Upsizing ground if a single run doesn't need to be upsized but mutiple runs do

Status
Not open for further replies.

aknorth

Member
Location
United States
Hey everyone thanks for checking this out and for your help. Had a bit of debate at work here's what we have

So one building is being fed from another building for a run of 400' with 400 amps of capacity at 120/208 3 phase accounting for voltage drop to be no more than 3%, and copper conductors. So the equipment grounding would apply in this case There are two scenarios that would work

1) 3 - 600 kcmil H 1 - 600 kcmil N and 1 - #3 ground

This option did not need to be upsized

2) For economy sake 2 sets of 3 - 300 H 1 - 300 N and 1 - #1/0 ground

This option was upsized from 3 - #4/0 1 - #4/0 and 1 -#3 ground to account for voltage drop and then the ground was increased proportionally

Here's where the problem came in. On one side my colleague says because we didn't have to increase the conductor size for the 600 kcmil even if we use option 2) which would cause us to up size conductors the equipment ground would stay a #3 because the 600 kcmil is your starting pointing and if that didn't need be up-sized then the ground for nay multiple runs would also not need to be up-sized. My position was that you would need to upsize the ground proportionally because you are up-sizing your ungrounded conductors, if you went with option 2. We agree that having to up-size the ground for option two would probably be more ground than the system needs. However it is my position that's what the code requires. What are your thought and opinions?

Thanks
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Is your colleague saying that a pair of 300's is the same as a single 600? Is he (or she) also saying that because a 600 ungrounded would work with a #3 EGC, one may infer that a pair of 300's would also work with a #3 EGC? If so, I disagree with both assertions. The first is wrong because although the cross sectional area of a single 600 is the same as a pair of 300's, the ampacity of a 600 is far less than the ampacity of a pair of 300's. The rules are not all about areas, they focus mainly on ampacity. The second is wrong for the reason you cited: that the code is not written that way. I also suspect that a #3 EGC would be sufficient to trip the supply breaker on a fault, and that upsizing to 1/0 or beyond is probably not necessary from a technical perspective. But the code is what the code is, and the code would not allow a #3 for option 2.

Let me suggest, however, that for option 2 you are not upsizing from a pair of 4/0's to a pair of 300's, but rather from a pair of 3/0's to a pair of 300's. I didn't do the math to see of your 1/0 EGC is the right size.

Welcome to the forum.
 

aknorth

Member
Location
United States
Charlie thank you for the input. I believe his reasoning is the second. He's saying because with a single set of 600 we don't need to up size for voltage drop therefore a #3 ground is sufficient, if we choose to do multiple sets a #3 would still be sufficient because it was good enough with the one set of 600. The only time we would need to up-size is if a 600 wasn't sufficient and it had to be up sized.
 
Status
Not open for further replies.
Top