Back-fed breaker

[Danielt]

I'm still pretty new to the world of solar. I have this on the plans:


I'm saying this is incorrect per 2017 NEC 705.12(B)(2)(3)(b). The 175a breaker is more than 20% of the busbar, 160a. Am I correct? Or are there other variables I should be considering?

 

[wwhitney]

705.12(B)(2)(3)(b) doesn't refer to the breaker size, it refers to 125% of the inverter continuous output current. So if the maximum inverter output current is 128A or less, then 125% is 160A or less, and 705.12(B)(2)(3)(b) is satisfied. As breakers only come in standard sizes, if the inverter output current is 120A or more, then a 150A breaker would be too small, so the install would require bumping up to a 175A breaker.

Cheers, Wayne

 

[Danielt]

Great! Thanks for the clarification

 

[Phil Timmons]

. . . . which then makes it more than 800 Main + 175 Backfeed = more than 120% of the Bus Bar. . . . meaning the WHOLE THING IS A NO GO.

Common fix would be a 200 amp External (to the Panel) Over-current -- such as 200 Amp Disconnect with 175 Amp Fuses -- and tie that in ABOVE the 800 Amp Main -- getting you entirely OUT of the Panel.

Most local utilities also require a External, Knife Blade, Visible LOTO type disconnect -- so we combine these requirements of Overcurrent for the Solar + LOTO Disconnect for the Utility, and tap in above the existing Main. Makes for a 2nd Main Exterior Disconnect -- which if the AHJ is sharp -- creates another problem of the the Main Disconnects NOT BEING GROUPED TOGETHER . . . . if the Existing Main Disconnect is indoors.

 

[wwhitney]

. . . . which then makes it more than 800 Main + 175 Backfeed = more than 120% of the Bus Bar. . . . meaning the WHOLE THING IS A NO GO.

Yeah, that's not what 2017 NEC 705.12(B)(2)(3)(b) says.

Edit: looking back at older NECs, you would be correct under the 2011 NEC. Fortunately this oversight got fixed in the 2014 NEC.

Cheers, Wayne