Generator Bonding

[Fryguy]

I've got a good question for you all.
A 120/240 V, 3 phase generator with a main breaker feeds a MLO panel mounted on the generator. A 3 pole breaker in that panel feeds a 60amp receptacle that feeds an extension cord that hooks up to a main breaker in a panel inside of a Ground Control Shelter. THe Ground control shelter has the only ground rod that is supplied. All of the feeders have 3 hots, a neutral and a ground.
Question: Where should the grounds and neutrals be bonded? The most common answer is nowhere, because the generator is the main panel and all of the other panels are subpanels. But 250.24 (C) says Where an ac system at less than 1000Volts is grounded at any point, the grounded conductor(s) shall be run to each service disconnecting means and shall be bonded to each disconnecting means enclosure.

Am I reading too much into this?
And I know the generator should be grounded too. Its a long story!

 

[hurk27]

Generally the bonding of the grounded conductor can be at any point from the generator to the first means of disconnect for the building, this main point of bonding is also where the grounding electrode conductors must be landed also.

Please see 250.30(A)(1)

also the breakers on the generator might not serve as the disconnect for the building

 

[Lcdrwalker]

Generator Supply

Generator Supply

My number one concern is the generator connection to the existing supply system. From your brief description, it sounds like a small generator used as a standby. If this is the case, the service is already grounded. If this is plugged into the generator ' do not ground or interconnect the N-G leads at the generator. An appropriate transfer switch would have to be installed to isolate the sources although the grounding systems can be common.

 

[kwired]

Is the generator the only supply or is it a standby supply?

 

[zbang]

And one more thought- if there are more than one circuit off the generator (a MLO panel is mentioned), then the generator won't have a MBJ when it's not connected to the building. Is this a fixed or portable/vehicle-mounted genny?

 

[Fryguy]

Its a mobile generator on a tralier and it is the only power source for this shelter. The shelter is not a fixed building.

 

[Smart $]

Its a mobile generator on a tralier and it is the only power source for this shelter. The shelter is not a fixed building.

The generator grounding is covered under...

250.34 Portable and Vehicle-Mounted Generators.

(A) Portable Generators. The frame of a portable generator
shall not be required to be connected to a grounding
electrode as defined in 250.52 for a system supplied by the
generator under the following conditions:
(1) The generator supplies only equipment mounted on the
generator, cord-and-plug-connected equipment through
receptacles mounted on the generator, or both, and
(2) The normally non?current-carrying metal parts of
equipment and the equipment grounding conductor terminals
of the receptacles are connected to the generator
frame.

(B) Vehicle-Mounted Generators. The frame of a vehicle
shall not be required to be connected to a grounding electrode
as defined in 250.52 for a system supplied by a generator
located on this vehicle under the following conditions:
(1) The frame of the generator is bonded to the vehicle
frame, and
(2) The generator supplies only equipment located on the
vehicle or cord-and-plug-connected equipment through
receptacles mounted on the vehicle, or both equipment
located on the vehicle and cord-and-plug-connected
equipment through receptacles mounted on the vehicle
or on the generator, and
(3) The normally non?current-carrying metal parts of
equipment and the equipment grounding conductor terminals
of the receptacles are connected to the generator
frame.

(C) Grounded Conductor Bonding. A system conductor
that is required to be grounded by 250.26 shall be connected
to the generator frame where the generator is a component
of a separately derived system.

FPN: For grounding portable generators
supplying fixed wiring systems, see 250.30.

The shelter's system will need to be grounded, likely with 2 rods and the grounding electrode conductor(s) bonded to the grounded conductor in the shelter's panel. A main bonding jumper is also required in the shelter's panel.

A grounding conductor in the cord between generator/trailer and shelter does not appear to be required.

 

[erickench]

NEC 250.34 state's that the system conductor to be grounded shall be connected to the generator frame if it is separately derived. NEC 250.34(B)(3) state's that if the non current carrying metal parts and the EGC terminals of the receptacles are connected to the generator frame then a grounding electrode shall not be required to be connected to the generator frame. The implication is that the SBJ is connected from the neutral to the generator frame which is bonded to the EGC. Follow these two rules and you will have a fault current return path.

 

[Fryguy]

Ok, you guys are much smarter than I. I have got the grounds and neutrals bonded in the gen panel, and bonded in the shelter panel. People say I am wrong, but I think they need to be bonded in those locations because of the ground rod to the shelter panel.

 

[kwired]

Ok, you guys are much smarter than I. I have got the grounds and neutrals bonded in the gen panel, and bonded in the shelter panel. People say I am wrong, but I think they need to be bonded in those locations because of the ground rod to the shelter panel.

regardless of what code says I think you should bond at one or the other but not both. This would create parallel neutral path which is not what you want.

That said I think the proper way is bonding jumper installed in the shelter panel, and equipment grounding conductor installed between shelter panel and generator to bond the generator frame. There should be no bonding jumper installed in the generator - I have not seen any newer portable generators that have any bonding jumper in them either, if connected to a building system they are bonded through the building bonding jumper.

It is no different than a feeder circuit it must have an equipment grounding conductor that is separate from the grounded circuit conductor.

 

[RUWired]

That said I think the proper way is bonding jumper installed in the shelter panel, and equipment grounding conductor installed between shelter panel and generator to bond the generator frame. There should be no bonding jumper installed in the generator -

.

By suggesting not to install the system bonding jumper at the generator and install the main bonding jumper at the shelter will open up violations of 250.30 and 250.34(B).

A ground rod at the vechicle mounted generator can be installed although not required and the ground rods at the shelter are required to be tied into the equiment ground bar iolated from the neutral.

Rick

 

[Smart $]

Ok, you guys are much smarter than I. I have got the grounds and neutrals bonded in the gen panel, and bonded in the shelter panel. People say I am wrong, but I think they need to be bonded in those locations because of the ground rod to the shelter panel.

Equipment grounding conductors and non-current-carrying metal parts located on the trailer are to be connected to the frame. The Code does not say exactly how this is to be accomplished.

One way is to consider the frame as taking the place of a premises/building/structure GES, and make the connection(s) thereto as if the frame were a grounding electrode. The other way is to consider grounded terminals and metal parts solidly "connected to frame" via mechanical means (bolts, screws, welds, etc.) and jumper only those parts which are electrically isolated from the grounded conductor.

In either of the above scenarios, a bonding jumper is likely to be used at the generator panel, and is compliant regardless of a system bonding jumper located elsewhere on the trailer.

 

[kwired]

By suggesting not to install the system bonding jumper at the generator and install the main bonding jumper at the shelter will open up violations of 250.30 and 250.34(B).

A ground rod at the vechicle mounted generator can be installed although not required and the ground rods at the shelter are required to be tied into the equiment ground bar iolated from the neutral.

Rick

My mistake is not considering this a separately derived system but after further reading I believe it needs to be considered a SDS.

I don't see a violation of 250.34(B) but instead violation of 250.34(C) in my previous advice.

250.30(A)(1) tells where the system bonding jumper is to be located.