250.122(B) & (F) - Parallel feeders upsized for voltage drop, re: impact to EGC

[cdcengineer]

I'm upsizing a 225A run which would've been run in (1) set of 300AL w/ #2 AL EGC. The size based on 800' run we are settling on is (3) sets of 330AL. Referring to NEC 250.122(B) & (F), I use table 8 - [(3x350)/300]=3.5
3.5x66360 (table 8 area for #2) = 232,260. I would size each parallel run with a #250kcmil AL EGC. An associate is telling me I need to divide the 232,260 by 3 (# of parallel runs). Result would be 77,420 or #1AL in each parallel run.

Thoughts and input are always greatly appreciated.

 

[skeshesh]

I'm upsizing a 225A run which would've been run in (1) set of 300AL w/ #2 AL EGC. The size based on 800' run we are settling on is (3) sets of 330AL. Referring to NEC 250.122(B) & (F), I use table 8 - [(3x350)/300]=3.5
3.5x66360 (table 8 area for #2) = 232,260. I would size each parallel run with a #250kcmil AL EGC. An associate is telling me I need to divide the 232,260 by 3 (# of parallel runs). Result would be 77,420 or #1AL in each parallel run.

Thoughts and input are always greatly appreciated.

I believe you're correct and your peer is not. Parallel feeders are fed from the same source so I believe EGC in each run should be sized based on the total possible fault current.

 

[Dennis Alwon]

You would not divide by three. You would need one egc in each pipe-- full sized.

 

[Dennis Alwon]

Just so I am clear you need one 250kcm in each parallel run if you need an egc. Of course, with metal conduit no egc is needed.

 

[cdcengineer]

Thanks to all for proving me right.

FYI - We want an EGC since this run is for the EM system. Too far to be comfortable depend on the pipe for EGC.

Thx

 

[Dennis Alwon]

Thanks to all for proving me right.

FYI - We want an EGC since this run is for the EM system. Too far to be comfortable depend on the pipe for EGC.

Thx

If, the big if, the pipe is installed properly it may be a better egc then the conductor, however I totally understand your concern.

 

[cdcengineer]

Thanks for the input Dennis

 

[iwire]

Dennis is correct, in almost all cases the steel conduit provides a lower impedance path than the wire EGC you will pull inside it.

 

[George Stolz]

I'm not trying to pick your statement apart, I'm just learning here: you said "almost". Is there an instance that comes to mind where the wire is lower impedance than the raceway?

 

[Open Neutral]

I'm not trying to pick your statement apart, I'm just learning here: you said "almost". Is there an instance that comes to mind where the wire is lower impedance than the raceway?

When the conduit rusts, perhaps?

 

[bob]

I'm upsizing a 225A run which would've been run in (1) set of 300AL w/ #2 AL EGC. The size based on 800' run we are settling on is (3) sets of 330AL. Referring to NEC 250.122(B) & (F), I use table 8 - [(3x350)/300]=3.5
3.5x66360 (table 8 area for #2) = 232,260. I would size each parallel run with a #250kcmil AL EGC. An associate is telling me I need to divide the 232,260 by 3 (# of parallel runs). Result would be 77,420 or #1AL in each parallel run.

Thoughts and input are always greatly appreciated.

It is my understanding that you could get by with a 225 amp ckt using 1 300 kcm AL. According to the table you would only need a #2 EGC.
Due to distance you have to install 3 # 350 AL per phase and a EGC 250 AL with each circuit.
The ampacity of 3 # 350 AL = 750 amps. If you installed a 750 amp breaker, you would only need a 3/0 EGC with each circuit instead of a 250 kcm EGC.
This really make a lot of sense.

 

[Dennis Alwon]

That's a good point and I have done that myself on a much smaller scale. Now you have to weigh the breaker cost difference vs the wire-- with that distance I suspect the breaker would be cheaper.

 

[bob]

I was not trying to suggest that the op install a 750 amp breaker. I was just pointing out the absurdity of this table in the NEC. I've pointed this out before.
There is no science behind the upgrading of the EGC when phase conductors are parallel. I think the EGC should be chosen based on the circuit breaker listed in the table. That makes more sense to me. If the circuit is long, as in this case, the eng'r could increase the size of the egc if he thought is was needed.

 

[Dennis Alwon]

I was not trying to suggest that the op install a 750 amp breaker. I was just pointing out the absurdity of this table in the NEC. I've pointed this out before.
There is no science behind the upgrading of the EGC when phase conductors are parallel. I think the EGC should be chosen based on the circuit breaker listed in the table. That makes more sense to me. If the circuit is long, as in this case, the eng'r could increase the size of the egc if he thought is was needed.

Apparently under certain conditions there can be an issue over long distance if the egc isn't up sized. I feel the same way as you esp. since using a larger breaker will satisfy the issue. I couldn't understand why a larger breaker didn't have the same problem.

 

[cdcengineer]

Interesting way to look at it. I won't be increasing the breaker, but it does make one think about the cost of this requirement.