Neutral Current paths

[Tidus]

Starting off with a simple question, Does the neutral current flow back to source or to ground?

In my situation I have a generator that has multiple main breakers, one feeding an ATS for NEC 700 and one backfeeding the Main switchboard. The neutral and ground are only bonded at the main switchboard. The rest is a 3 pole system with the neutral solidly bonded through the ATS.

Is there an issue with there being "multiple" paths to ground at the main switchboard. One path through the ATS to the generator and through the backfeed to the ground point at the main switchboard. Another path through the ATS to the main switchboard to ground.

 

[wwhitney]

Starting off with a simple question, Does the neutral current flow back to source or to ground?

Current always flows back to the power source. Ground or earth is only part of the equation when one of the circuit conductors is grounded/earthed, as is common with a neutral circuit conductor. If you have a solidly connected neutral everywhere, and there is only one N-G bond anywhere in the wiring system, then there's no grounding/earthing issue, but:

While I only glanced at your diagram, it appears to me you have created a neutral loop. That would result in an NEC 300.3(B) violation. [All conductors of a circuit to be in the same cable/raceway. If neutral current can go two different ways around a loop, then the circuit is effectively split between those two routes.] One general way to deal with this is by collapsing the loop into a tree by routing multiple circuits in the same circuit/conduit with a single shared neutral.

Or if that is not possible, because e.g. Article 700 circuits have to be kept separate from other circuits, then you must use at least one ATS that switches the neutral to avoid any loops. At which point you have to pay more attention to your N-G bond(s) to ensure that in all configurations of all ATSs there is exactly one N-G bond in the system.

Cheers, Wayne

 

[Tidus]

Current always flows back to the power source. Ground or earth is only part of the equation when one of the circuit conductors is grounded/earthed, as is common with a neutral circuit conductor. If you have a solidly connected neutral everywhere, and there is only one N-G bond anywhere in the wiring system, then there's no grounding/earthing issue, but:

While I only glanced at your diagram, it appears to me you have created a neutral loop. That would result in an NEC 300.3(B) violation. [All conductors of a circuit to be in the same cable/raceway. If neutral current can go two different ways around a loop, then the circuit is effectively split between those two routes.] One general way to deal with this is by collapsing the loop into a tree by routing multiple circuits in the same circuit/conduit with a single shared neutral.

Or if that is not possible, because e.g. Article 700 circuits have to be kept separate from other circuits, then you must use at least one ATS that switches the neutral to avoid any loops. At which point you have to pay more attention to your N-G bond(s) to ensure that in all configurations of all ATSs there is exactly one N-G bond in the system.

Cheers, Wayne

So lets say I didn't have the backfeed to the main switchboard from the generator, only the feed to the ATS. The system is a 3-pole system, wouldn't the currently still be spilt between the generator and the switchboard? or are you saying that it would only go back to the generator? Doesn't the neutral get its reference point where its grounded?

Also not entirely sure that NEC 300.3(B) applies, the neutral has to go back in both directions already, is the concern is that there is the backfeed to the generator?

 

[wwhitney]

So lets say I didn't have the backfeed to the main switchboard from the generator, only the feed to the ATS. The system is a 3-pole system, wouldn't the currently still be spilt between the generator and the switchboard?

If you have an ATS with solid neutral with two sources of supply, a generator and a utility connection, and with only one N-G bond in the system (at the utility service disconnect, no N-G bond at the generator), then all is proper. When the load is on utility power, all neutral current flows back to the utility; the neutral connection to the generator is a "dead end" that doesn't lead back to the source. And when the load is on generator power, vice versa (except that the utility N-G bond is still part of the fault current path in the event of an L-G fault).

Now if the generator also had an N-G bond, that would be a problem, as the 2 N-G bonds would create a neutral loop where a portion of the loop is through the grounding conductor system. So for either state of the ATS, all neutral current from the ATS loads would divide between the two possible routes, one just on the neutral conductor and the other the other way around the loop, partially through the grounding conductor.

Doesn't the neutral get its reference point where its grounded?

Connecting the grounding conductor (EGC) to the neutral allows the grounding conductor to act as a fault current path and trip a breaker when you have an L-G fault. Connecting the earth to the neutral will stabilize the neutral's voltage relative to earth. But a conductive body (either earth or the non-current-carrying bonded metal parts connected to and including the grounding conductor) won't affect current flow paths when the circuit conductors have only a single connection to that conductive body.

Cheers, Wayne