3 phase generator wiring

[quinn77]

ok...generators are one of my weaknesses so bare with me.
get to a job that another company started to finish it up. 3 phase 480 volt wye POCO secondary, thru disconnect, thru metercan, to 3 pole manual transfer switch. transfer switch has a neutral bar iso'd from case, with bonding jumper to case. generator is 125 kva 3 phase 480 wye trailer mounted. the connection compartment has 5 busses. 3 phase conductors, 1 grounded(neutral), and 1 grounding with bonding finger between them. this system is feeding a 480 volt mcc with NO line to neutral load, and in the transfer switch, the conductors are phased like a delta(no neutral, just a #2 ground). ps the neutral conductor from POCO is bonded in disconnect. then my boss explains that from the generator to load of transfer switch that only the 3 phases are needed and that I need to drive another ground rod near the trailer and ground the case...wich in turn is bonded to neutral in compartment. i asked for code reference and he replied "figure it out". really trying to wrap my head around the theory and code application of this install.

thanx
quinn

 

[hurk27]

ok...generators are one of my weaknesses so bare with me.
get to a job that another company started to finish it up. 3 phase 480 volt wye POCO secondary, thru disconnect, thru metercan, to 3 pole manual transfer switch. transfer switch has a neutral bar iso'd from case, with bonding jumper to case. generator is 125 kva 3 phase 480 wye trailer mounted. the connection compartment has 5 busses. 3 phase conductors, 1 grounded(neutral), and 1 grounding with bonding finger between them. this system is feeding a 480 volt mcc with NO line to neutral load, and in the transfer switch, the conductors are phased like a delta(no neutral, just a #2 ground). ps the neutral conductor from POCO is bonded in disconnect. then my boss explains that from the generator to load of transfer switch that only the 3 phases are needed and that I need to drive another ground rod near the trailer and ground the case...wich in turn is bonded to neutral in compartment. i asked for code reference and he replied "figure it out". really trying to wrap my head around the theory and code application of this install.

thanx
quinn

Ok first you have a Separately Derived System so 250.30 will apply because you not switching the neutral conductor, the problem happens when you try to leave the ground to neutral bond in the generator as this will cause a parallel neutral path, second you mention a disconnect ahead of the meter? Is this a non-fused meter disconnect, if so your going to need a service rated disconnect between the utility and the transfer switch, this means it will have over current protection. also this is where you should install the MBJ, which will also be the place to land your GEC's, Since the neutral is not needed, the neutral can also land here and end. but remember you must loose the neutral to ground bond at the generator, and run both a neutral and a grounding conductor (could be metal pipe) to the transfer switch where they will be bonded by the MBJ.

 

[quinn77]

Ok first you have a Separately Derived System so 250.30 will apply because you not switching the neutral conductor, the problem happens when you try to leave the ground to neutral bond in the generator as this will cause a parallel neutral path, second you mention a disconnect ahead of the meter? Is this a non-fused meter disconnect, if so your going to need a service rated disconnect between the utility and the transfer switch, this means it will have over current protection. also this is where you should install the MBJ, which will also be the place to land your GEC's, Since the neutral is not needed, the neutral can also land here and end. but remember you must loose the neutral to ground bond at the generator, and run both a neutral and a grounding conductor (could be metal pipe) to the transfer switch where they will be bonded by the MBJ.

ok...i understand the application of 250.30, but still not sure why i drive another ground rod for generator case that is not tied in to system gec...wouldnt this create a diff in potential. code ref please.

 

[quinn77]

ps...disconnect is fused.

 

[kwired]

Ok first you have a Separately Derived System so 250.30 will apply because you not switching the neutral conductor, the problem happens when you try to leave the ground to neutral bond in the generator as this will cause a parallel neutral path, second you mention a disconnect ahead of the meter? Is this a non-fused meter disconnect, if so your going to need a service rated disconnect between the utility and the transfer switch, this means it will have over current protection. also this is where you should install the MBJ, which will also be the place to land your GEC's, Since the neutral is not needed, the neutral can also land here and end. but remember you must loose the neutral to ground bond at the generator, and run both a neutral and a grounding conductor (could be metal pipe) to the transfer switch where they will be bonded by the MBJ.

If the neutral of the generator is not disconnected by the transfer switch he does not have a separately derived system.

See Separately derived system definition in article 100.

 

[quinn77]

If the neutral of the generator is not disconnected by the transfer switch he does not have a separately derived system.

See Separately derived system definition in article 100.

thats what i was thinking...removing the mbj in generator wouldnt allow fault current ( line to case) to return to source...but my boss told me to only install phase conductors from transfer to gen...then install ground rod and gec to case only...still confused about this.

 

[kwired]

thats what i was thinking...removing the mbj in generator wouldnt allow fault current ( line to case) to return to source...but my boss told me to only install phase conductors from transfer to gen...then install ground rod and gec to case only...still confused about this.

If your load had a neutral you would install a neutral and an equipment ground to the transfer switch, and since you do not have a separately derived system no grounding electrode conductor is required. The neutral to ground bond for the generator is also the neutral to ground bond at the service disconnect.

Since you have no neutral load and no separately derived system I think you can connect the generator XO to the equipment grounding conductor but I am not entirely sure of this. There should be no current on it which is why it needs isolated if there were neutral load.

 

[quinn77]

If your load had a neutral you would install a neutral and an equipment ground to the transfer switch, and since you do not have a separately derived system no grounding electrode conductor is required. The neutral to ground bond for the generator is also the neutral to ground bond at the service disconnect.

Since you have no neutral load and no separately derived system I think you can connect the generator XO to the equipment grounding conductor but I am not entirely sure of this. There should be no current on it which is why it needs isolated if there were neutral load.

if no gec is required...where would ground fault current go if there is no path back to source? assuming generator is source at this point?

