EGC size?

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  #1  
Old 04-16-05, 11:48 AM
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EGC size?

Hi,

I wanted to run 16-18 circuits through 10 - 15 feet of 1-1/2" conduit from the main panel to a junction/pull box. All of the circuits will be 15 or 20 amp. I wanted to only pull 1 equipment grounding conductor. What size should it be? I was thinking of using one 12 AGW green wire for all of the circuits.

I plan on pulling 15 10agw and 9-12 12 agw THHN wires plus the ground. After this junction box I will be using 12 and 14 AGW NM-b. Can I install a ground bus bar in the junction box or should I wire nut/ crimp the EGC together?
 
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  #2  
Old 04-16-05, 12:21 PM
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Because of ampacity derating rules, you almost certainly do not want to run that many wires through the same conduit. But in any case, the grounding wire should be the same size of the largest of the current-carrying conductors.

You are now probably going to ask about the ampacity derating rules. I suggest you read up on them in your several home wiring books, but the bottom line is that you are going to want to limit yourself to no more than 9 current-carrying conductors in any one conduit. There's a lot of math behind this, and we can get into it if you really want to.

So the solution is to run multiple parallel conduits rather than one large one.
 
  #3  
Old 04-16-05, 02:48 PM
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John

Thanks for the reply. I was planning on derating 50%. That is why I was going to use 12 AGW for 15 amp and 10 AGW for 20 amp circuits. I will be running 16-18 hot conductors and 8-9 neutrals with the ground. Am I missing something.

In addition, I don't know if the code allows for any diversity. The panel only has a 100 main. I may be cheating without diversity because my plans do not adjust for temperature and a couple feet of the conduit will be on an exterior wall that could go up to 100° F ambient.

Running two smaller conduits with 8 circuits each would be a real PIA. The cost difference is almost a wash.

Thanks again.
 
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Old 04-16-05, 02:57 PM
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You have twice as many hots as neutrals! Are these all multiwire circuits?
 
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Old 04-16-05, 03:10 PM
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Fred,

Yes - all multiwire. The reaon for all of the multiwire circuits is to reduce the current carrying conductors used for derating. Without multiwire circuits, I would need to go up two wire sizes and then I would run into other problems and I might as well put in a sub-panel in an easier to wire location. Right now, the panel is on an exterior stucco wall with the tub enclosure on the other side of the wall and the attic is virtually inaccessable above the tub.
 
  #6  
Old 04-16-05, 03:30 PM
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You're okay if you up the wire sizes as you stated. It sounds like you've already done your homework.
 
  #7  
Old 04-16-05, 06:38 PM
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It looks like you've done your homework on the derating. The point that you make about a 100F ambient is important, and often ignored. You should ask your inspector what temperature derating factors should be used in your location. If the ambient will only hit 100F now and then, then you likely do not need to derate, but this will be the inspectors call. If you need to derate for ambient temperature, then you will need to use 10ga for your 15A circuits, and 8ga for your 20A circuits.

You only need one ground wire, sized for the largest circuit, however if your conductors are increased in size for any reason, (voltage drop or derating), then you are required to similarly increase the size of the ground wire. If you use a 10ga wire for a 20A circuit, then you will need to use a 10ga ground wire.

-Jon
 
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