We are back feeding 80 amps ,I wish I new how to post pictures ,but I will try my best. The main service is split buss with two 200 amps, there is distributin on both 200 amps one of them has minimal load on it four 20 amp breakers, what I want to do is move those four 20 amp breakers to the other 200 amp bussing, the only load one the other 200 amp bussing would only be my backfeed 80 amp solar breaker, my question is I'm I still only able to backfeed 120% of the bussing thanks
Split buss 400
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jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Carultch
Senior Member
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- Massachusetts
On my iPhone
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
You basically treat these as separate panels. If you want to move the loads from one to the other it's okay as long as you do the load calc to verify that it can handle the load. You can't just move them without checking.
So if you have one bus with nothing on it but your back feed CB then what you do depends on the code you are on:
2011: you are stuck with the 120% rule.
2014: you can use the PV combiner section that allows the sum of all the CBs other than the main to add up to the bus rating.
Many people on 2011 still use the method from 2014 but it's not compliant so you need to get approval from the AJH.
So if you have one bus with nothing on it but your back feed CB then what you do depends on the code you are on:
2011: you are stuck with the 120% rule.
2014: you can use the PV combiner section that allows the sum of all the CBs other than the main to add up to the bus rating.
Many people on 2011 still use the method from 2014 but it's not compliant so you need to get approval from the AJH.
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
Carultch
Senior Member
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- Massachusetts
FWIW, I have been going by the 2014 rule since 2009.
I've been going by simply 125% of total inverter current for all panelboards that contain no loads or PV-specific loads of negligible amperes, ever since I started in 2010.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Me, too, or something like that. Not the 120% rule, in any case; that would have made some of my AC combiners ridiculously large. FWIW, none of the AHJ's I dealt with raised any issues.
FWIW, I have been going by the 2014 rule since 2009.
These are all interesting points to me. I agree that I see a lot of designs that have ignored the 120% rule when sizing panels that are just combining PV inverters together. Otherwise it requires significantly over sizing the panel prior to 2014 NEC.
My question is, did you explicitly call this out to the AHJ and ask permission to comply with the code using alternative methods or did you just design them that way and see if the AHJ picks it up in plan review? I think most people do the design and wait to see if it gets questioned route. I've always tried to go with requesting the alternative methods route and many times have been told by AHJs to not worry about it.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I just designed them and submitted the plans. Forgiveness is more easily obtained than permission.
That would be the most common action from my experience.
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