Need A Little Help Sizing Wires

[Bib Overalls]

Need A Little Help Sizing Wires ** SOLVED!

For most of the electrical jobs a home and shop owner is faced with I am a pretty capable electrician; I respect electricity, I understand the basics, I am **** about the details, I know when I need help, and I am not cheap.

After a bit of on line research and consulting my Ugly's Electrical Reference I still need a bit of wire sizing assistance. With the current price of copper I am reluctant to over engineer a project that requires long runs. But I don't want to undersize either.

Here is my situation. I have 100 amps at the sub panel in my shop. I am installing a 3.5 ton "package" heat pump. The run from the sub-panel is about 75 feet. I plan on using NM cable. I can surface mount this cable if necessary but would prefer to run it inside walls that will eventually be insulated.

The unit specs call for 40 amps of protection and 29 amps run. Can I use NM 2/10/WG or will I need NM 2/8/WG?

The unit has three 5 KW heat strips with one 30 amp and one 60 amp breaker. I know that I do not have sufficient amps at the panel to support this load and only plan to wire up the 30 amp strip. Again, the question is can I use NM 2/10/WG or will I need 2/8/WG.

If I am reading my references right my situations are right at the limit of #10. What would you use?

 

[MrMark]

You need 8 gauge cu for 40 amps. 10 gauge is limited to 30

 

[Norcal]

If you were using THHN/ THWN, 10 AWG would be fine w/ the 40A circuit breaker because of the rules in NEC article 440, the values in 310.16 can be used w/ lists 10 AWG THWN at 35A, but w/ NM cable (Romex®) your limited to the 60 degree ampacity, so 8/2 NM will have to be used.

The A/C equipment manufacturers have done the work, so one can wire to the minimum & fuse to the max values listed on the label.

 

[MrMark]

29 fla X 1.25 = 36.25, which is more than the 35A listed for 10 AWG THWN CU at 75 degree temp rating

 

[Norcal]

29 fla X 1.25 = 36.25, which is more than the 35A listed for 10 AWG THWN CU at 75 degree temp rating

That is already accounted for. The manufacturer has already done the work...

 

[MrMark]

That is already accounted for. The manufacturer has already done the work...

I think I understand what you are saying that if the manufacturer lists the required branch circuit selection current you use that - he has done the work. But here, the I see 29 amps running listed. Is this not the rated load current to which my calculation above would have to be applied to?

 

[Norcal]

I think I understand what you are saying that if the manufacturer lists the required branch circuit selection current you use that - he has done the work. But here, the I see 29 amps running listed. Is this not the rated load current to which my calculation above would have to be applied to?

You don't need to calulate anything, the listed minimum amperes have been given.

 

[MrMark]

What about 440.32? Single motor compressor?

If the listed minimums are 29 as you apparently agree, why is 10 gauge NM not appropriate? 10 AWG at 60 degrees is 30 amps.

 

[PRH44]

What type unit do you have?
Is this a single unit? heat only? or is a heat pump used for AC and heat with electric back up?

 

[Aceman]

If you were using THHN/ THWN, 10 AWG would be fine w/ the 40A circuit breaker because of the rules in NEC article 440, the values in 310.16 can be used w/ lists 10 AWG THWN at 35A, but w/ NM cable (Romex®) your limited to the 60 degree ampacity, so 8/2 NM will have to be used.

The A/C equipment manufacturers have done the work, so one can wire to the minimum & fuse to the max values listed on the label.

Exactly, Norcal saved me some typing.

 

[Bib Overalls]

What type unit do you have?
Is this a single unit? heat only? or is a heat pump used for AC and heat with electric back up?

I have a Goodman 3.5 ton heat pump with a three element backup resistance heat add on kit. The unit is for my shop.

 

[MrMark]

This seems authoritative:

http://www.dantespeakheatingcooling.com/upload/Mike_Holt_NEC.pdf

 

[MrMark]

Can you show a picture of your rating plate. Does it say 29 amps RLA?

 

[brewchief]

Can you show a picture of your rating plate. Does it say 29 amps RLA?

I would expect it to say 29 amps minimum circuit ampacity with the RLA listed for the compressor, indoor fan and condenser fan motor listed separately.

 

[MrMark]

I would expect it to say 29 amps minimum circuit ampacity with the RLA listed for the compressor, indoor fan and condenser fan motor listed separately.

Perhaps, but he stated it was "running amps" in his post.

Even if it listed as the minimum circuit ampacity - and thus the manf has done the calc - I fail to see why 10 gauge nm is not sufficient.

 

[Bib Overalls]

Went to Lowe's today and purchased my wire. I was still undecided between #8 and #10 and tossed it back and forth between the sides of my brain as I drove along. I was thinking #8 most of the time thanks to you all but I was wavering on that a bit. At Lowe's, the answer revealed it self. For reasons unfathomable to me #10 was actually several cents higher than #8. So I upgraded and saved a couple of bucks in the process.