Heat Pump service outlet ground

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john14

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San Jose, CA
Hi guys,
I'm having 4 roof top heat pumps installed in a building and I have grounding question. AC contractor wants to run 3-#8 (35A 3phase), 1-#10 gnd, 1-#12 hot, 1-#12 Neutral (per unit). Can the service outlet attached to side of AC disconnect switch be grounded to the #10 ground that is there for the heat pump? Heat pump and service outlet circuits run in same conduit. I found lots of posts on service outlets but I don't think any mention the ground.
 
john14 said:
Hi guys,
I'm having 4 roof top heat pumps installed in a building and I have grounding question. AC contractor wants to run 3-#8 (35A 3phase), 1-#10 gnd, 1-#12 hot, 1-#12 Neutral (per unit). Can the service outlet attached to side of AC disconnect switch be grounded to the #10 ground that is there for the heat pump? Heat pump and service outlet circuits run in same conduit. I found lots of posts on service outlets but I don't think any mention the ground.
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Is 35 amps MCA or MOCP? If it is MCA but MOCP is over 60 amps (and over 60 amp device is used) then 10 AWG would be too small of an EGC.

Otherwise only one EGC is needed sized per the largest circuit in the raceway.
 
Per the heat pump spec sheet including the aux heat strips: MCA: 33.2A, MOCP:35A
Installing a 35A breaker for heat pump and 15A breaker for the service outlet.
 
john14 said:
Per the heat pump spec sheet including the aux heat strips: MCA: 33.2A, MOCP:35A
Installing a 35A breaker for heat pump and 15A breaker for the service outlet.
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Okay.

You are using #8 where #10 would be sufficient.

Voltage drop maybe, since you are using #12 where #14 would work.

If the HVAC conductors were upsized from #10 to #8, does not the EGC have to be upsized also, 250.122(B), meaning a #8 EGC would be needed?
 
I don't think we have a voltage drop issue. Run is about 50'.
I'm not an electrician but the engineer in me has me checking everyone's work. From what I can tell reading thru these forums and the codes, the wire size is barely adequate after all the derating.
Please correct me if I'm wrong, and I certainly could be. The elect code is much more complicated than I ever thought.

#8 THHN/THWN-2 90C wet rated 55A. 310.15(B)(16)
Derate .80 for 4-6 conductors in the conduit 310.15(B)(3)(a)
and .82 for 50C temperature 310.15(B)(2)(a) (conduit on the roof 3.5"-12"-Copper Development Association tables-Hayward, CA).
#8 ends up only good for 36A.

#12 THHN 75C wet rated 25A.
Derate .80 for 4-6 conductors in the conduit and .75 for 50C temperature.
#12 ends up only good for 15A.
 
Yes, temp and derating may still be an issue so running 10 is not a bad idea. From I hear it sounds like the #10 equipment grounding conductor is fine to use- no need for 13 awg
 
You are considering the #12 as THHN only. Almost all THHN These days is THHN/THWN-2.
That would allow you to use the 90C rating as the starting point for adjustments and corrections.

Sent from my XT1585 using Tapatalk
 
john14 said:
I don't think we have a voltage drop issue. Run is about 50'.
I'm not an electrician but the engineer in me has me checking everyone's work. From what I can tell reading thru these forums and the codes, the wire size is barely adequate after all the derating.
Please correct me if I'm wrong, and I certainly could be. The elect code is much more complicated than I ever thought.
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I'm glad you noticed that. The electric code is getting ridiculously complicated and not for reasons that have anything to do with safety or reliability.

#8 THHN/THWN-2 90C wet rated 55A. 310.15(B)(16)
Derate .80 for 4-6 conductors in the conduit 310.15(B)(3)(a)
and .82 for 50C temperature 310.15(B)(2)(a) (conduit on the roof 3.5"-12"-Copper Development Association tables-Hayward, CA).
#8 ends up only good for 36A.
Click to expand...

Key phrase right there. Interesting how the Copper Development Association managed to get some temperature derating factors added to the code that required the use of more copper. I haven't noticed any of the rooftops in my fair city bursting into flames because temp corrections factors were not used in the '60s, '70s, '80s, '90s and early 2000's; have you?

#12 THHN 75C wet rated 25A.
Derate .80 for 4-6 conductors in the conduit and .75 for 50C temperature.
#12 ends up only good for 15A.
Click to expand...

Your 12 AWG would be just fine on a 20A breaker and 10AWG would be more than just barely adequate already, so if you want to go on up to 8 AWG knock yourself out.

On the other hand, a lot of electricians get worried about spending somebody else's money. I don't. Get me over to your place and I'll spend every dollar you have to get this job to whatever level you want to go to to make sure that is much, much more than barely adequate.
 
I didn't mean barely adequate like its borderline hazardous, but from a code standpoint there isn't much margin. But I'm certain the code has lots of margin built in. Like you said, I'm sure the smaller wires would be fine, but any money saved would quickly be eaten up if the inspector questioned the sizing.
 
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