<html><body><span style="font-family:Verdana; color:#000000; font-size:10pt;"><div>We went over this stuff in class last year.. I'm hoping some if it stuck... seems to me you're on table 450.3(B) of the NEC.. if you choose to protect the primary only, you're restricted to 125% of primary FLA. IF you choose to protect both primary and secondary, you can size to 250% of the primary, (For inrush), and 125% of the secondary.. (and you're allowed the next size larger fuse). Hope I'm not getting too far out in the rhubarb here, but I'm thinking you may be able to do a line side tap to a SE rated knife switch fused with a slow blow fuse... maybe? db<br></div><div><br></div><div><br>Dan Brown<br>President<br>Foxfire Energy Corp.<br>Renewable Energy Systems<br>(802)-483-2564<br>www.Foxfire-Energy.com<br>NABCEP #092907-44<br><br></div>
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-------- Original Message --------<br>
Subject: Re: [RE-wrenches] Transformer inrush<br>
From: Richard.L.Ratico@valley.net (Richard L Ratico)<br>
Date: Thu, May 13, 2010 8:01 am<br>
To: re-wrenches@lists.re-wrenches.org<br>
<br>
Sorry for the additional post here, but it includes a typo correction:<br>
<br>
Bill,<br>
<br>
Not a lot of personal experience with this issue. The one time I had to deal<br>
with it, we were able to leave the circuit on, once we got it energized. <br>
As you know, the crux of the problem is the nature of the voltage relative to<br>
it's cycle when connection is attempted.<br>
<br>
An internet search resulted in this explanation that was very helpful. See:<br>
<br>
<a target="_blank" href="http://www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm">http://www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm</a><br>
<br>
As I understand it now, counterintuitively, you don't want to switch at the zero<br>
crossing. SS relays aren't my strong suit either, but if they connect at that<br>
moment.......<br>
<br>
Is your step up transformer actually designed by the manufacturer to be a step<br>
up unit, or are you using a step down unit in reverse? <br>
Again, according to information found in my search, not personal experience, the<br>
latter can contribute to higher than expected inrush.<br>
<br>
Dick Ratico<br>
<br>
<br>
--- You wrote:<br>
Thanks for the reply. To clarify, we have tried his with one transformer <br>
and with two transformers. Our success rate is lower powering one <br>
transformer. My hunch is that the capacitance of the feeder is buffering...<br>
<br>
I wonder if I use a large solid state relay with zero crossing switching I <br>
might have better luck.<br>
<br>
<br>
William Miller<br>
<br>
<br>
At 06:23 PM 5/12/2010, Richard L Ratico wrote:<br>
>Bill,<br>
>Your branch circuit breaker is pretty small.<br>
>Are you energizing both transformers simultaneously? If so, it might help to<br>
>energize them individually.<br>
><br>
>The branch breaker can be up to 250% of the step up transformer's primary<br>
>rating. 10kVA/240V=41A.<br>
>41A x 2.5=100A. (round numbers). To do this you would also need overcurrent<br>
>protection on the transformer secondary.<br>
><br>
>Of course, you've already got the 100A main tripping. That's why I asked if<br>
>you're<br>
>energizing them simultaneously.<br>
><br>
>Transformer inrush can be a real problem.<br>
><br>
>Hope this helps.<br>
><br>
>Dick Ratico<br>
>Solarwind Electric<br>
><br>
>--- You wrote:<br>
>Friends:<br>
><br>
>I have on off-grid client that is trying to eliminate winter generator run<br>
>time. He has a rental on another corner of the property that has grid<br>
>power. When it was installed, he put in an extra meter, hoping to use it<br>
>to run a utility back up feeder to charge batteries.<br>
><br>
>We are now installing this feeder. We purchased two 10KVA 240/600 volt<br>
>transformers to bump the voltage to 600 and then back down to 120/240.<br>
><br>
>Here's the problem: when we energize the circuit breaker that feeds the<br>
>transformer, the circuit breaker AND the main breaker trip. The<br>
>transformer is 10KVA, the main is 100 amps and the branch breaker is<br>
>60A. The breakers hold sometimes but trip other times. This is obviously<br>
>an inrush problem. How do I correct this problem?<br>
><br>
>Thanks in advance,<br>
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--- end of quote ---<br>
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