Neutral/Ground bonding for Gen/ATS

[maghazadeh]

I have a code question, please help me with your design and installation
experience.

In the past, most of transfer switches were 3-pole. In a 4-wire system,
there will be three neutral wires present at the ATS, one from normal
source, one from generator and one from load side, how do you connect
these three neutral wires? Do you solidly connect them together? If yes,
when we are connected to normal source there will be a live neutral
going back to off-line generator which is bonded to ground at there.
This means that we have neutral/ground bonding at two different
locations, isn't it code violation? Will it send wrong signal to ground
fault sensor at the normal source service point to cause tripping?

Thanks,

 

[steve66]

Normally, with a 3 pole ATS, you connect all three neutrals together at the ATS, and then you have to remove the neutral to ground bond at the Generator.

If you can't remove the N-G bond at the generator, then you should use a 4 pole ATS and swtich the neutral with the phase conductors in the ATS.

Steve

 

[ceb58]

I have a code question, please help me with your design and installation
experience.

In the past, most of transfer switches were 3-pole. In a 4-wire system,
there will be three neutral wires present at the ATS, one from normal
source, one from generator and one from load side, how do you connect
these three neutral wires? Do you solidly connect them together? If yes,
when we are connected to normal source there will be a live neutral
going back to off-line generator which is bonded to ground at there.
This means that we have neutral/ground bonding at two different
locations, isn't it code violation? Will it send wrong signal to ground
fault sensor at the normal source service point to cause tripping?

Thanks,

What you have is a non-separately derived system. The neutrials would be tied together and isolated from the ATS can. As Steve said the N-G must be separated at the gen. you would be creating a parallel neutral. As far as returning current on the gen. neutral under normal utility you would not. The current wants to go back to the source which would be the utility supply.
But you have mentioned GFP. If your system has GFP on the service then it should be installed as a separately derived system. The utility-gen. neutral should be switched. And the gen. N-G would be bonded in the gen.