Have plans for a system, panels themselves have individual micro inverters. AC disconnect out at the panels with 2-2 pole 20 amp circuit breakers, from here direct burial 250mcm alum feeding a AC disconnect service rated circuit breaker 100 amp. This then goes into main load center. What size conductors should feed the load center from the 100 amp disconnect? Hope I explained this correctly.
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8kw DC PV system
- Thread starter VIC1958
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jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I don't see the need for conductors rated for more than 40amps under conditions of use. Assuming equipment has 75C terminals, and you do not need to derate for temperature, they could be #8.
Why are you using a 100 amp disconnect when your output is significantly less than that? (And why are you using 250mcm aluminum? Although I haven't done a calc, that seems excessive even for reducing voltage drop on a very long run.)
Why are you using a 100 amp disconnect when your output is significantly less than that? (And why are you using 250mcm aluminum? Although I haven't done a calc, that seems excessive even for reducing voltage drop on a very long run.)
The reason for the 250 mcm is that the run from the 2 arrays is over 700', I have no idea why they are using a 100 amp fused disconnect (they like to spend money?) from the 100 amp disconnect they are feeding the 200 amp panel to a 2 pole 40 amp breaker. Shouldnt this breaker be a backfed breaker?
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
The reason for the 250 mcm is that the run from the 2 arrays is over 700', I have no idea why they are using a 100 amp fused disconnect (they like to spend money?) from the 100 amp disconnect they are feeding the 200 amp panel to a 2 pole 40 amp breaker.
Yeah, I would use a 60amp disconnect with 40amp fuses. Again, I know of no reason why the conductors between the disconnect and panel have to be sized for more than the required ampacity under conditions of use, leaving aside voltage drop considerations.
Shouldnt this breaker be a backfed breaker?
Yes, but see 705.12(D)(5) and (6). Usually you can use an ordinary breaker and install it the ordinary way (no tie down).
Marvin_Hamon
Member
- Location
- Alameda, CA
Since the interconnect breaker in the panel is 40A the conductor between the panel and the AC disco only needs to be large enough to be protected by the 40A breaker. The fuses in the AC disco will be useless in this installation.
As for why a 100A disco, my guess would be that was the smallest AC disco that someone could find that the 250MCM conductors could land in, and I'm surprised you can land 250MCM in a 100A disco.
As for why a 100A disco, my guess would be that was the smallest AC disco that someone could find that the 250MCM conductors could land in, and I'm surprised you can land 250MCM in a 100A disco.
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