Clarification on NEC 690.64(B)(2)

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mike007

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I dealing with a solar grid tie system here

The explanation on the links isn't too clear. they state that during the daytime you will have current from both the power company and from the solar array across the bus bar. I would have to ask if this has been tested.

If my array lets say 5KW 240 volt is generating full power at noon and lets say 10.6 amps across the 2 hot bars. That power is first going to feed the devices in the house and any excess would be pushed out to the grid. The dependancy on the grid has been reduced.

How can you have both and over power the bus bar ? Even if its early in the morning or late in the evening most of the load is covered by the power company but as in my situation by mid day I'm pushing almost 16 amps back to the grid and the remainder of the 21 amps ( 5 amps ) is taking care of the phantom loads in the house. Nothing is being taken from the Grid power system.


The requirement is that on a "dwelling unit"
the sum of circuit breakers feeding current to a conductor or busbar cannot exceed 120% of the rating of the busbar [NEC 690.64(B)(2) Exception]. What this means is that on a typical 200A residential service the maximum sum of breakers is 240A. If the main breaker for the service panel is 200A that leave a maximum of 40A for for?grid-tie Inverters
 
Re: Clarification on NEC 690.64(B)(2)

Mike007,

My read on this is to protect the busbars from overload by PV system. By summing all the c.b. values it insures they will not get overloaded.

The excp. for dwellings allows 20% more to this sum. So 200a busbar times 120% equals 240a, You just made it,if the sum of the c.b.'s was 260a you would need to increase the size of the busbars(panel).

Mike, I'm no expert in the PV field,but I believe this is the intent.
 
Re: Clarification on NEC 690.64(B)(2)

I can see the intent of the rule.
However,
If the initial load in the house were almost 190 amps and the solar was putting out max 40 amps then you could assume that you would see 230 on the bus bar.

I can see the solar reducing the inbound power by 40 amp effectively you would only have 150 amps from the power company and 40 from the solar array. The over all load is still 190 amps.
 
Re: Clarification on NEC 690.64(B)(2)

Mike,

You've got it right,if total connected load is 190a,the busbar will never see more than that under normal conditions (ie),fault---etc.
 
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