just curious.....

[g-and-h_electric]

Hey guys, I am running into an issue that makes no sense to me. I am hoping maybe someone here has an answer.

I am the supervising electrician for an HVAC+ Electric shop, so all electrical problem (and their solutions) fall on me.

I have been running into an issue where the customer wants a heat pump system to replace a conventional gas furnace and central ac system. The estimator and sales staff are going "size for size. By that I mean the furnace ( which is still gas) has the same BTU output, and the A/C has the same tonnage. The problem that I am running into is that the outdoor unit (on a 3 ton system) is now wanting a 42 Amp circuit, 45 Amp breaker. We agree that this needs #8 wire.

My question is why with the outdoor unit being more efficient and having variable speed and inverter drives, am I needing a larger circuit???? I would have expected the exact opposite. I am confused.


Howard

 

[Hv&Lv]

Electric supplemental strip heat and/or defrost cycle?

 

[BarryO]

You're still going to need to size the circuit for when the compressor is running 100%, even if it will be doing that infrequently. That sounds about right for a 36K BTU compressor.

 

[ptonsparky]

Guessing the circuit needs to be sized @ 125% of the VFD current, not the compressor current.
Similar to 430.122(A) that we would use in the field.

 

[Frank DuVal]

or defrost cycle?

Defrost is just reverse the refrigerant flow, i.e. run it as AC and heat the outdoor coils.

Indoor unit is where the electric strip heat is located.

The compressor does have a little sump heater, but that is 100W or less.

 

[Hv&Lv]

Defrost is just reverse the refrigerant flow, i.e. run it as AC and heat the outdoor coils.

Indoor unit is where the electric strip heat is located.

The compressor does have a little sump heater, but that is 100W or less.

Indoor heat strips, correct

My thoughts on defrost cycle were the increased head pressure on some units.