Non separately derived system wiring for a Generator in a 3 wire system (No neutral)

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jesusrperezd

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Location
California
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Engineer
Hi all, I need help understanding the proper connection of a generator in an existing system.

Existing system
The existing system has a Service Panel Wye (480V-277V, 3ph, 4w) where the Neutral and ground are bonded. Downstream feeders are all 3W+EGC; there are no 277V loads, so neutral is not required.

Some of these feeders supply the normal source input of 3-pole ATSs.

The existing generators feed a Switchboard, 3phase 3w (no neutral). The feeder coming from the Generator to the SWBD is 3ph + EGC.

Then, the ATSs are supplied from the SWBD (backup source) via 3W+EGC feeders.

Generator and Questions
I understand this is a NON separately derived system due to the 3pole ATSs and the feeders coming from the service and generators which are 3W. is this correct?

The generator has a CB in its cabinet, so the G conductor from the generator to the SWBD is an Equipment Grounding Conductor EGC. Is this correct?

If there is no neutral conductor from the generator connected to the Service neutral, my interpretation is that the system is floating (code violation for 480V?). Is that correct?

What should be the proper way to design the grounding and neutral treatment of the generator?

Is the neutral and ground bond required at the generator? supply side body jumper? GEC?

If the EGC from the generator is connected to the neutral point at the generator, is that a code violation since this is a Non sep. derived system? Note that the ground is connected to the frame.

Thank you so much!
 
Hi, I am still investigating that, but at this stage, we can assume it is 3 wires from a wire connection.

One thing I forgot to mention at the begining is that the Circuit breaker at the new generator output is 1200A rated. It will require ground fault protection per code. Is this going to affect the use of three-pole ATS? The existing system consists of two generator sets with 500A outputs each, so they didn't provide ground fault protection; we are replacing those with two generator sets with 1200A breakers each.
 
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jesusrperezd said:
I understand this is a NON separately derived system due to the 3pole ATSs and the feeders coming from the service and generators which are 3W. is this correct?
Click to expand...
Sounds like an SDS to me. To be a non-SDS, you would need to have a direction connection between one of the service circuit conductors and one of the generator circuit conductors. You have not mentioned any such connection.

Usually the only way you'd have such a connection is if the generator has a neutral conductor which is directly connected to the service grounded conductor. That is a common arrangement when both the service and the generator are wye systems, and so you encounter the idea that 3 pole ATS = non-SDS, 4 pole ATS = SDS. But that idea only applies when the 3 pole ATS is accompanied by a direct connection of the generator neutral to the service grounded conductor.

Cheers, Wayne
 
jesusrperezd said:
Hi all, I need help understanding the proper connection of a generator in an existing system.

Existing system
The existing system has a Service Panel Wye (480V-277V, 3ph, 4w) where the Neutral and ground are bonded. Downstream feeders are all 3W+EGC; there are no 277V loads, so neutral is not required.

Some of these feeders supply the normal source input of 3-pole ATSs.

The existing generators feed a Switchboard, 3phase 3w (no neutral). The feeder coming from the Generator to the SWBD is 3ph + EGC.

Then, the ATSs are supplied from the SWBD (backup source) via 3W+EGC feeders.

Generator and Questions
I understand this is a NON separately derived system due to the 3pole ATSs and the feeders coming from the service and generators which are 3W. is this correct?
Click to expand...
No, not correct as you've described. If there are no neutrals from the two systems connected together, then they are separately derived systems even though the ATS is only 3 pole. See the definition of an SDS.

jesusrperezd said:
The generator has a CB in its cabinet, so the G conductor from the generator to the SWBD is an Equipment Grounding Conductor EGC. Is this correct?
Click to expand...
Yes.

jesusrperezd said:
If there is no neutral conductor from the generator connected to the Service neutral, my interpretation is that the system is floating (code violation for 480V?). Is that correct?
Click to expand...
No, not necessarily. You haven't told us if the generator has system grounding or not. Also strictly speaking it is not a code violation for a 480V system to be grounded, it can be ungrounded if it has ground detectors.

jesusrperezd said:
What should be the proper way to design the grounding and neutral treatment of the generator?

Is the neutral and ground bond required at the generator? supply side body jumper? GEC?

If the EGC from the generator is connected to the neutral point at the generator, is that a code violation since this is a Non sep. derived system? Note that the ground is connected to the frame.
Click to expand...
We need you to answer Don's question definitively to provide further guidance here. Is the generator a wye or a delta? In other words, does it have a neutral terminal at all (regardless of whether anything is connected yet) or does it completely lack a neutral terminal?
 
jesusrperezd said:
Hi, I am still investigating that, but at this stage, we can assume it is 3 wires from a wire connection.

One thing I forgot to mention at the begining is that the Circuit breaker at the new generator output is 1200A rated. It will require ground fault protection per code. Is this going to affect the use of three-pole ATS? The existing system consists of two generator sets with 500A outputs each, so they didn't provide ground fault protection; we are replacing those with two generator sets with 1200A breakers each.
Click to expand...
Assuming that "wire" was meant to say "wye". You need a conductor from the wye point of the generator to the neutrals that supply the circuits that are connected to the line side of the transfer switches to provide a fault clearing path.
 
don_resqcapt19 said:
Assuming that "wire" was meant to say "wye". You need a conductor from the wye point of the generator to the neutrals that supply the circuits that are connected to the line side of the transfer switches to provide a fault clearing path.
Click to expand...
Did you mean "load side" of transfer switches? The OP indicates there are no neutrals in the load side circuits.

I believe the assumed generator neutral has to be run to its first means of disconnect for the SBJ, but then the feeder from that disconnect to the ATS need not have a neutral conductor?

