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Old 03-07-2014, 12:35 PM   #76
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Want to upgrade home from 200A to 400A service


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Originally Posted by dmclean701 View Post
Since this thread has spun off into space, I will add the little I know about the fuses on the pole top transformer. A lineman told me they are typically 15 amp on a 12000 volt line. These are just there to protect the power companies lines. The 240 volt customer side of the transformer gives a turns ratio of 50:1. 15 amps times 50 is 750 amps. That will burn down the house drop, into molten metal, long before the fuses on the primary side will blow. If I got this wrong please correct it.
The typical voltage is 7200 phase to ground (12,470V phase to phase). For a 15KVA xfmr, the typical fuse size is 3A fuse. Some places use a 6A; it depends on the fuse type. A 25KVA xfmr would typically have a 6A fuse.

The ratio is 7200/240=30.

I think most utilities run a 2/0 AL service drop to the house. I believe this is rated for ~200A @ 90 degC.

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Old 03-07-2014, 12:37 PM   #77
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Want to upgrade home from 200A to 400A service


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I do not know what service drop size is, or how to check it.
It should be printed on the side of the cable every 12 or 24 inches.
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Old 03-07-2014, 12:47 PM   #78
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Want to upgrade home from 200A to 400A service


Thanks for the correction. My understanding of the transformer wiring is not clear. Anyway, the up sizing of service to 320 or 400 amps would require a larger drop line at minimum, correct?
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Old 03-07-2014, 01:35 PM   #79
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Want to upgrade home from 200A to 400A service


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Thanks for the correction. My understanding of the transformer wiring is not clear. Anyway, the up sizing of service to 320 or 400 amps would require a larger drop line at minimum, correct?
Most likely. And, in some cases may require a larger xfmr. See, a 400A service is two-200A services. If a pole or pad xfmr is already at its limit and you try to add another 200A, the xfmr limit may be exceeded and a larger one will have to be swapped out.
It also helps to look up the conductor size required to carry 400A continuous. You'll see why most 400A services switch to using CTs instead of running all that current through a meter and base.

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