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Discussion Starter · #1 ·
I'm running some 8/2 NM-B to a new pullout disconnect about 35' from my panel. My most direct route is definitely through the crawlspace. First question is does 8/2 qualify as a large enough cable to run on the bottom of joists or does it fall into the same category as 12/2 and require running boards or bored holes in joists? Second question is what size THHN green ground wire should I use in conduit from disconnect to heat pump along side the two 8 gauge THHN hots? Thanks
 

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A running board is required for cable run across the top of ceiling joint in an attic. I know of no requirement for a running board for a cable attached to the bottom of a floor joist in a crawl space.
The purpose of the running board in the attic is to prevent you from stepping on the cable. No way you are going to step on a cable attached to the bottom of a floor joist.
 

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The requirement is NEC 334.15(C)

In Unfinished Basements and Crawl Spaces. Where cable is run at angles with joists in unfinished basements and crawl spaces, it shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists. Smaller cables shall be run either through bored holes in joists or on running boards.
 

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Discussion Starter · #4 ·
Ok, thanks for the code based advice. What I'll do then is run the 8/2 along the side of the sill plate where feasible and through bored holes in joists or on a running board where necessary. Any advice on THHN ground wire size in conduit to match the ground wire in 8/2 NM-B?
 

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Ok, thanks for the code based advice. What I'll do then is run the 8/2 along the side of the sill plate where feasible and through bored holes in joists or on a running board where necessary. Any advice on THHN ground wire size in conduit to match the ground wire in 8/2 NM-B?
What do you mean by the "ground wire". And the conduit? Match the cable?
The 8/2 has a ground wire.
 

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Discussion Starter · #6 ·
I mean 8/2 with ground is running to pullout disconnect. From there it'll be (2) hot 8 gage THHNs in conduit to the heat pump. Curious to know the size of ground wire in 8/2 so i can match the size from disconnect to heat pump. I know it's not 8 gauge, it's something smaller right?
 

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For separate conductors such as THHN, a 10 gauge ground wire goes with 8 gauge (and 6 gauge and 10 gauge) current carrying conductors.
 
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Are you sure the unit needs #8? That would be a rather large unit. Look at the minimum circuit ampacity.
 
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