EMT Capacity

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  #1  
Old 05-16-04, 08:31 AM
Gr Gramps
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EMT Capacity

I am wiring a woodshop using EMT on the surface. When completed it will be inspected here in NC. One of my runs in 3/4" EMT will contain a 20 amp (120 volt) circuit for general purpose receptacles and a 20 amp (240 volt) circuit for the table saw. My concern is the number of current carrying conductors in the conduit. Reading the NEC has been a real challenge for the uninitiated (and has given me great respect for electricians).

My understanding is that I will have only 3 current carrying conductors among the 6 wires in this run. 310.15(4) says the neutral conductor carrying only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of
310.15(B)(2)(a). I interpret this to mean that I will have only 3 current carriers in this run.

Am I on track? Will use 12 awg THHN so I think I'm OK regarding conduit fill?

I appreciate your help.

Roy
 
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  #2  
Old 05-16-04, 10:15 AM
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Join Date: Jan 2004
Location: Oregon
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You will have _4_ current carrying conductors. For the 120V circuit, you have the hot and the neutral return. On the 120V circuit, the neutral must carry _all_ of the current in the hot.

For the 240V circuit, you have two hots, each counted.. If you also have a neutral, then this neutral will carry any unbalanced current between these two hots. It is this sort of neutral which you don't have to count. It is not clear to me that you even need this neutral; if the table saw is 240V, then _zero_ current will flow on the neutral. However it doesn't hurt to run the neutral, and this provides flexibility if you want to connect a 120/240V load.

With _4_ current carrying conductors, you are required to derate. However for purposes of derating, remember that 12ga THHN wire has an amp rating of _30A_. You are required to protect 12ga wire with a 20A (or smaller) circuit breaker, for reasons separate than overload protection. This means that you have some headroom to work with. Because you have 4 conductors, you are required to derate the ampacity of the wire, but the derated ampacity is still greater than 20A, so for 12ga 20A circuits, this derating doesn't have a practical effect until you get to 10 or more current carrying conductors.

6 #12 THHN wires in 3/4 EMT is no problem at all.

-Jon
 
  #3  
Old 05-16-04, 03:23 PM
Gr Gramps
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Jon, your explanation is much appreciated. I approach this job with a bit of apprehension since my work will be judged by the inspector. You have helped to remove some of the pressure. Thanks,

Roy
 
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