Something I ran into...

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jason

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Someone fed a 100 amp subpanel with a 6/3 and an 8/3 copper paralleled. These were protected (or not) by a 100 amp breaker. This is not part of the job I'm there to do, but it may be serious enough to fix regardless. I haven't mentioned it to the homeowner yet. This is a residence. However, it got me wondering. Lets say the load was 60 amps. How would the current be distributed on each wire? 30/30 40/20 60/0 ??? I have no idea and I know it's not right, just wondering.
 
jason said:
Lets say the load was 60 amps. How would the current be distributed on each wire? 30/30 40/20 60/0 ??? I have no idea and I know it's not right, just wondering.
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The conductor size will not limit current - so 30/30...
 
iwire said:
Whoa, of course the conductor size will have an impact on the current split. :smile:
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In a fault condition yes... Under a 60a load not by much, certainly not 40/20.... Doubt 1a could be measured in difference under a moderate load.
 
stickboy1375 said:
I find it interesting that someone would run two feeders vs 1 correctly sized feeder, I would love to know the whole story on this one...
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I don't know either but it looks like it'll be a pain feeding a #2 into that panel. The panel is on an outside wall that a joist runs right beside. I was wiring a spa and caught hell fishing the 6/3 to it. Luckily there was a spare 10/2 that I could tie on to.
 
jason said:
I don't know either but it looks like it'll be a pain feeding a #2 into that panel. The panel is on an outside wall that a joist runs right beside. I was wiring a spa and caught hell fishing the 6/3 to it. Luckily there was a spare 10/2 that I could tie on to.
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Do a load calc and see if you can just eliminate the 8-3...
 
stickboy1375 said:
I'm also going to assume they doubled up the lugs on both ends as well...
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Ok, although it's really crazy, let me clarify. This is what it is. A 100 amp QO breaker in the main panel. Using a reducer/splicer, they put the #6 and the #8 on one side and a #2 at the other side and going to the breaker. Then once under the house they put a junction box and also spliced just as they did in the panel to a #2 and went to the sub panel from there. So the sub panel has 40' of #2 SER going to it, just as you'd expect.

Maybe this was to fool the home owner?
 
jason said:
Ok, although it's really crazy, let me clarify. This is what it is. A 100 amp QO breaker in the main panel. Using a reducer/splicer, they put the #6 and the #8 on one side and a #2 at the other side and going to the breaker. Then once under the house they put a junction box and also spliced just as they did in the panel to a #2 and went to the sub panel from there. So the sub panel has 40' of #2 SER going to it, just as you'd expect.

Maybe this was to fool the home owner?
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Using up left over material? :confused: Wow, hey just so you know #2 ser is only good for 90 amps...
 
Percent difference.

Percent difference.

iwire said:
I don't know by how much but I agree 40/20 seems imposable.
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1000' of #8 has .8 ohms resistance
1000' of #6 has .5 ohms resistance

.8-.5=.3
.3?.8=.375 or 37.5% difference in resistance.

That sound right?
 
stickboy1375 said:
Using up left over material? :confused: Wow, hey just so you know #2 ser is only good for 90 amps...
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Not as a sub feed. 100 amps if supplying a sub panel, right? I've done this hundreds of times and have never had it turned down.
 
jason said:
Not as a sub feed. 100 amps if supplying a sub panel, right? I've done this hundreds of times and have never had it turned down.
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Your trying to use table310.15(B)(6)... this table is only for the "main power feeder", so 100 amps if its a dwelling service, 90 amps for a sub-panel.
 
stickboy1375 said:
Your trying to use table310.15(B)(6)... this table is only for the "main power feeder", so 100 amps if its a dwelling service, 90 amps for a sub-panel.
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Thanks. Lots of improper sub panels out there.
 
jason said:
Thanks. Lots of improper sub panels out there.
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Yes, all over the place, the NEC has been changing the wording of the section to try to clear up the confusion.

Only the conductors that supply 100% of the dwelling units power can be sized using table 310.15(B)(6).
 
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