subpanel - 4AWG = 90A breaker?

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  #1  
Old 09-29-08, 12:46 AM
R
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subpanel - 4AWG = 90A breaker?

Short version -

Feeder for non-service rated panel in detached garage - can I use a 90A breaker for 4AWG CU THWN-2 through conduit with 3 current carrying conductors and no derating required with 75 rated terminals, or 80A breaker?

Thorough version -

I'm running a non-service rated panel to my detached garage. I was going to go with 100A (off of my 200A service) and my inspector said to use 4AWG CU. When he said this, I (basically) clarified that he's allowing 310.15(B)(6) and not 310.16 and he said yes. (were on 2002 here)

So I bought my 4AWG CU, but have been debating this. With the 2008 code clarifications on 310.15(B)(6), I personally don't believe that 310.15(B)(6) should be allowed for a garage feeder (4AWG with a 100A breaker). I also don't plan on staying in this house for real long, so I am also concerned that some home inspector will flag this during an inspection when I'm selling. So I'd like to just do it right to begin with. Either buy new wire or buy a different breaker.

Since 310.6 lists 4AWG @ 75 = 85A, I have to either round up or down. If I can use a 90A, then I'll just buy that, run the 4AWG, and return the 100A breaker I already bought for a total of ~$20.00 additional. If I have to go to a 80A, well then I may try to get some 3AWG and go with the 100A breaker and ebay the 4AWG. This will probably end up costing more than $20.00... I just think that 80A will be just a tad bit closer to pushing it than I'd like, but 90A should be OK.

Essentially, my question comes down to if code allows the OCPD to be sized for the wire (which I believe it does, thus 90A), or if the wire needs to be sized for the OCPD.

Thanks,
Rick
 
  #2  
Old 09-29-08, 09:18 AM
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You are allowed to round-up to the next standard breaker size when the calculated wire ampacity falls between two standard breaker sizes. So in this case the 90A breaker would satisfy the '08 code; however I also don't believe that there would be any problem using the 80A breaker. An 80A panel is still far more power than most anyone could use in a detached garage.

In either case, your ground wire should be #8 cu.
 
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Old 09-29-08, 09:19 PM
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Thanks for the confirmation, I'm going to go with the 90. The garage will also be used for woodworking, so there will be extra loads.

Thanks,
Rick
 
  #4  
Old 10-02-08, 11:08 AM
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Just a slight clarification: You are only allowed to round up on the breaker size if the calculated load on the conductor is less than the conductor ampacity. Your conductors are rated for 85A. The 'round up' rule doesn't change that 85A value. But if your _load_ is less than 85A, you may protect the conductor with a 90A breaker.

-Jon
 
 

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