Sizing Storage Battery Feeder

HoosierSparky

Senior Member
Location
Scottsdale AZ
I have a storage battery that per it's specs has an output of 32A and required circuit breaker protection at 30a. Applicant is calling out #10. Do I size the feeder at 125% of output? Also there is a temp derate to 0.84 for our ambient. I'm thinking #8.
 

SceneryDriver

Senior Member
Location
New York, NY
#10 is acceptable. THWN is rated at 40A the 90C column. 40A * 0.84 = 33.6A.

Not sure you need to size to 125%. Is this considered a continuous load?

Either way, I'd probably go with #8 wire. It will be more efficient and the losses will be lower - especially important for a battery system.


SceneryDriver
 

jaggedben

Senior Member
Those are pretty weird specs.

Notwithstanding that the NEC prohibits using 10awg with a larger than 30A OCPD, it also lists the 75C ampacity at 35A and the 90C at 40A. So say for THWN-2 in EMT, 10awg would meet all requirements, if it's not considered continuous.

However I'm pretty sure that should be considered continuous. So I agree with 8awg. I don't know now to address the conflict between the NEC and the manufacturers spec for the max breaker size.
 

HoosierSparky

Senior Member
Location
Scottsdale AZ
Those are pretty weird specs.

Notwithstanding that the NEC prohibits using 10awg with a larger than 30A OCPD, it also lists the 75C ampacity at 35A and the 90C at 40A. So say for THWN-2 in EMT, 10awg would meet all requirements, if it's not considered continuous.

However I'm pretty sure that should be considered continuous. So I agree with 8awg. I don't know now to address the conflict between the NEC and the manufacturers spec for the max breaker size.
Battery discharge is considered continuous. It discharges until it is "dry" or receiving a charge. That's why I apply the 125%. When derating, you have to go with the 90C column. It's close, but I think I'll require the #8. Math works. Specs are from TESLA. Go figure!
 
Overcurrent protection should be selected according to the wire size and the load. It doesn't matter whether the battery's capable of delivering 32A or 32 kA.

If this is a battery charger capable of putting 32A into the battery, it's probably reasonable because the battery will absorb its maximum current only when it's completely discharged, and only for a brief while. Once there's some charge in there, the charge current will taper off. It won't absorb 32A continuously.
 

jaggedben

Senior Member
Battery discharge is considered continuous. It discharges until it is "dry" or receiving a charge. That's why I apply the 125%. When derating, you have to go with the 90C column. It's close, but I think I'll require the #8. Math works. Specs are from TESLA. Go figure!
If that's a Tesla Powerwall, you are looking at the peak output, not the continuous output. So you're making a mistake. You don't apply a continuous factor to a peak output that's not continuous. Code specifically says that you use the continuous output rating. That would explain why the specs don't make sense, you're looking at the wrong spec.
 
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