# NEC Transformer Secondary Issues

#### david luchini

##### Moderator
Staff member
with 1/0 ground in each parallel set
The "ground" in each parallel set could be #2 AWG, per 250.102(C)(2)

#### tecorp

##### Member
The "ground" in each parallel set could be #2 AWG, per 250.102(C)(2)
isn't it 2/0 since the "equivalent area for parallel conductors" is 500MCM + 500MCM? or am i understanding this wrong

this is copper btw

##### Senior Member
But that is something you may want to ask the AHJ since this issue is something they will be paying close attention to.
No, I would just ask him if the "minimum" wire size is enough. I wouldn't open another can of worms asking about non-linear loads.

#### jim dungar

##### Moderator
Staff member
yup. i think we're just gonna do this.

so with 500A mcb ahead, we must have wire sized for 625A (500A/0.8) since we have 4 current carrying conductors.

that means we'll need 2 parallel sets of 400MCM wire.

using 250.66 we'll need 2/0 ground on each set of the feeder.

You do not need to size your conductors any larger than the protective device.

#### david luchini

##### Moderator
Staff member
isn't it 2/0 since the "equivalent area for parallel conductors" is 500MCM + 500MCM? or am i understanding this wrong

this is copper btw
It's #2 because the size of the conductors in each conduit is 250mcm.

(The equivalent area for parallel conductors is 250+250, not 500+500)

#### Tulsa Electrician

##### Senior Member
yup. i think we're just gonna do this.

so with 500A mcb ahead, we must have wire sized for 625A (500A/0.8) since we have 4 current carrying conductors.

that means we'll need 2 parallel sets of 400MCM wire.

using 250.66 we'll need 2/0 ground on each set of the feeder.

This would be you SSBJ.
Size it according.
The "ground" in each parallel set could be #2 AWG, per 250.102(C)(2)
Understanding the correct verbage will help with correctly sizing that conductor.

Do not use 250.66 for an SSBJ
(Supply Side Bonding Jumper).
yup. i think we're just gonna do this.

so with 500A mcb ahead, we must have wire sized for 625A (500A/0.8) since we have 4 current carrying conductors.

that means we'll need 2 parallel sets of 400MCM wire.

using 250.66 we'll need 2/0 ground on each set of the feeder.

Since you going with two sets and have read 250.102 (C) (2).
You would size your SSBJ for each based on the ungrounded wire size using table 250.102 (C) (1).
Yea I was leaning on the heavy side since I don't want to piss off the inspector anymore. But yes most loads are nonlinear.

So then I'll go with 2 sets of 250MCM wire with 1/0 ground in each parallel set
Over 3/0 through 350 cu
Is a #2 cu.
If in two raceways.

If a Single race way then it would be sized for 500 (250*2) Mcm cu and the table says 1/0 CU.

This is where terminology helps. Sizing for an EGC will differ for parrelle conductors in separate race ways versus an SSBJ.

So try not to use the word ground when sizing or you will be lead astray. As there is no table for ground(the earth) in artical 250.

#### tecorp

##### Member
This would be you SSBJ.
Size it according.

Understanding the correct verbage will help with correctly sizing that conductor.

Do not use 250.66 for an SSBJ
(Supply Side Bonding Jumper).

Since you going with two sets and have read 250.102 (C) (2).
You would size your SSBJ for each based on the ungrounded wire size using table 250.102 (C) (1).

Over 3/0 through 350 cu
Is a #2 cu.
If in two raceways.

If a Single race way then it would be sized for 500 (250*2) Mcm cu and the table says 1/0 CU.

This is where terminology helps. Sizing for an EGC will differ for parrelle conductors in separate race ways versus an SSBJ.

So try not to use the word ground when sizing or you will be lead astray. As there is no table for ground(the earth) in artical 250.
Thank you very much for the clarification. I understand now. 250.66 is meant for services but the "separately derived system" threw me off.

#### Knightryder12

##### Senior Member
yup. i think we're just gonna do this.

so with 500A mcb ahead, we must have wire sized for 625A (500A/0.8) since we have 4 current carrying conductors.

that means we'll need 2 parallel sets of 400MCM wire.

using 250.66 we'll need 2/0 ground on each set of the feeder.

We usually do (2) sets of 4 #300kcmil cu and 1 #2 cu e.g. in 3"c ea for this scenario.

#### infinity

##### Moderator
Staff member
We usually do (2) sets of 4 #300kcmil cu and 1 #2 cu e.g. in 3"c ea for this scenario.
Two sets of 300's is only 570 amps so that is too small if he needs 625 amps. Parallel 350's at 310 amps each (310+310=620) would also be too small so he's correct in saying that he needs 400's. There are no equipment grounds run with the secondary conductors. The conductors run in each parallel raceway would be SSBJ's.

#### tecorp

##### Member
Two sets of 300's is only 570 amps so that is too small if he needs 625 amps. Parallel 350's at 310 amps each (310+310=620) would also be too small so he's correct in saying that he needs 400's. There are no equipment grounds run with the secondary conductors. The conductors run in each parallel raceway would be SSBJ's.
I went with 2 sets of 250s since the neutral is not considered a current carrying conductor.

#### infinity

##### Moderator
Staff member
I went with 2 sets of 250s since the neutral is not considered a current carrying conductor.
That works. It is extremely rare that the neutral ends up being a CCC on a transformer secondary.