Electrical Upgrade

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Old 07-09-09, 10:05 PM
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Electrical Upgrade

I am helping out a friend of mine with his electrical upgrade in his home. He recently added a garage onto the house. He currently has (2) sub-panels that were added years ago that he wants to eliminate. We decided to add a 200 amp panel in the garage and run (3) 2" PVC conduits under the floor over to the house and to a large pull station/ junction box in place of the subpanels. Once the utility company disconects the old service, upgrades the xfrmr & runs his the new drop to the new panel & meter base, we were going to run THHN from the new panel to the pull station. At this time the sub panels are goin to be removed and the pull station installed. The 2" PVC conduits are going to carry the THHN from the new panel to the pull station to be wire-nuted to the existing house ccts. The inspector stopped by today for the service inspection. While he was there I had explained to him our plan to make sure we were going to be compliant. Once he looked in and saw the aging Wadsworth & Federal panels, he agreed the upgrade was a good idea. The pull stations size was ample, but he was concerned over the conduits sizing in relation to the number of ccts we had to pull through. I checked a couple of NEC tables and we have (27) ccts we will be running which makes a total of approx (56) wires. I looked this up in some of the tables and it appears this can all be done with only (1) of the 2" conduits with the 40% conduit fill taken into account. The inpector had a different idea and told me that I would have to de-rate the wire for the heat withing the conduit. I understand this is a factor as well, but after he explained it to me, we would have to use all (3) conduits with all the ccts split up between them. I was confused and his frustration was mounting, so I let it go and decided to try to get educated on my own. Both electrical supply houses in our area were out of code books and were also confused by my conversation with the inspector. Sorry for such a long post, I just wanted to make sure I gave all the info I could. Can anybody shed any light on this for me to help me understand? Thanks!!!
 
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Old 07-09-09, 10:34 PM
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The 40% conduit fill is only half the story. The inspector is right about having to de-rate the conductors in the conduit based on heat. You first need to look in table 310.15(B)(2)(a) to get the de-rating factor based on the number of current-carrying conductors (hots and neutrals, not grounds) in the conduit:

4-6 conductors = 80%
7-9 = 70%
10-20 = 50%
21-30 = 45%
(hopefully that's enough!)

Then you go to table 310.16 and look at the 90 degree column for ampacity and multiply it by the de-rating percentage. This is the only time you actually use the 90 degree column -- please note these ampacities are for derating calculation only NOT what you can use in regular circuits.

#14 = 25A
#12 = 30A
#10 = 40A
#8 = 55A
#6 = 75A

Finally, if you are dealing with #14, #12 or #10 wire you round down to the maximum allowed breaker sizes of 15A, 20A and 30A respectively. This means that even if your derating calculation shows that #14 could be used with a 20A breaker, you still must only use the 15A breaker.

I'll do a quick example. You have 20 #12 ccc's (ten circuits) in the conduit which gives you a de-rating factor of 50%. In table 310.16, you see the 90 degree ampacity of #12 copper is 30A. Multiplying, you get 30A * 50% = 15A. That means you cannot use that #12 wire in the conduit for more than 15A whereas normally #12 would carry 20A. If you needed those ten circuits to actually be 20A circuits, you would need to upgrade to #10 wire (40A * 50% = 20A) instead of #12 wire.
 
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Old 07-09-09, 11:05 PM
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Electrical Upgrade

Thanks Bob, that helped a lot.

The majority of the ccts are 15amp and therefore according to the chart, I need to run #12. Makes sense. So then I would be able to use the (2) 2" PVC conduits to run all (56) current carrying conductors if I split them up then correct?
 
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Old 07-10-09, 12:36 AM
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If you have 56 current carrying conductors that would mean 28 conductors in each of two conduits. You would need to derate the conductors to 45% of the maximum rating and that means for a 15 ampere circuit you would need to use #10 conductors, not #12.

If you use all three conduits then your conductor count per conduit drops to 18.79 or 19 current carrying conductors. This would allow the derating factor of 50% which ends up as 15 amperes, just right.

Note that IF you have any three-wire circuits (240/120 volt) you do not need to count the neutral wire for that particular circuit as it only carries the unbalanced current between the two "hot" conductors.
 
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Old 07-10-09, 09:04 AM
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Also important to note that few household panels have only 15A circuits. There are usually some 20A and probably a 30A if you have any major electric appliances. Pay close attention to those ones as you may need to upsize one or two wire sizes.
 
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Old 07-10-09, 10:00 AM
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With all the derating required now (and in the future for any new circuits on that end of the house), you may be better served by running multiple 3/4" or 1" conduits instead of 1 or 2 (or 3) large ones since the 2" conduits will never really be 'filled'. It also could make a case for simply running individual NM-B (Romex) cables for each circuit with stack-ems or something similar.

I'd also consider the use of MWBC for certain circuits... there are some downsides, but it can save quite a bit of wire.
 
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Old 07-11-09, 08:35 AM
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Smaller Pipes

That makes a lot of sense with the de-rating now, using several smaller pipes instead of a couple large ones. Thank you to everybody for clarifying this up for me. I appreciate it!!! Beer 4U2
 
 

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