Fixed space heater conductors

[mavrck]

I’m looking into installing a 15kw residential heater. It requires a 60 and 30 amp circuit.

I know standard practice is to pull a 6-2 nm conductor and a 10-2nm conductor.
But I noticed the min circuit amps is 55 and max ocpd is 60
Nec require this to be continuous load calculations on conductors with increases amps to 68.75. Which would rule out the 6-2nm

how do people get away with using a 6-2 and not a 4-2? Thx

 

[mavrck]

Does the nameplate take that into consideration? If so is that refereed in the code book?

 

[ptonsparky]

Post a picture of the label, but generally if it states MCA, you do not need to do anything further.

 

[mavrck]

Here is the nameplate on the air handler.

 

[mavrck]

This is better picture

 

[mavrck]

This air handler I posted is an example not actual unit I’m installing.
looks to be 10 kw with 1.3 fan amp load . 53.38 would be total load with 125% figured in as well. So I would say that the nameplate is already configured.

 

[Little Bill]

The nameplates already take the 125% into account. You wire to the MCA and provide OCPD per the max OCPD. This may say max fuse, or max fuse or breaker

 

[Eddie702]

Agree with @Little Bill

Follow the nameplate and your fine

 

[Fred B]

Agree with @Little Bill

Follow the nameplate and your fine

This is one that has puzzled me, large motor loads, mfg indicated cord set size, field installed, that would be greatly undersized per breaker sizing from mfg. For ie motor, had one the mfg calls for 40A 240V breaker, but also indicates a 12AWG cord set, or 10AWG if direct wired.

 

[James L]

This is one that has puzzled me, large motor loads, mfg indicated cord set size, field installed, that would be greatly undersized per breaker sizing from mfg. For ie motor, had one the mfg calls for 40A 240V breaker, but also indicates a 12AWG cord set, or 10AWG if direct wired.

Depends on the temperature column you're using, and could include an ambient temperature adjustment of 5-10% additional ampacity.

#12 could handle 33 amps or more. That would most likely go on a 40 amp breaker for a motor

 

[Dennis Alwon]

If the unit min cir amp is 55 amps then run a #6 .

This is a 10kw unit not 15 kw. It looks like it needs one cir at 55 amps so a #6 would work

 

[kwired]

This is one that has puzzled me, large motor loads, mfg indicated cord set size, field installed, that would be greatly undersized per breaker sizing from mfg. For ie motor, had one the mfg calls for 40A 240V breaker, but also indicates a 12AWG cord set, or 10AWG if direct wired.

The "small conductor rule" in 240.4(D) doesn't apply to motor circuits (see 240.4(G)).

Not sure exactly why 12 allowed for cord set but 10 needed for direct wiring, but different wiring methods and different temperatures can make a difference on selecting conductor size.

AC motors typically need higher short circuit and ground fault protection level to allow them to hold during starting, you still need overload protection for the motor and this will also protect the conductor from overload.