G/B Generator fed MDP feeding multiple buildings

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Sdr96

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We have a freestanding MDP that feeds mulitple buildings on a site, all under single management. Generator fed, no utility service. One of our electricians produced the sketch below and reportedly has used this setup before in similar applications. Ingoring the fact that the TS is not shown, the idea as he explained it was to take only a 3-wire feeder from the MDP to each out building and treating each outbuilding as and "SDS". At first, It just looked a bit odd. The more I look at it, the more I think it's just flat out wrong.

1. N-G bond in genset and MDP would create a parallel path.
2. There's no bonding between MDP and the out panels. Feeders are are cable and/or nonmetallic conduit.
3. There's a pretty good distance between the MDP and out buildings, which could potentially play havoc with the phase-N and phase-G voltage level at the out buildings.

Seems like, at the very least, it violates 250.32 B requiring an EGC with the supply conductors. I would take a 5-wire feeder (ABC, N, EGC) from MDP to out buildings and connect EGC to GES (rod, water pipe, building steel, concrete-encased in slab) without a N-G bond. Am I nuts?

4wirebondedDrawing39.jpg
 
I would start with this.

200.3 Connection to Grounded System.
Premises wiring shall not be electrically connected to a supply system unless the latter contains, for any grounded conductor of the interior system, a corresponding conductor that is grounded. For the purpose of this section, electrically connected shall mean connected so as to be capable of carrying current, as distinguished from connection through electromagnetic induction.
Click to expand...

I would doubt the intent was to allow this connection to be made through the earth.
 
Sdr96 said:
Generator fed, no utility service.

1. N-G bond in genset and MDP would create a parallel path.
Click to expand...

Article 250.30(A)1 exception # 2 allows a bonding jumper both at the source and the fist OCPD providing there are no metallic paths between equipment.
SDr said:
2. There's no bonding between MDP and the out panels. Feeders are are cable and/or nonmetallic conduit.
Click to expand...
Thats a problem. article 250.4(A)3 and 4 requires an effective ground fault return path.
SDr said:
Seems like, at the very least, it violates 250.32 B requiring an EGC with the supply conductors. I would take a 5-wire feeder (ABC, N, EGC) from MDP to out buildings and connect EGC to GES (rod, water pipe, building steel, concrete-encased in slab) without a N-G bond. Am I nuts?
Click to expand...
Yes it does violate 250.32(B).
icon14.gif
but I can't say if your nuts or not.
icon7.gif
 
sdr,

This place sounds familiar :)

IMO, the conductors from gen to MDP are considered feeders, so 5 wire gen-mdp, remove

N/EGC at mdp.

Is sdp a 3ph-3wire or 3ph-4wire ? If it is 3/3 the feeder needs to be 3 hots/1egc or if it is

3/4 the feeder needs to be 3hots/ 1N/ 1/egc. Also if 3/3 remove the N bar in sdp.

The ground rods are ok
 
benaround said:
sdr,

This place sounds familiar :)
Click to expand...

I wonder why. ;). Actually, this was part of an attempt to standardize an installation and quit letting others reinvent the wheel each time we hit a new site.

benaround said:
IMO, the conductors from gen to MDP are considered feeders, so 5 wire gen-mdp, remove N/EGC at mdp. Is sdp a 3ph-3wire or 3ph-4wire ? If it is 3/3 the feeder needs to be 3 hots/1egc or if it is 3/4 the feeder needs to be 3hots/ 1N/ 1/egc. Also if 3/3 remove the N bar in sdp.

The ground rods are ok
Click to expand...

That's what I was thinking. Either it works, or we're both nuts.
 
benaround said:
sdr,

This place sounds familiar :)

IMO, the conductors from gen to MDP are considered feeders, so 5 wire gen-mdp, remove

N/EGC at mdp.

Is sdp a 3ph-3wire or 3ph-4wire ? If it is 3/3 the feeder needs to be 3 hots/1egc or if it is

3/4 the feeder needs to be 3hots/ 1N/ 1/egc. Also if 3/3 remove the N bar in sdp.

The ground rods are ok
Click to expand...
The N/G bonding jumper needs to be removed in both the MDP and the SDP (unless applying the 250.30(A)(1) Exception No. 2 mention by RU).

I don't believe the N bar is required to be removed when not used.
 
Smart $ said:
Why would you need a transfer switch if the gennie is the only source for on-site power?
Click to expand...

There's two generators, similar to the other thread. This was more of what happens past the MDP and how we'd feed multiple buildings on the site. .
 
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