Single backup generator for two services

[fusedneutral]

Client has two buildings, 120/240 service (one 200A, one 400A), each with a separate meter, fed from the same on-premise pad pig, building grounds bonded at the main breaker service panels in each building. Client wants to put in a single 10K standby generator for emergency power in both buildings. 70ft between generator & each building. Control wiring for two transfer switches to one generator is a solved problem, no questions there.

If we use two pole transfer switches and connect all the neutrals together, that's parallel neutrals (generator neutrals in parallel with service neutrals). Not something we want to do.

If we use three pole transfers and a separately derived ground at the genny, then the ground wires between the genny and subpanels (also connected to the main breaker panels) become a parallel path to the service neutral because they are bonded to the service neutrals at the main panels. Really not something we want to do.

My initial thought is to use one two-pole transfer and one three pole transfer switch. This way all the neutrals are a single circut with hard connections (assuming they run to the same neutral bar on the pad pig), but the three pole transfer breaks the parallel neutral loop. The generator stays nonseparately derived, with ground conductor run to the two-pole building, no loops. So far so good. My question is about the fault current from the other building. In the building with the three pole transfer, the fault current path back to the genny (ground still bonded to the service panel) is now via the service neutrals, pad-pig buss bar, and panel/transfer in the other building to get to the genny.

Is this even reasonable? Are warning labels on the transfer switches & generator disconnect sufficient? Done many searches, but haven't found anything that quite scratches this itch.

 

[tom baker]

A 10K generator for 2 buildings with ATS, gennny is required to carry entire connected load of both buildings.
would that change design ?

 

[fusedneutral]

Intent is to only transfer a subpanel of critical circuits in each building. Each building is on a separate meter with a separate feeder from the pad pig. Perhaps I should have said "emergency generator", not "standby".

 

[tom baker]

Ok.
Are you aware of restrictions on Art 700 emergency wiring? could you install one transfer switch for both buildings?

 

[fusedneutral]

Would 702 (optional standby) apply? This is residential backup power (refrigerators, smoke detectors, well pump, selected general lighting, internet/phone).

With two separate meters for the main house & accessory building, I'm not sure how a single transfer switch would work. I imagine that would mean moving accessory building loads to the backup subpanel in the main house (or vice versa) to use a single transfer switch. Moving the loads to the main house defeats the purpose of having separate metering, and the client won't agree. Client won't agree to combine both buildings on one service either.

NEC 250 requires an effective ground-fault current path but seems a bit vague on implementation. In my initial solution: For the house with a two pole transfer switch, the fault current path for backup loads (e.g. refrigerator) is GEC to main panel, bonding screw to house main panel neutral, to generator neutral, wiring to ATS neutral bar, to generator. Normal installation, normal operation. In the accessory building with three pole ATS and switched neutral, the fault current path for backup loads is GEC to accessory bldg main panel, bonding screw to accessory building main panel neutral, accessory bldg service neutral to pad pig X0 terminal, house service neutral to house main panel, wiring to house ATS neutral bar, to generator. It's a longer path with many hops. The ampacity of the path (vai service neutrals) is sufficient to carry any fault currents the generator can produce. It looks like it meets NEC 250, but it seems like a wonky solution.

 

[fusedneutral]

This thread here at least talks about fault current with a generator feeding two buildings with switched neutrals (to avoid parallel neutrals), but the discussion breaks down when they realize switched neutrals breaks the fault current path.

Genny Genny who can I turn to

Would it be possible to run two independant services off one generator? The customer has two houses on his property each with its own 200amp service. My thought was to build a pedestal with the two meter sockets and two service entrance rated ATS's near the generator.Would there be code issues...

forums.mikeholt.com

My idea is to only switch the neutral in one of the buildings, preserving a fault current path through the service/feeder neutrals (grounded conductors)/unswitched building...and preventing parallel neutrals.

 

[fusedneutral]

If we do a separately derived system, bond at the generator, and run both building GEC to the generator, that still forms a parallel neutral path to the existing service neutrals during normal operation because the grounds are also bonded to the main panels. Perhaps it doesn't matter. In some cases it's permissible to tap multiple disconnecting panel grounds (each of which are bonded to neutral) off a single grounding electrode...which looks exactly like a parallel neutral via the grounding taps.

I want to keep the clients safe but without having to propose something massive like rebuilding their service (which might cause them to go with a cheaper lousy solution). It also answers the question for every other "two meters one generator" threads here.

 

[EC Dan]

This thread here at least talks about fault current with a generator feeding two buildings with switched neutrals (to avoid parallel neutrals), but the discussion breaks down when they realize switched neutrals breaks the fault current path.

Why would the fault current path be broken with switched neutrals? Unless I'm missing something, the entire system will be bonded together, so fault current will either return to the generator source or the service source via the EGC pathway depending on which source is switched active and providing the current.