Seperately derived system

[joeyww12000]

On my job we are using a 2500a main, I know at the service entrance you must bond neutral conductor to ground. When a transformer is used to step down from 277/480 to 120/208 doesnt the neutral created by the transformer need to be grounded also? The reason I ask is my boss says only bond neutral to ground at the service entrance and thats it, but it was my understanding that at each seperately derived system you must do it also. If I check voltage from neutral to ground on any of my low volt panels I read anywhere from 70 to 100 volts. The system isnt grounded correct? I jumpered a piece of wire from neutral straight to ground and got a little spark, but it grounded the neutral and I read no voltage. Am I correct or is my boss right on this one?

 

[LarryFine]

Am I correct or is my boss right on this one?

You is.

 

[brian john]

Your boss is an electrician?, For a job of this magnitude he really should have this BASIC knowledge.

 

[benaround]

Joey,

250.30 is where you have to go for 'Grounding Seperately Derived AC Systems'.

You need a system bonding jumper from XO to your ground bar in the transf.

sized from 250.66.

You need a GES sized from 250.66 from grounding electrode(s) to the grounded

conductor of the SDS.

Joey, If someone gets hurt from work performed by you, you can be held

liable, even if your 'boss' tells you to do it a certian way. So it's good that

you took the effort to find out what to do in this case. good luck.

 

[tryinghard]

?I know at the service entrance you must bond neutral conductor to ground. When a transformer is used to step down from 277/480 to 120/208 doesnt the neutral created by the transformer need to be grounded also??it was my understanding that at each seperately derived system you must do it also?

Yes either at the transformer or the first disconnect from the transformer but not both places, at the transformer is most common, 250.30(A)(1). The neutral ?XO? bond also determines where the GEC will be terminated they have to be at the same location, 250.30(A)(3).

The bonding of the grounded (neutral) conductor to case frame allows fault current a route to source, without it fault current - dangerous touch voltage - will not have a clear path to source and the circuit will remain on, OCP will not operate. The bonding jumper completes the circuit for fault current.