct but I guess he is stuck. I don't know why manufacturers do that. It used to be that a 10kw unit could be wired with #6 nm then the units were coming in with fans that would bring the loads up to 56 and 57 amps ...

10 kW of heat only leaves you about 3 amps for a blower. If there is only 10 kW of total heat, maybe that is all the blower needed. Need to consider how much air is needed for any cooling coil also, plus the unit in discussion here has another 10 kW bank of heat.