Another sub panel question


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Old 06-16-04, 09:30 PM
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Another sub panel question

A little knowledge can be a dangerous thing... I thought I had it all figured out, then I read a lot of other threads, and now aren't so sure of my assumptions. I have a 200 amp main breaker panel (technically a sub panel, I believe, because I have a 200 amp disconnect under the meter, 2 hot, 1 neutral, 1 ground between them). I have mounted a 100 amp sub panel next to my 200 amp panel, and plan to feed it with a 100 amp 220 breaker. I thought I could use #8 THWN copper, rated at 50 amps, according to the "home wiring" book that I purchased, to connect with, but other threads that I've referred to indicate that #6 should be used. I'm going such a short distance that it makes little difference, cost wise, but I wonder now if my book is giving me bad information?
 
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Old 06-17-04, 03:51 AM
rlrct
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Actually, for a 100 subpanel and based on the ampacity tables, you need to run either #3 THHN (copper) or #1 if you're running aluminum.

#6 would be good for 65 amps.

These ampacity ratings are from table 310-16. Here's a link to an online version of it, Ampacity Tables, which you might find interesting. You need to use the 75&#176 column because that's what the terminals on your breakers are rated for.
 
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Old 06-17-04, 07:59 AM
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How can something that should be basic get so confusing? The 100 amp breaker that I have is actually 2 100 amp breakers connected, the trip levers bonded so if one side trips, the other side is disconnected as well; I'm assuming then, that each leg of the power supply can provide 100 amps of current through the respective breaker, thus requiring wiring capable of carrying the 100 amp load for each phase. Am I right so far? If that's true, then the 125 amp panel that I bought is inadequate, and should be protected by, perhaps, 60 amps per leg, rather than 100 amps per side; otherwise I'm exceeding the rated capacity of the panel. If I use the #3 THHN, as suggested, will the stranded wiring actually fit in the clamp of the breaker? #1 Aluminum seems like it would be a real challange to fit into those clamps, as well.
 
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Old 06-17-04, 08:05 AM
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Actually a 100A double pole breaker supplies 100A on each leg. 200A at 120V, 100A at 240V. So if all you have is 120V loads you can actually pull 200A total. You will need to run a #3 copper as your feeder (2 hots, neutral and a #4 ground). The neutral and ground can't be bonded in the new panel, and you need separate neutral and ground bus bars.
 
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Old 06-17-04, 08:23 AM
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So the "Siemans load center 125 amp 20x20" requires which breaker to prevent exceeding the rated capacity.?... The 100 amp double pole would actually allow 200 amps, the 60 amp double pole allows 120 amps. I thought the "rated" capacity of the load center would be the maximum amperage allowed, not the amps per side... understand what I'm getting at??
 
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Old 06-17-04, 08:28 AM
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The 125A rating is a 220V rating. In another words the size of the breaker used to feed the panel.
 
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Old 06-17-04, 09:24 AM
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It is misleading to say that a 100-amp breaker allows 200 amps. No one wire is ever carrying more than 100 amps. The 125-amp panel is fine.

To say that it is providing 200 amps is like saying that your car is going 100 miles per hour because each of the four wheels is going 25 miles per hour. The amps on the two hot wires cannot be added.
 
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Old 06-18-04, 06:43 AM
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I guess I'm still learning proper "forum etiquette"... I asked my next question under a new thread "making the connection". Sorry for the inconsistancy...
 
 

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