calculation [RE-wrenches]

[William Miller]

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At 12:44 PM 1/18/2006, you wrote:

Keith:

I reread your post and it raised more questions in my feeble mind.  Based 
on the responses of some of our colleagues, I'm not sure I understand the 
scenario:

You say that your client has a 200 A service and the engineer calcs the 
loads at 298 A.  Is this load calc at 220 VAC or 110VAC?

If it is at 220 VAC, it sure sounds like the service needs to be upgraded, 
regardless of what offsetting a grid-tie system provides.  One can not 
count on any PV system to offset loads when designing a service, for 
obvious reasons.

If the 298 A loads described are at 110 VAC, then you are within the 
capacity of the 200 A service (assuming the duty cycles of the loads are 
accounted for).  Then you have a simple question:  Can the buss provide 
enough reserve ampacity to be fed by two sources: the feeder breaker and 
the grid-interactive system.  The math is easy and you are allowed a 
combined ampacity of 120% of buss rating.

If the answer is: "No, there is not enough buss ampacity", then, as pointed 
out by others, you have two choices:  Reduce the feeder breaker size until 
the total source ampacity is equal to or less than 120% of the buss rating, 
or, tap the feeders, if there is a legal way to do that.  There are 
disadvantages to each option:  The former means that you reduce the 
ampacity available to the customers loads.  The latter requires that you do 
not violate the listing of the service device and that over current 
protection is provided as per the tap rule.

Seems simple to me.  Am I missing something?  If the service is too small, 
it needs to be upgraded and trying to use an RE system to avoid that 
reality is problematic.  (As I pointed out earlier, a battery system is 
sometimes a load instead of a load offset).

Hope my ramblings make sense and help you in some way.

William Miller




>We have a client that has an existing 200 amp electrical service. They are
>doing a complete remodel, addition with some heavy AC and other stuff-
>pools, water features etc.
>
>
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>The engineer did a calculation of 298 amps. The work involved to bring the
>service up to 325 amps (next standard size here) is lengthy, costly, and
>somewhat challenging.
>
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>We are installing 10KW of PV, 4 - 3048 Outback's, and a small battery
>system.
>
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>They are eager to not upgrade to 325 amps, and I know the limitations
>outlined, Per NEC- 690.64 B 2 exception regarding the busbar rating.
>
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>Any thoughts on this?
>
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>(Bill..you might have some good suggestions..)
>
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>Thanks
>
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>
>
>Keith Cronin

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