# 208 Volt Solar Backfeed?????? What do I do?



## jsmart84 (Nov 20, 2011)

Alright Ive got a a commercial building with a 400 amp Cold sequence Ct cabinent. The Power company wants me to be after the Ct's problem with that is I have 140 amps of backfeeding current from 136 -Enphase 215 Inverters. If I use the 125 % rule Im only allowed 80 amps worth of backfeeding . I could downsize the main breaker or upsize the entire service in a whole. The Power companys rep told the solar company that because it was 208 volts on three phases the 140 amps is split between all three . That really didnt sit well with me , if I was feeding a 200 amp 3 phase panel i wouldnt use a 3-pole 75 amp breaker. Does anyone have any input.


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## bbee (Dec 15, 2011)

jsmart84 said:


> Alright Ive got a a commercial building with a 400 amp Cold sequence Ct cabinent. The Power company wants me to be after the Ct's problem with that is I have 140 amps of backfeeding current from 136 -Enphase 215 Inverters. If I use the 125 % rule Im only allowed 80 amps worth of backfeeding . I could downsize the main breaker or upsize the entire service in a whole. The Power companys rep told the solar company that because it was 208 volts on three phases the 140 amps is split between all three . That really didnt sit well with me , if I was feeding a 200 amp 3 phase panel i wouldnt use a 3-pole 75 amp breaker. Does anyone have any input.


Is this a 3-phase service? If so you don't have 140 amps of solar, it would be 100 amps. Each M215 inverter outputs 208 volt 1-phase power. Easiest way would be to tap off the ct cabinet ahead of the main breaker, but after the ct's with a 100 amp fused disconnect. Then you don't need to apply the 120% rule and you fall under NEC 705.12(A) and not 705.12(D)(2)


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## jsmart84 (Nov 20, 2011)

Here in CT were still working out of 05 NEC. It doesnt have 705.12( d)2. The feed back on the voltage side i found out to be right i forgot about the 1.73 multiplier for three phase. If were down stream from tge main howdoes the 125% not apply ?:


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## Louieb (Mar 19, 2007)

Could you elaborate more on your service 400A 208V 3 Phase ,what size main breaker,Disco are you planning to dump the solar into is this panel,disco near the service entrance?


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## jsmart84 (Nov 20, 2011)

Its a 400amp main breaker gear with 400 amp main breaker. Originally i specd a 200 amp 3 phase ;panel and 200 amp disco.diaconnecting means is right next to ct meter outside. Main gear is behind meter.


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## don_resqcapt19 (Jul 18, 2010)

jsmart84 said:


> Here in CT were still working out of 05 NEC. It doesnt have 705.12( d)2. The feed back on the voltage side i found out to be right i forgot about the 1.73 multiplier for three phase. If were down stream from tge main howdoes the 125% not apply ?:


 I think the rules would be in 690.64 in the 2005 code, but not sure as I don't have a copy handy.


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## bbee (Dec 15, 2011)

don_resqcapt19 said:


> I think the rules would be in 690.64 in the 2005 code, but not sure as I don't have a copy handy.


Your right, in 2005 it's 690.64(A) for connecting on the line side of the main disconnect and 690.64(B) for connecting on the load side of the main.


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## bbee (Dec 15, 2011)

jsmart84 said:


> Here in CT were still working out of 05 NEC. It doesnt have 705.12( d)2. The feed back on the voltage side i found out to be right i forgot about the 1.73 multiplier for three phase. If were down stream from tge main howdoes the 125% not apply ?:


Are you talking about the 125% for continuous use or the 120% rule for the bus bar rating? The 120% rule in 690.64 only applies if you tie in down stream of the main disconnect. If you do that you'd have to reduce your main breaker to 350 amps if the load calc on the building allows it. 

Assuming the bus bar is rated at 400 amps, 400*1.2 = 480 amps. If you do a 100 amp pv breaker that give you 380 amps remaining for the main and the closest smaller breaker would be 350 amps.


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## bbee (Dec 15, 2011)

jsmart84 said:


> Here in CT were still working out of 05 NEC. It doesnt have 705.12( d)2. The feed back on the voltage side i found out to be right i forgot about the 1.73 multiplier for three phase. If were down stream from tge main howdoes the 125% not apply ?:


Are you talking about the 125% for continuous use or the 120% rule for the bus bar rating? The 120% rule in 690.64 only applies if you tie in down stream of the main disconnect. If you do that you'd have to reduce your main breaker to 350 amps if the load calc on the building allows it. 

Assuming the bus bar is rated at 400 amps, 400*1.2 = 480 amps. If you do a 100 amp pv breaker that give you 380 amps remaining for the main and the closest smaller breaker would be 350 amps.

Because of this it's usually simpler to tie in on the line side of the main so the 120% rule doesn't apply.


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