Auto Transfer switch/Generator

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David Rogers

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Lake County, California
I have been asked by a volunteer community group to help with installing a new 800 amp, 120/208v, 3 ph, wye, 3 pole, automatic transfer switch. This was supplied by volunteers donation in the community. The generator is matching size, and voltage. It appears to me that Article 250.11 of the 2014 NEC allows a shared neutral (since the ATS is not four pole) with the serving utility and the output of the generator. It also appears that the switchboard, ATS, and generator are bonded, and grounded at the utility service point, and that is the only ground rod. My question is: Does the neutral of the generator connect to the frame of the generator, or is that supposed to be isolated?

The other place I am not sure about is the conductor size. I plan on using nema 1 gutter to route the new conductors. Can I calculate the current carrying capacity based on conductors in free air?

Thanks for helping our community...........
 
There is only one bond in a non-SDS and it is at the service, everything else is treated like a sub panel, so the neutral is isolated at the generator.

You don't have conductors in free air so there is no reason to size them for that condition.
 
David Rogers said:
I have been asked by a volunteer community group to help with installing a new 800 amp, 120/208v, 3 ph, wye, 3 pole, automatic transfer switch. This was supplied by volunteers donation in the community. The generator is matching size, and voltage. It appears to me that Article 250.11 of the 2014 NEC allows a shared neutral (since the ATS is not four pole) with the serving utility and the output of the generator. It also appears that the switchboard, ATS, and generator are bonded, and grounded at the utility service point, and that is the only ground rod. My question is: Does the neutral of the generator connect to the frame of the generator, or is that supposed to be isolated?

The other place I am not sure about is the conductor size. I plan on using nema 1 gutter to route the new conductors. Can I calculate the current carrying capacity based on conductors in free air?

Thanks for helping our community...........
Click to expand...

Question: Is the generator located inside the building or outside the building? If outside, outdoors, read 250.30 (A) (1) and 250.30 (C). The neutral shall be bonded at the generator. Therefore the ATS neutral shall be switched.
 
There are rules for portable generators (UL, CPSC) which may mandate bonding, but the generator being mounted outside does necessarily not make it portable.

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I agree with the others. Just because the gen set is outdoors does not make it a SDS. That determination is made by the transfer switch (switched neutral or un-switched).
The Sections you quoted address how the installation is to be treated IF {first word of 250.30(C)} the system is a SDS.
No switched neutral = no SDS = No neutral bonding
 
Last sentence.

If the generator is permanently installed outdoors.

250.30(A)(1)
IF the source is located outside the building or structure supplied, a system bonding jumper shall be installed at the grounding electrode connection in compliance with 250.30(C)

250.30(C) Outdoor source. If the source of the separately derived system is located outside the building or structure supplied, a grounding electrode connection shall be made at the source location to one or more grounding electrodes in compliance with 250.50. In addition, the installation shall comply with 250.30(A) for grounded systems or 250.30(B) for ungrouned systems.
 
Jamesco said:
Last sentence.

If the generator is permanently installed outdoors.

250.30(A)(1)
IF the source is located outside the building or structure supplied, a system bonding jumper shall be installed at the grounding electrode connection in compliance with 250.30(C)

250.30(C) Outdoor source. If the source of the separately derived system is located outside the building or structure supplied, a grounding electrode connection shall be made at the source location to one or more grounding electrodes in compliance with 250.50. In addition, the installation shall comply with 250.30(A) for grounded systems or 250.30(B) for ungrouned systems.
Click to expand...

IF it's an SDS. A generator with an unswitched neutral is not an SDS.
 
IMO, the configuration of the generator determines if it is a SDS or not. If the neutral and EGC are connected at the generator it is a SDS, and would require a transfer switch that switches the neutral.
 
If the premises wiring does not require a SDS generator often times its a lot easier to remove the Neutral-Equipment Ground + Bonding Jumper than it is to change the transfer switch.
The two work in conjunction to determine the SDS ststus.
 
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