calculation [RE-wrenches]

[Bill Brooks]

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Keith and William,

There are two separate issues here. First is the fact that the remodel has
increased load on the service to the point that a service upgrade is
required. There is no way around this issue without getting rid of load,
which the owner is unlikely to allow. Service upgrades can be expensive and
difficult. Likely the existing service is 4/0 AWG Aluminum, 200-A service
drop. An additional 100-A service from the utility is required. This is
accomplished through a new 2 AWG Aluminum service drop to a new panelboard. 

My last house in North Carolina was set up this way. It actually had two
200-amp services with a CT-meter measuring both services simultaniously to
provide one meter entrance. The call between running a 100-amp or 200-amp
additional service is between you and your client. If there is any chance of
expansion, 200-amp probably makes some sense.

Once you put a new 100- or 200- amp service on the house, the second
question can be addressed (the one everyone jumped on answering first)--how
to connect to the service? This seems quite straightforward to me. Assuming
a 100-amp new service is run (on a single house meter for both services),
the PV could be installed in a 200-A(likely 225-A bus)panelboard with a
100-amp main. This provides tons of room to put both supply (PV) breakers
(actually supply/load breakers for Outback), and load breakers. If a 200-amp
service is run, assuming a 225-amp busbar, this can handle 70-Amps of supply
breakers in compliance with the 120% 690.64(B)(2) Exception. However,
connecting on the load side of the panel with 4 3048 Outback inverters
pushes that number of 70-amps (not positive, but I think you need 2-50-Amp
two-pole breakers (even though we know that the system can only produce a
total 50-amps at 240V continuous).

So what does all this mean? I think it means a fair amount of new equipment
and reconfiguring of the existing service to pull it off. Guess what?--your
just the guy to do it!

Several circuits are going to need to come out of the existing 200-amp panel
and be placed in the the standby subpanel for the backup system. This will
free up room to put the new pool, hot tub, A/C crap on the old 200-amp
service. All the backup loads will go on the new service panel. That (I'm
thinking 100-amp service and panel given the size of the inverters--200amp
is more expensive but has potential for growth) new service and service
panel will need to be right next to the existing 200-amp panel so that all
the service disconnects are grouped (less than 6 disconnects). The new panel
could be a 200-amp panel with a 100-amp main and two 50-amp breakers (one
for each pair of inverters). You may find a cheaper way to do it, but this
is what comes to mind first to me. Good luck.

Bill.


-----Original Message-----
From: William Miller [mailto:wrmiller at charter.net] 
Sent: Thursday, January 19, 2006 12:23 AM
To: RE-wrenches at topica.com
Subject: Re: calculation [RE-wrenches]


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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.
>
>
>
>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.
>
>
>
>We are installing 10KW of PV, 4 - 3048 Outback's, and a small battery 
>system.
>
>
>
>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.
>
>
>
>Any thoughts on this?
>
>
>
>(Bill..you might have some good suggestions..)
>
>
>
>Thanks
>
>
>
>
>
>Keith Cronin

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