back-feeding 150 amps on a residence

JoeNorm

Senior Member
Location
WA
Doing a fairly large system on a residence. The back-feed amperage is around 150 amps before multiplying for continuous duty.

My question is to folks who have done residential systems this large, how did you design it? How did you interconnect? What are your voltage drop parameters? Anything to take into consideration that I might be missing?

We are back feeding a 400 amp panel with a 200 amp main breaker.
 
Doing a fairly large system on a residence. The back-feed amperage is around 150 amps before multiplying for continuous duty.

My question is to folks who have done residential systems this large, how did you design it? How did you interconnect? What are your voltage drop parameters? Anything to take into consideration that I might be missing?

We are back feeding a 400 amp panel with a 200 amp main breaker.


Yeah thats pretty big for resi. What is that, around 40kw DC? Largest residential system I have done is 25 KW DC. Not much different really, just larger. I dont know what is there for existing, but you dont necessarily need 400 amp equipment for that. I like to use 230.40 exception #3 with a class 320 socket for larger ground mount PV (which could just be called a supply side connection, but a different way to think about it, although it would allow you to run three wire if your area's interpretation is that a supply side gets 4 wire). I just used a 200 A MB panel at the array and connected the inverters to it load side per 705.12(D)((2)(3)(c). This would work with your size system too. If this is a roof mount then none of this would apply.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Doing a fairly large system on a residence. The back-feed amperage is around 150 amps before multiplying for continuous duty.

My question is to folks who have done residential systems this large, how did you design it? How did you interconnect? What are your voltage drop parameters? Anything to take into consideration that I might be missing?

We are back feeding a 400 amp panel with a 200 amp main breaker.
If your numbers are correct you have the electrical headroom in the MDP to connect through a backfed breaker, but you'd need a 200A breaker and that will likely be a problem. Is it possible to split the system into two ~75A pieces? If so, ask Your AHJ if they will allow two interconnection breakers. Failing all that it will have to be a line side connection under 705.12(A). If you go that route I'd advise consulting the AHJ as to whether or not they consider a PV system thus interconnected to be a service; that will have an impact on your design.

Voltage drop parameters are maximum inverter current, distance, and AC wire resistance.
 

PWDickerson

Senior Member
Location
Clinton, WA
I have done a few systems that size. Like electrofelon said, you don't necessarily need to use a 400 A panel if you supply-side connect to the service conductors ahead of the main breaker in a 200A panel. We use Burndy clear taps to make the splice. Hint, if you use the 2-port double-sided taps good for up to 350 wire, you can remove a section of insulation from the service conductor and slide the splice down the wire to tap into it without cutting it. This saves some space as the 2-port taps are not nearly as wide as the 3-port taps. Run the tap conductors into a 200A main breaker panel and wire it like a service (that's what the inspectors want to see around here, other areas are different so check with you AHJ). If you don't put any load breakers into the panel, you can qualify the bus under the "sum of the breakers" rule, 2017 NEC 705.12(B)(2)(3)(c). Also, check with your utility to make sure their transformer can handle 150A of backfeed for a 3-hour stretch.
 
200A branch breakers do exist.
He said it was a 400 amp panelboard, so that would be a true panelboard, not a load center, which brings up some interesting points.

1. As far as 200 amp breakers go , there are certainly options, but I think it will depend on what brand it is. Some (all?) manufactures will make a 200 amp plug on that takes up 6 standard spaces. However the panelboard might be bolt on. Square D NQ panelboards will take either bolt on or plug on, and there is a QO2200 breaker. For Siemens, their panelboards are all bolt on and I am pretty sure their standard bolt on only goes up to 125 (for single phase), According to their panelboard catalog, their P1 only goes up to 125 amp branch device (which is probably that breaker). A P2 will take up to a 400 amp branch device, but anything over 125 is certainly a different frame and would require different bussing parts than the standard bolt on fingers. I am not very familiar with other brands.

2. Because it is a panelboard, subfeed and feed thru lugs are almost certainly available which could make for some convenient options as well.
 

JoeNorm

Senior Member
Location
WA
We did it with a 400 amp panel board from SQD and a 200 amp main. Was not hard to find. Pretty big panel to be sitting in a 1600Sqft, 1bedroom house though!
 
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