New 30A circuit through existing EMT

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Old 11-08-14, 03:52 PM
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New 30A circuit through existing EMT

Hello, I'm running a new 220V, 30A ckt, and I'd like to do it through existing EMT. I found NEC tables online stating how many conductors are allowed in 3/4 and 1/2" EMT, but theyre all for a single gauge.

If I add the circuit I'd have a short section of 3/4" EMT with seven 12g conductors and five 10g conductors. Then I'd have a longer section of 1/2" EMT with three 12g conductors and two 10g conductors. This includes the new circuit.

The NEC table shows the limit of 16 12g conductors and 10 10g conductors for 3/4" and 9 12g and 5 10g for 1/2".

Finally, its ok to use the existing 12g ground wire thats going to an existing 120V outlet on the new 30A, 220V circuit correct? Only the two hots need to be 10g? The EMT is also grounded.

Thanks
Marshall
 
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Old 11-08-14, 04:16 PM
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It will be a 240 or208 not 220. My rule of thumb is to use the largest size conductor. If I have three #12 and 2 #10 then I treat it as five #10s. Slightly oversize is never wrong. Yes, you usually need only one ground for all the circuits but with the ground sized to the highest amp circuit. A 30 amp circuit requires #10 (Cu).
 
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Old 11-08-14, 04:26 PM
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I agree oversize is never wrong (I work in the medical field -- safety first!). But if I use your rule of thumb I'll have to run new conduit, which I really prefer not to do. And there'll never be a time when all circuits are running at capacity. (Its a woodshop, and i can only use so many tools at one time )

Would it be against code to have 12 total conductors in the 3/4 (seven 12g, five 10g) and 5 total conductors in the 1/2 (three 12g, two 10g)?

I thought I had read somewhere that the ground conductor could be undersized on the 30A circuit? Technically do I need it at all since the EMT is grounded?

Thanks!
 
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Old 11-08-14, 04:30 PM
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Continuous EMT meets code grounding requirements. The rest of your questions I'll leave to the pros.
 
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Old 11-08-14, 06:28 PM
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When calculating wire fill for different sized conductors you need to take the cross-sectional area of all conductors and add them together. Then take take 40% of the cross sectional area of he conduit and see if the conduit area is larger than the conductor area. Since the area of the conductors is dependent upon the insulation type you need to know that before consulting the tables.

In your particular case, ASSUMING type THHN/THWN insulation you are okay. The area of seven #12 and fie #10 THHN conductors is 0.20 inches and the area of 3/4 inch EMT is 0.213.

With the 1/2 inch the numbers are 0.08 and 0.122 for the conductors and conduit respectively.


As for the equipment grounding...yes, a properly connected continuous run of conduit CAN be used for equipment grounding BUT if you add a separate EGC then that separate conductor MUST be sized for the largest circuit. That leave you the option of of either removing the #12 or changing it to a #10. I did not run the calculation for changing it to a #10 so you would have to do that. #12 cross section is 0.0133 and #10 is 0.0211.
 
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Old 11-08-14, 07:30 PM
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The issue you will have is needing to derate the conductors since you have so many in the conduit. This may mean some are undersized for the ampacity.
 
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Old 11-08-14, 09:35 PM
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Good point, PC.

Marshall, for more than three current carrying conductors (you don't count the equipment ground or a neutral in a multi-wire circuit) up to six conductors you derate to 80% of the Ampacity shown in column three of Table 310:16 (if I remember the table number correctly ) For seven through nine conductors derate to 70%. For over nine conductors derate to 50% of column three.

That would mean for the 3/4 inch conduit with a total of 12 current carrying conductors the #12 would have to be protected to no more than15 amperes and the #10 to no more than 20 amperes.

In the 1/2 inch conduit since you have only four current carrying conductors the derating factors are still above the maximum allowable protection of 20 amperes for #12 and 30 amperes for #10 so no problem there.

IF the 3/4 inch conduit is equal to or less than 24 inches in length then no derating is required.
 
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Old 11-10-14, 10:25 AM
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Thanks for the replies all.
 
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