Sub panel from panel fed transformer question.

[dftmaint]

Maintenance guy in factory here just trying to learn.

We have a 480v panel that supplies a 25kva transformer. Single phase 480v primary, 240/120 secondary. The neutral of the transformer is not bonded to the case or ground. There are two panels getting power from this transformer, one has a main breaker (100A) and the other, a GE load center has a backfed (60A) breaker. Both of these panels have the grounds and neutrals together and the panel with the main has the neutral/ground bar still in it. Both panels are for additional office space.

I know if this were my house and i was installing a sub panel this would be wrong. Remove the ground/neutral bar as to not provide a parallel path. Is this ok or not?

Thanks

 

[charlie b]

It sounds OK to me. Downstream of the utility service or downstream of a transformer, the neutral and ground must be bonded at only one location. But that location can be where the first overcurrent device is installed, or at the transformer, or anywhere in between. A transformer is allowed to serve more than one panel in the manner you have described. Each of those panels has to have an N-G bond. If you don't do that at the transformer, then you have to do it at both of the panels.

Welcome to the forum.

 

[kwired]

I think you would need to do the bonding at the transformer, if two secondary overcurrent devices is even allowed- I don't think it is with your arrangement. You have 160 amps of secondary protection on a secondary only rated at 104 amps - you could have up to 125% which would be 130 amps.

See note 2 to table 450.3(B). Multiwire secondaries other then a 3 wire delta require secondary protection. A two wire secondary doesn't require secondary protection either. Both instances must still have secondary protected by primary device and take primary to secondary voltage ratio into consideration when selecting this device.

 

[charlie b]

I should have checked the math. Kwired is right about the 160 total being too much. You could go as high as 150 amps (the next higher standard rating above the 130 amps that represents 125% of the rated secondary current). But I still believe that supplying two panels is acceptable, and that the N-G bond can take place at each panel. But you need to reduce one of the panels' main breaker (or backfeed breaker), so that they add up to no more than 150.

 

[ActionDave]

I know if this were my house and i was installing a sub panel this would be wrong. Remove the ground/neutral bar as to not provide a parallel path. Is this ok or not?

Thanks

I agree with charlie b. You don't have a sub panel like you have in your house, you have a Separately Derived System. Look at 250.30 and if you have access to an NEC handbook there are some good illustrations and there might be some at mikeholt.com too.