[RE-wrenches] down-sizing main breaker

William Miller william at millersolar.com
Sat Aug 1 13:07:46 PDT 2009


Dick:

I don't believe you have solved Brian's dilemma yet.  It is my 
understanding that if one is going to install a load side tape, the back 
feed calculations need to work for every link of the distribution system 
upstream to the meter.  In the scenario you suggest, that will not be the 
case.  The 100 amp meter main on Brian's project will still  accept only 20 
amps of PV AC feed, still not satisfying his requirements.

I may not have read Brian's original post closely enough.  If Brian needs 
to feed 40 amps of PV derived AC and he wants to do it via the load side 
connection allowed by 690.63 (B), he has two choices:

         A. He can upgrade all of the distribution to 200 amps  from the 
meter to his point of connection.  The arithmetic is simple:  200 * 120% = 
240 amps.  240 -200 = 40 amps of PV feed allowed.
         B. He can upgrade all of the distribution to at least 125 amps 
capability and leave the 100 amp main breaker.  This would require that all 
of his distribution be upgraded, from the bus in the main disconnect to the 
feeder wire to the sub-panels. 125 * 120% = 150 amps. 150 - 100 = 50 amps 
of PV feed allowed.

Logistically, there is not a lot if difference between A and B.

See our web site, http://millersolar.com/, for a point-of-connection 
calculator.

Thanks for the advice on Milbank.  I will not be needing any 200 amp 
breakers as there is no need to up-size a breaker, instead I would be 
generating more spare 200 amp main breakers.

William


At 12:20 PM 8/1/2009, you wrote:
>William,
>
>I stand corrected. Thank you.
>
>This is not a supply side tap. I should have said it was a tap on the supply
>side of the panelboard main breaker. That said , I think it works and meets
>code, provided the conductors spliced to the feeder are properly sized. The
>panelboard busbar is protected by it's main breaker and cannot be 
>overloaded by
>the solar contributuion.
>
>On your other point, Milbank makes a 200 amp MMC that that can be retrofitted
>with a 100 amp breaker. I have done this twice, but not because of a solar
>install. We've had a couple 100 Amp, 300', underground service laterals that
>required oversized conductors because of voltage drop. The Milbank 100 Amp
>equipment is physically too small to do this. In both cases we replaced 
>the very
>expensive 200 amp breaker with an also very expensive 100 amp breaker, all by
>Milbank and labeled as such. I was told by my supplier it was impossible to
>purchase a 200 amp Milbank MMC with the 100 A breaker preinstalled. The 100 A
>breaker is the same physical size as the 200 and fits right in, no 
>modifications
>required. I have two 200 amp Milbank breakers on my shelf if you need 
>them. The
>MMC units are very expensive to begin with. Here in VT / NH a basic meter only
>enclosure is more often pared with a separate disconnect enclosure through an
>offset or straight nipple. This combination might work for you, substituting a
>load center for the disconnect.
>
>Dick Ratico
>Solarwind Electric
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