Sharing Eqipment Grounding conductor

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dmanda24

dmanda24

Senior Member
here is the scenario:
- Plans call for 3#6 for each 60 A AC unit
- Ac units are located about 150 feet from main panel
- Plans call for the circuits to be run in 3/4" emt

From table 250.122 I see that I must use at least a #10 as equipment grounding conductor.

Questions:

1) Must I use 3/4" emt, or is that a minimum and I can use 1" emt and run both circuits in one conduit, in order to save material and time?

2) If I am allowed to use the 1" or in the case that the plans did not specify the 3/4" emt, do I have to run 2 #10 for the equipment grounding conductor, one for each circuit, or can i just run one #10 and use it for both circuits?

I do not know what the code says about this, but in my opinion it should be fine since there is no current on the equipment grounding conductor, and if one AC fails the equipment grounding conductor will clear the fault, open the breaker, and be ready in case the second one fails, the only picture I can see where it could be troublesome is if they both fail at the same time which is highly unlikely, unless of course lightning but if that happens I don't think separate equipment grounding conductors would make much of a difference in the end result.

I appreciate your help, please include code references. Thanks a lot.
 
480sparky

480sparky

Senior Member
Location
Iowegia
dmanda24 said:
1) Must I use 3/4" emt, or is that a minimum and I can use 1" emt and run both circuits in one conduit, in order to save material and time?
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If the plans call for 3/4" EMT runs, you may need to obtain a variance from the engineer/designer. This may be a spec that cannot be changed without authorization. As far as the NEC is concerned, you can go to 1", but check to make sure derating doesn't affect your conductor ampacity as required by the units' nameplates. Table 310.15(B)(2)(a). You may end up needing to up the wire size a notch, forcing you to a larger grounding conductor. 250.122(B).
dmanda24 said:
2) If I am allowed to use the 1" or in the case that the plans did not specify the 3/4" emt, do I have to run 2 #10 for the equipment grounding conductor, one for each circuit, or can i just run one #10 and use it for both circuits?
Click to expand...
You won't need separate grounds. One #10 will do for both. The odd of both units shorting to ground before one breaker opens is mathematically close to impossible to calculate.
 
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roger

roger

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Read 250.122(A) before trying to use the Table.

Roger
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
My 1/50th of a $:

1. The NEC sets minimum requirements. You can always upsize a conduit, and it's done for various reasons. But, you might have to go even larger for six conductors, as they may have to be upsized in order to still be rated to carry the load.

2. As you surmised, and others confirmed, a single EGC sized for the largest circuit sharing it will suffice, and as also said above, if not specified, the conduit can eliminate the requirement for any separate EGC conductor.


You basically have to start with the desired ampacity, and see what size conductor still qualifies after derating, and then size the conduit for the results. Two conduits can sometimes be cheaper than a single shared one, especially if it means larger conductors.
 
dmanda24

dmanda24

Senior Member
ampacity of number 6 = 75 from 90 degree column table 310.16
(I was told to use the 90 degree column when derrating and applying correction factors, is it true?)
two 3 phase 60 amp ac--> 6 conductors
from table 310.15b2a 80 % of the ampacity
so 75*.8 amp = 60 amp so ampacity is ok

2) plans do not call for a separate EGC so use emt to disconnect and from disconnect run a #10 EGC from the case of the disconnect to AC unit


Is that correct?
 
480sparky

480sparky

Senior Member
Location
Iowegia
dmanda24 said:
ampacity of number 6 = 75 from 90 degree column table 310.16
(I was told to use the 90 degree column when derrating and applying correction factors, is it true?)
two 3 phase 60 amp ac--> 6 conductors
from table 310.15b2a 80 % of the ampacity
so 75*.8 amp = 60 amp so ampacity is ok
Click to expand...

Is 60amps equal or greater than the minimum circuit ampacity listed on the nameplate?

dmanda24 said:
2) plans do not call for a separate EGC so use emt to disconnect and from disconnect run a #10 EGC from the case of the disconnect to AC unit


Is that correct?
Click to expand...

Yes
 
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