25 kva electric furnace- circuit sizing question

[sw_ross]

Most of the time when I see a 25kva electric furnace it usually will have 3 branch circuits- (2) 60-amp & (1) 30-amp circuits.
This setup is comprised of (2) 10 kva & (1) 5 kva heating elements setup to make a total of 25 kva.

In the last year I’ve seen 2 furnaces of this size that were set up with (2) branch circuits- (1) 15 kva & (1) 10 kva. This requires (1) 80-amp circuit (4-awg) & (1) 60-amp circuit (6-awg).

In my opinion, regarding the electrical requirements and installation for this new furnace, this is a royal PITA! 4-awg THHN is expensive!

Is there any specific reason that the 2 branch circuit setup is being used over the 3 branch circuit furnace?
Both furnaces that I encountered were update/retrofit furnaces that were using this new furnace to replace an existing furnace, so there was already the proper wiring there for the 3 branch circuit furnace, but now I have to try and retro-install 4-awg wire to feed the 15-kva circuit!

 

[Dennis Alwon]

The manufacturers don't care about our end. I used to hate those 10kw units where the nameplate was 56 amps. For residence we use nm and #6 is only good for 55 amps so we had to run #4.... just for one amp.

You should think about running se cable if you can and install a sub panel next to the unit

 

[sw_ross]

The one I’m dealing with now luckily has 1-1/4” EMT to it so I’m not too worried, but I’ll have to pull out the existing #6’s (6) and re-pull the #4’s (2) and (1) set of #6’s.

I think my main issue is that the HVAC installer doesn’t inform the customer that this needs to be done because of the equipment that they chose. All the sudden the customer gets a bill from me for 100’ of #4 thhn and ~2-3 hours labor when it could’ve all been avoided if the HVAC company chose their equipment appropriately!

 

[infinity]

The one I’m dealing with now luckily has 1-1/4” EMT to it so I’m not too worried, but I’ll have to pull out the existing #6’s (6) and re-pull the #4’s (2) and (1) set of #6’s.

You need to check your calculations now that you have 4 CCC's in the raceway your conductors might be too small.