confusion of neutral bonding

[zyhml]

very clear! thanks. Another question is if there is any restriction on neutral bonding of step down transformer in building. The upstream transformer has neutral bonding according to NEC.

 

[JohnJ0906]

very clear! thanks. Another question is if there is any restriction on neutral bonding of step down transformer in building. The upstream transformer has neutral bonding according to NEC.

A transformer is a seperately derived system. See 250.30.

 

[zyhml]

Got it, thanks! According to NEC, it seems multiple self derived system could be bonding to common GEC, i wonder if it apply when these systems have different voltage level.

 

[tryinghard]

If your transformers don't have any direct connection to supply conductors (inductive), including neutral grounded conductor, then your transformers are considered Separately Derived Systems -- notice Definitions.

The grounding electrodes must all connect to form a grounding electrode system in keeping with 250.58. If you have multiple services (utilities) this is important as Mike Holt noted in one of his newsletters, "current flows on the grounding electrode conductor when multiple service disconnecting means are grounded to the same electrode. This is because neutral current from each service can return to the utility via the common grounding electrode and its conductors." The common electrode connections allow this safely, and the lack of causes the current to wonder dangerously where it does not belong.

Voltage doesn't matter for grounding; each SDS must be grounded (as Defined)

Tapping to a correctly sized grounding electrode conductor is fine, 250.30(A)(4), follow 250.30.(A)(4)a through c in doing so.

 

[zyhml]

got it, thanks!