I'm having trouble determining if the standby generator should be hooked up as a separately derived system or not and specifying the correct plug and receptacle. The application; mobile 100kw 3 phase 480/277 volt diesel generator set to supply one of three lift stations. These are located at different parts of the city. The concept is to move the genset to where it's most needed, quickly hook it up via plug and receptacles and manual transfer switch. The lift station is normally fed with 480v 3 phase power via overhead transformer bank, no neutral load, a dry type transformer 480/120 is used for 120 volt loads. My intention is to use a 4 pole plug/receptacle, 3 for the phase conductors, 1 for the ground. Is it still considered a "Separately Derived System" even though I'm not using the neutrals? Thanks.
Standby 3 phase Generator
- Thread starter pward
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bphgravity
Senior Member
- Location
- Florida
Yes. See the definition of Separately Derived System in Article 100 of the NEC.
Standby 3 phase Generator
I?m still a bit confused on this issue. Normal utility supply to the lift stations is 3ph 3 wire 480 Vac, no neutral load. The generator is 3 phase 4 wire, wye, and the neutral is not used and currently isolated from the frame/ground. My understanding of article 250.20 (B), for my application, doesn?t require grounding. Is this correct? Also, in reference to article 250.34 (B), Vehicle-Mounted Generators states that the frame shall be permitted to serve as the grounding electrode for system supplied by a generator located on the vehicle under certain conditions. Would a 3 pole 4 wire plug and cord arrangement that connects the phase conductors to a transfer switch, and bonds the generator frame to the transfer switch fulfill the requirements of this article?
I?m still a bit confused on this issue. Normal utility supply to the lift stations is 3ph 3 wire 480 Vac, no neutral load. The generator is 3 phase 4 wire, wye, and the neutral is not used and currently isolated from the frame/ground. My understanding of article 250.20 (B), for my application, doesn?t require grounding. Is this correct? Also, in reference to article 250.34 (B), Vehicle-Mounted Generators states that the frame shall be permitted to serve as the grounding electrode for system supplied by a generator located on the vehicle under certain conditions. Would a 3 pole 4 wire plug and cord arrangement that connects the phase conductors to a transfer switch, and bonds the generator frame to the transfer switch fulfill the requirements of this article?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Forget about _grounding_ for a moment. Ask about _bonding_.
1) Is the electrical system bonded to ground, or do you have an ungrounded system?
2) When operating on generator power will you lose your electrical system bond (if the system is grounded), or will you get an unexpected bond (if the system is supposed to be ungrounded), or will you have _multiple_ bonds between electrical system and ground?
As far as grounding, even if the generator frame does not need a grounding electrode system, the building certainly does.
-Jon
1) Is the electrical system bonded to ground, or do you have an ungrounded system?
2) When operating on generator power will you lose your electrical system bond (if the system is grounded), or will you get an unexpected bond (if the system is supposed to be ungrounded), or will you have _multiple_ bonds between electrical system and ground?
As far as grounding, even if the generator frame does not need a grounding electrode system, the building certainly does.
-Jon
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
All electrons that leave a source, want to go back to that same source.
That said, for the overcurrent devices to trip, the neutral conductor for the utility does not go back to the generator. You need the neutral conductor from the generator to the service disconnect enclosure so that the overcurrent devices will open, especially since this is a SDS. This is regardless of the fact that the loads do not utilize a grounded (neutral) conductor.
That said, for the overcurrent devices to trip, the neutral conductor for the utility does not go back to the generator. You need the neutral conductor from the generator to the service disconnect enclosure so that the overcurrent devices will open, especially since this is a SDS. This is regardless of the fact that the loads do not utilize a grounded (neutral) conductor.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
pward said:
I do not have the necessary training or experience to determine what would be 'better'. I do research on electric motors. The best I can do I point you to questions to ask to evaluate the situation.
The standard solution that you suggest would have the benefit of being easily understood, but the downside of a seemingly 'extra' conductor. The load doesn't need the neutral, after all.
But if you leave off one of these 'extra' conductors, then you need to provide the necessary neutral-ground bond, without creating any sort of parallel path.
Beyond this, I am out of my league.
-Jon
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