Correct size conductor

[1Wiredtaz]

This one should be pretty cut and dry,But i am posting due to a disagreement with my x boss so her it is:

What is the min size THHN conductor for a HVAC unit with the following specs, Min circuit amps= 62,Max Overcurrent device 70 Amps,480 volts 3PH.
This conductor is being feed from a 70 amp breaker in the panel to a fused disconnect mounted on the HVAC unit with 70 amp fuses installed.The distance is approx 285 feet

Would this be classified as a feeder being that it is feeding the last OCPD?

Thanks' Don

 

[iwire]

What is the min size THHN conductor for a HVAC unit with the following specs, Min circuit amps= 62,Max Overcurrent device 70 Amps,480 volts 3PH.

Speaking code minimum only and assuming this is THHN in pipe not NM.

6 AWG.

This conductor is being feed from a 70 amp breaker in the panel to a fused disconnect mounted on the HVAC unit with 70 amp fuses installed.The distance is approx 285 feet

That is fine code wise but is really pushing things as far as voltage drop at start up.

Would this be classified as a feeder being that it is feeding the last OCPD?

No, it is not a feeder, it is a branch circuit as the OCPDs in the disconnect at the unit are protecting the unit not the branch circuit conductors.

 

[dahualin]

Based on the MCA, the wire size shall be #6. Since 285 feet is a long distance, you shall do a voltage drop calculation. Maybe #4 is the correct answer (I guess, I didn't do the calculation).

 

[infinity]

I agree with Bob. The conductors are sized according to 310.16 based on the MCA. #6 THHN at 75 degrees is rated for 65 amps. Voltage drop is a different issue regarding the design of the circuit. A quick calculation tells me that with #6 copper the voltage at the unit will be approximately 465 volts which should be well within the running range of the 480 volt unit. Start up might be a different scenario however.

 

[iwire]

Since 285 feet is a long distance, you shall do a voltage drop calculation.

Shall or Should?

The NEC does not require a VD calculation.

But I agree 4 AWG or 2 AWG may be much more appropriate for good service.

 

[1Wiredtaz]

It was my understanding that if the circuit is under 100 amps that you must use the 60 Deg table for sizing.

Also What is protecting the branch circuit? The 70 amp breaker? Your saying that since the fuses are protecting the unit that it would be legal to say for instance to feed the #6(in this case) with a 125amp breaker?

 

[infinity]

It was my understanding that if the circuit is under 100 amps that you must use the 60 Deg table for sizing.

Also What is protecting the branch circuit? The 70 amp breaker? Your saying that since the fuses are protecting the unit that it would be legal to say for instance to feed the #6(in this case) with a 125amp breaker?

Almost all equipment has lugs that are rated for 75 degrees C, so the 75 degree ampacity may be used. The branch circuit CB needs to be sized to allow the unit to start. It does not have to conform to the ampacities of 310.16. It simply provides ground fault and short circuit protection.

 

[1793]

No, it is not a feeder, it is a branch circuit as the OCPDs in the disconnect at the unit are protecting the unit not the branch circuit conductors.

If the disconnect is protecting the unit not the branch circuit, then would the branch circuit/feeder not be a feeder. What is protecting the conductor between the OCPD and the disconnect, the branch circuit/feeder?

 

[1793]

Man, I need to type faster.

 

[infinity]

If the disconnect is protecting the unit not the branch circuit, then would the branch circuit/feeder not be a feeder. What is protecting the conductor between the OCPD and the disconnect, the branch circuit/feeder?

This has been a long debated question because of the Article 100 definitions. If you consider the conductors from the CB to the unit the branch circuit, the fuses at the unit would be supplementary protection.

 

[iwire]

It was my understanding that if the circuit is under 100 amps that you must use the 60 Deg table for sizing.

You are correct and I should have mentioned that.

You must use the 60 C column under 100 amps unless the terminations are listed 75 C which in any modern electrical equipment they are likely listed for 75 C use. (Some cheap HVAC pull out disconnects are not rated 75 C so you should check first)

Also What is protecting the branch circuit?

The 70 amp breaker is protecting the conductors from short circuits and ground faults.

The motor overload devices inside the heat pump protect the conductors from overload.

Your saying that since the fuses are protecting the unit that it would be legal to say for instance to feed the #6(in this case) with a 125amp breaker?

That could be true if the unit was marked like this, but it has noting to do with the fuses.

Maximum breaker size 125 amps

Minimum circuit ampacity 62 amps.

However that is a large 'spread' and I don't think you will really find a unit marked like that.

By the way unless that unit requires 70 amp fuses you could have used a non fused disconect.

 

[1Wiredtaz]

Thanks All for clearing this up