Tap Rules

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TVC

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I have a air handler with a 20 KW heater and a 3 h.p. motor. The air handler has a factory mounted disconnect and overload protection for the blower motor. On the other end I have mounted a 100 amp non-fused disconnect for the heaters. The unit is protected by a 70 amp three pole breaker at the panel and fed with 4 awg THHN wire. Is it permitted to tap the supply with #6 THHN to feed the blower motor. The name plate on the unit claims the MCA of the unit is sized for both the heater and the fan motor but the wiring gutter on the fan motor appears not to have adequate room to terminate the # 4 THHN. :confused:
 
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TVC

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Tap Rules

Trevor,

No the wiring is not in the same compartment. The heaters are located within the air handler but the terminations are in a what I would call a seperate enclosure. Another reason I ask is the wiring on the load side of the factory disconnect is 10 awg THHN.
 
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FrancisDoody

Senior Member
Location
Durham, CT
Calculation

Calculation

20KW heater 208 volt supply = 69.39 AMPS
3 HP Blower Motor 208 volt = 10.6 AMPS
Both units draw = 80.0 AMPS
Continuous use 1.25% =100.0 AMPS
#4 THHN CU is good for = 85.0 AMPS
85 AMP CB is nonstandard = 90.0 AMPS
So your branch circuit conductors are not protected. The disconnect and overload for the motor is in place. You have mounted a disconnect for the heaters. The heating elements are subdivided within the unit, So each unit most likely has a set of cartridge fuses for each heater. #10 THHN is rated at 35 AMPS. Factory installed so you don't have to consider this to be a problem.
You must state the lenght of the tap to figure that caculation. This tap will be provide with overcurrent protect of 70 amps at the start. (panelboard) # 6 THHN is a lot of conductor for a 11 AMP tap even at 25 feet. But let us know the lenght?
Thanks,
Fran
 
S'mise

S'mise

Senior Member
Location
Michigan
As I see it;
10.6x1.25=13.25amps min. needed for the conductors to your motor.
13.25+69.39=83amps so #4 is fine. That 70amp OCPD does seem small, I calculate 10.6X2.5+69.39=96amps, next size up is 100A CB.

The 25' tap rule says the tap would have to be at least 1/3 of your 100A OCPD or 33amps. The #10 sounds fine to me as long as it's not over 25'.
If you keep the 70A OCPD you could getaway with a #12.
 
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