 

[hurk27]

Yea kind of got that one backwards, it is because it is a non-SDS, that you have to remove the bond at the generator, because of the parallel path of the neutral and grounding, if you put the main bonding jumper in the service disconnect, or in the transfer switch if it is service rated, then you will have an EGC from the generator, and a neutral that will be bonded together at the service disconnect, that will be your fault current path. along with the EGC to the MDP, you just can not bond at the generator and the service disconnect without switching the neutrals on the transfer switch (SDS), as far as installing a ground rod at the generator, I don't feel it will do anything, the GEC bonded at the service disconnect will already be bonded to the generator frame, and the EGC from the generator will be sized per 250.66, not 250.122.

 

[kwired]

if no gec is required...where would ground fault current go if there is no path back to source? assuming generator is source at this point?

Through the equipment grounding conductor. GEC in general is not for ground faults it is for lightning protection and for "earth" reference point. This has already been established at the service equipment. If it were a separately derived system it would have no connection to the service supplied system and would then need a grounding electrode conductor that is also connected to the service GEC in some way. Just like a separately derived system supplied by a transformer has to be done.

 

[hurk27]

If your load had a neutral you would install a neutral and an equipment ground to the transfer switch, and since you do not have a separately derived system no grounding electrode conductor is required. The neutral to ground bond for the generator is also the neutral to ground bond at the service disconnect.

Since you have no neutral load and no separately derived system I think you can connect the generator XO to the equipment grounding conductor but I am not entirely sure of this. There should be no current on it which is why it needs isolated if there were neutral load.

250.24(B) will still require that the neutral be brought to the service disconnect, in which so will an EGC/GEC that will give the frame of the generator a fault path through the MBJ.

 

[kwired]

250.24(B) will still require that the neutral be brought to the service disconnect, in which so will an EGC/GEC that will give the frame of the generator a fault path through the MBJ.

I see 250.24(B) applying to the grounded service conductor but do not see how it applies to a generator that is not a separately derived system.

Only thing I find so far is 250.35. If the generator is not SDS it is treated no different than a branch circuit or feeder. If there is no neutral load you need no neutral conductor from what I can tell.

 

[Rick Christopherson]

If there is no neutral load, then there is no neutral conductor. Everything downstream from the first bonding point essentially has two grounding conductors and no grounded conductor. It doesn't matter where the wire originates, what color it is, or what you wish to call it, there is no neutral downstream of the bonding location without a neutral load.

It is not necessary to switch the grounded conductor at the transfer switch because there is none. Furthermore, because there is no grounded conductor, this is in fact a separately derived system.

 

[hurk27]

I see 250.24(B) applying to the grounded service conductor but do not see how it applies to a generator that is not a separately derived system.

Only thing I find so far is 250.35. If the generator is not SDS it is treated no different than a branch circuit or feeder. If there is no neutral load you need no neutral conductor from what I can tell.

Well after chasing this down which it can get confusing, 445.13 sends us to 250.30(A) for conductors that will carry fault current, 250.30(A) again requires using table 250.66, which is basically what the 2002, and 2005 required, But 250.35 requires use to use table 250,122 if after the OCPD of the generator, or 250.66 if ahead?? so since a GEC is required which is the bigger of the two, why not just use it, and let it serve as both?

Oh and I'm kind of agreeing on the NEC not requiring the neutral being brought to the service point? not sure where I picked that up, but that was the way I been wiring them? can't see a harm, I just don't see where the X/O of the generator being bonded at the generator would be allowed unless only one 250.66 sized grounding conductor was ran to the service point?

As a design point I like bringing the neutral to the Service point, in case of later wanting 277v lighting, but other then that I don't see the need.

I have done a few 480 volt lift stations where I didn't need a neutral, but I always brought the neutral from both the generator and the utility to the service disconnect, many times just bonding them on the grounding bar, but the generator bond was also always lifted, and a EGC was also ran to this same grounding bar from the grounding bar in the Kohler generator.

 

[jap]

I'm trying to follow this through in my head.
I picture a Service Rated Transfer Switch where the Neutral is not switched and the Neutral in this swich is bonded to the ground.(This is where the Grounding point Originates). Neutral and ground must be kept seperate from here on.
If a phase conductor from the generator was to short out to the generator frame, then the fault current would have to travel through an Equipment Grounding Conductor (Pulled with the Phase Conductos) to the Transfer Switch,,,,, if there was no Neutral conductor pulled from the Generator to the Service Rated Transfer Switch,,,, how would this fault ever open the Generator overcurrent protection device without a return path?

 

[hurk27]

I'm trying to follow this through in my head.
I picture a Service Rated Transfer Switch where the Neutral is not switched and the Neutral in this switch is bonded to the ground.(This is where the Grounding point Originates). Neutral and ground must be kept separate from here on.
If a phase conductor from the generator was to short out to the generator frame, then the fault current would have to travel through an Equipment Grounding Conductor (Pulled with the Phase Conductors) to the Transfer Switch,,,,, if there was no Neutral conductor pulled from the Generator to the Service Rated Transfer Switch,,,, how would this fault ever open the Generator over current protection device without a return path?

in the OP case there is no neutral needed except for a fault current path.
If only a equipment bonding conductor was pulled from the generator (sized to 250.66) then the generator neutral could be bonded in the generator, and there would be no parallel paths, and this would also give you a fault path back to this point from the transfer switch, but if there is a neutral load then there would have to be a neutral and a grounding conductor ran and no bond at the generator can take place. the bond would have to be at the service disconnect or service rated transfer switch.

 

[jap]

The explanation of installing the Neutral to Ground Bonding Jumper in the generator pretty well cleared this up for me.

Thanks,