Installation of non-separately derived standby generator

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t_van

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I'm designing a new emergency 3 phase generator for an existing station (please see attached schematic, clouded area). The existing ATS is 3-pole without switched neutral. According to the NEC, this configuration is deemed a non-separately derived system. I've done some studying on grounding of standby power systems but an still questioning my installation.

I have two questions:


1. If the utility has a supply-side bonding jumper, do I need to run the generator neutral or can I simply tie the generator EGC to the ATS EGC as shown in the schematic? In this scenario, it appears the fault current for a line to ground fault would only return to the utility neutral. On the other hand, bonding the generator neutral to the utility neutral in the ATS would allow fault current for a line to ground fault to return back to the generator.

2. Because the installation is not a separately derived system the NEC doesn't require a GEC. However, our utility standard calls for the generator to have a supplementary GEC. Is it better practice to install a separate grounding electrode system for the generator OR connect the generator GEC to the existing electrical enclosure grounding electrode system?

Thanks for the comments.
 
Your feeder from the gen needs to have a neutral run with the the feeder supply

bond the utility required electrode at the gen location to the equipment ground not the neutral
 
Need the neutral in the system from the ATS to the service also or there is no path for fault current while on generator.

The fault current path for the non-separately derived system is through the service main bonding jumper...hence the need for the warning sign under 702.7(B)
 
Since there is no neutral loads, you are ok with no N-G bond at the gen and no neutral from the gen to the ATS

Do you not have a breaker or fused switch on the generator output before the ATS?
 
ron said:
Since there is no neutral loads, you are ok with no N-G bond at the gen and no neutral from the gen to the ATS

Do you not have a breaker or fused switch on the generator output before the ATS?
Click to expand...

Generator feeders will terminate on manufacturer provided circuit breaker at generator and at ATS.
 
ron said:
If the generator is the source during the fault, fault current goes through the EGC to the utility N-G bond, through the solid neutral at the ATS and back to its source at the generator neutral.
Click to expand...


But in post #4 you said that the neutral is not needed? Maybe I don't understand your statement in post #4
 
if you do not bond the util and gen neut when on gen you have an ungrounded wye
looks ok except you need to bond gen neut to util neut in the xfer sw
 
david said:
Your feeder from the gen needs to have a neutral run with the the feeder supply

bond the utility required electrode at the gen location to the equipment ground not the neutral
Click to expand...

That is not correct. There aren't any line to neutral loads so a neutral from the generator is not necessary. It's like a subpanei with no neutral.
 
shortcircuit2 said:
But in post #4 you said that the neutral is not needed? Maybe I don't understand your statement in post #4
Click to expand...

The generator's Equipment Grounding Conductor is the fault path back to the main bond. The neutral would be a normal current path. There are neutral loads so you don't need a neutral. It's just like a subpanei with no neutral.
 
ron said:
Since there is no neutral loads, you are ok with no N-G bond at the gen and no neutral from the gen to the ATS

Do you not have a breaker or fused switch on the generator output before the ATS?
Click to expand...

I didn't catch that, so is it being said that the gen supply to the 480/277 service is going to be un-grounded supplied system and he is going to provide a ground fault detection warning system
 
ron said:
If the generator is the source during the fault, fault current goes through the EGC to the utility N-G bond, through the solid neutral at the ATS and back to its source at the generator neutral.

I'll try to find a picture, but most will show 4W loads.
Click to expand...

than he would have to bond the neutral at the gen
 
if you do neither of these
bond util and gen neut
bond gen neut to gen gnd
you will have an ungrounded system and no fault path to the gen X0/neut

I would just bond util to gen neut in the xfer switch
 
Neutral or no Neutral...

Neutral or no Neutral...

Good conversation. Seems the jury is still out on this one. I have no neutral loads. Should I run Generator Neutral to ATS and bond with Utility Neutral? Or is the generator EGC sufficient?
 
Bonding the generator ground to neutral at the gen-set would be the best IMO.

Shortest direct path for fault current to the separately derived source. Need GES at Gen-set.

Appears to be a 450-amp breaker at the gen-set and a 400-amp at the service?
 
shortcircuit2 said:
Bonding the generator ground to neutral at the gen-set would be the best IMO.

Shortest direct path for fault current to the separately derived source. Need GES at Gen-set.

Appears to be a 450-amp breaker at the gen-set and a 400-amp at the service?
Click to expand...

it was stated the transfer switch is three pole not four pole just want to make sure that is clear and the thread title is a non separately derived system
 
t_van said:
Good conversation. Seems the jury is still out on this one. I have no neutral loads. Should I run Generator Neutral to ATS and bond with Utility Neutral? Or is the generator EGC sufficient?
Click to expand...

Would you tell us more about this design?

What does the generator provide backup power for? Emergency or Optional?

I asked about the 450-amp breaker? How about some feedback.

Thanks!
 
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