Cheers, Wayne
 
don_resqcapt19 said:
Assuming that "wire" was meant to say "wye". You need a conductor from the wye point of the generator to the neutrals that supply the circuits that are connected to the line side of the transfer switches to provide a fault clearing path.
Click to expand...
Huh? He said there were no neutrals connected to the line side of the of transfer switches. He does not need to run a neutral if the loads do not use one. The EGC can be the fault clearing path.

He said the generator has a CB "in it's cabinet" which to me means he can start his EGC there.
 
jesusrperezd said:
...

If the EGC from the generator is connected to the neutral point at the generator, is that a code violation since this is a Non sep. derived system? Note that the ground is connected to the frame.
Click to expand...
Sorry, I kind of missed this the first time. If there is an EGC connected to the neutral point of the generator, but no neutral connected, that is not a code violation. In fact it is required. The EGC is the fault clearing path for the generator.
 
This is the way we design most buildings. We bond the N-G at the utility and the generator and bring 3Ph 3W downstream. That way we don't need to worry about which way the neutral current or GF current will go.
For GFP at the 1200A gen, just get GFP built into the set mounted gen breaker trip unit.
The very small amount of 277V loads are handled with a 480-480/277V transformer.
 
jesusrperezd said:
Hi, I am still investigating that, but at this stage, we can assume it is 3 wires from a wire connection.

One thing I forgot to mention at the begining is that the Circuit breaker at the new generator output is 1200A rated. It will require ground fault protection per code. Is this going to affect the use of three-pole ATS? The existing system consists of two generator sets with 500A outputs each, so they didn't provide ground fault protection; we are replacing those with two generator sets with 1200A breakers each.
Click to expand...
GFP wouldn't be required if generator is delta wound.
 
Hi all, thanks for your time and responses. I'm sorry for my misspelling and some wrong terms I might have used in the original message.

After reading all the interactions, I would summarize:
  • The system is a Separated Derived system since there is no connection between the service neutral and the generator neutral point.
  • The reason the ATSs are 3 poles instead of 4 is that the load side is 480V and does not require a neutral. Even though the ATSs are 3 poles, it is still an SDS.
  • I'm still investigating the winding connection with the vendor, but let's say it is Wye to be solidly grounded; we can take that as a premise for now.
  • The EGC will be the fault clearing path for the generator source.
  • The EGC will run from the generator's neutral point to the grounding bar of the SWBD and ATS. Should I connect the generator's neutral terminal to a local ground as well, let's say a rod? Is this optional?
  • The bond that connects the Generator frame with the neutral point (I believe the proper name is system body jumper) should remain in place, is that correct?
  • The ground fault protection can be provided by the MCCB with LSIG trip unit at the generator cabinet

I will share a simplified scheme / SLD soon to show our discussion.
 
jesusrperezd said:
...
  • The EGC will run from the generator's neutral point to the grounding bar of the SWBD and ATS. Should I connect the generator's neutral terminal to a local ground as well, let's say a rod? Is this optional?
Click to expand...
Yes, you should ground the system. If the generator is not at the same structure at the service then the generator gets its own electrode (a rod or two, say). If it's at the same structure as the service, then it's supposed to be connected to the same electrode(s) as the service. (Note that connecting the neutral terminals of both systems only via grounding conductors does not change whether they are separately derived systems, per the definition.)

(As I read the code, strictly speaking it's optional to ground such a system, but it would be very unusual not to ground a wye system like this; you'd need ground detectors for an ungrounded system, too.)
jesusrperezd said:
  • The bond that connects the Generator frame with the neutral point (I believe the proper name is system bonding jumper) should remain in place, is that correct?
Click to expand...
Correct.
 
ron said:
Very few off the shelf commercially available gens in the market I work in are delta wound. I would think it will be wye and a N-G bond, so only 3 wire would extend outbound.
Click to expand...
How many of this size or larger are truly "off the shelf"? If anything I could possibly see the alternator being somewhat off the shelf with 12 leads and are easily capable of being configured to meet the needs to finish assembly for final destination to fulfill orders as they come in.
 
Does having a System Body Jumper larger than the Grounding Electrode Conductor make sense?

In the case we have been discussing, the feeder is 3 sets 3x500kcmil + G 2/0AWG (the main breaker is 1200AF/1000AT; I'm selecting the 1000A row in table 250.122 for EGC).

So the EGC per 250.66 will be 3/0. However, SBJ per 250.102 will be 4/0...

It's curious to have multiple sizes:
2/0 for EGC
3/0 for GEC
4/0 for SBJ
 
jesusrperezd said:
Does having a System Body Jumper larger than the Grounding Electrode Conductor make sense?

In the case we have been discussing, the feeder is 3 sets 3x500kcmil + G 2/0AWG (the main breaker is 1200AF/1000AT; I'm selecting the 1000A row in table 250.122 for EGC).

So the EGC per 250.66 will be 3/0. However, SBJ per 250.102 will be 4/0...

It's curious to have multiple sizes:
2/0 for EGC
3/0 for GEC
4/0 for SBJ
Click to expand...

It doesn't particularly make sense but it is what the code requires. But also read the text of 250.66 as well as the table, and note that depending on the electrodes used a 3/0 may not be required.
 
jesusrperezd said:
Does having a System Body Jumper larger than the Grounding Electrode Conductor make sense?

In the case we have been discussing, the feeder is 3 sets 3x500kcmil + G 2/0AWG (the main breaker is 1200AF/1000AT; I'm selecting the 1000A row in table 250.122 for EGC).

So the EGC per 250.66 will be 3/0. However, SBJ per 250.102 will be 4/0...

It's curious to have multiple sizes:
2/0 for EGC
3/0 for GEC
4/0 for SBJ
Click to expand...
Yes, the system bonding jumper is a fault clearing conductor and the GEC is not.
 
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