Max Solar Backfeed on a 200A Residential Service

[jaggedben]

Coming summer 2014!!


anybody running into the capacity limitations of
NEC705. 12(B)(2)(3)(b) s
and working with the new combos from siemens?
seems like they are expensive
and with Al bus... but

with the MC0816S1200SCT
The solar attaches directly
to the load side of the meter with
the auxiliary disconnect built into the panel front.
...

otherwise, are there any 200a meter main combos with 225a buss?
...

All the major brands have a 225A bus meter/main. But thanks for mentioning those Siemens products, I thought they only had dpne 60A solar connection. Hopefully those are still being produced.

 

[jaggedben]

And as my gears grind, I would think that in theory a solar unit would need to be a maximum of only half the size of a service. As if a load draws unity among the two different systems with a max input of say 200 amps then the most a solar would offer is 100 amps (half current supplied by the POCO and half current from solar)

It doesn't work that way. Loads just draw based on the voltage they see, they have no way of knowing where it comes from. Solar inverters (typically) just output the power available from the panels, they don't know where it goes. So panelboard rules aside, the most you can output on a service is the rating of the service.

 

[Carultch]

If I’m not mistaken,
Unless your system can work in Island mode it won’t back feed power without the power coming from the power company to begin with. As the inverter needs the 60Hz to sync with for output.

You are correct. The inverter either needs the grid, or a grid-forming device, in order to operate. It follows the waveform of the already-formed grid, and produces slightly greater voltage in order to supply power. This power feeds on-site loads, and whatever is remaining is exported to the grid, if the policy and service agreement allow for it.

And as my gears grind, I would think that in theory a solar unit would need to be a maximum of only half the size of a service. As if a load draws unity among the two different systems with a max input of say 200 amps then the most a solar would offer is 100 amps (half current supplied by the POCO and half current from solar)

I disagree with this one.

The upper limit of your production capacity can be independent of on-site consumption, assuming there is no policy/economic reason to limit to what you can export. You might have load calculations that prescribe a 200A service, and to keep it simple, you'd most likely build something that is practical to interconnect on the same 200A service. But if it were desired, you could specify/upgrade to a service in excess of that 200A size, specifically for a much larger system.

Bookkeeping-wise, there often is a practical limit to what you can export, related to your loads. The utility most likely doesn't have a policy to write you a check if you keep spinning the service meter backwards year-after-year. You'd likely need a policy that allows you to virtually assign the surplus to another customer. As a result, it is common that the annual kW-hr usage will limit the "usable" annual kW-hr production.