Two switchboards

[Wire-Smith]

Nec 2014 Article 250.64(D)(2) does not mention bolting them together

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I agree.

It just says no objectionable current.

NEC is not a design manual, and puts that notice in the very front for extraordinary reason.

Remember my take on objectionable current, it's only objectionable if it will cause a problem. If they are not bolted together it May not cause a problem, or when an electrician goes to redo a disconnect he may get shocked by the GEC when he disconnects it, if you have danger label it might not be considered objectionable because he is likely to read it and not get shocked. I would at least have them connected by conductive strut supports or something like that, but again that is design, but design to be NEC compliant. NEC is not a how-to manual, it's a how-not-to

 

[Mr.Merl]

Objectional neutral current

Objectional neutral current

I would ground one panel and bond the metal pipe. With a separate ground wire to the separate ground bus to the other panel so you don't get an objectional current flowing through both

 

[hornetd]

you could eliminate the problem several ways

1.) disconnect(s) before panel boards, if you use one then that's where your only main bond will be. if you use two, group the disconnects and those will be your only main bonds.

2.)group panel boards beside each other

Gals and Guys keep in mind that the Fire Marshal gets to stick an oar in this pond as well. In many cases the Fire Pump Controller must not be exposed to the effects of the failure of any other equipment. In large area and High Rise buildings the fire pump many be required to be in a Fire resistive room or enclosure having a fire exposure resistance of up to 4 hours containing only Lighting, a servicing outlet, and the fire pump. You never place a disconnect which is not required by the NEC ahead of a Service Rated fire pump controller. The idea is to minimize the number of points of failure. Fire pump installations are designed so that the motor will destroy itself before it stops pumping water to the fire protection systems it supplies. That is why we size the supply to them by the locked rotor current. That is also why we use rigid conduit right up out of the pad to the controller which only becomes schedule 80 PVC below the slab or floor so it is outside the building.

 

[hornetd]

These two services in one building are hardly unique!

These two services in one building are hardly unique!

i can't think of a scenario where you can't get rid of objectionable current, if you would like to expand on what you are describing i would appreciate it. sometimes the easiest way is to add an additional transformer at the premises for the "normal electrical system", sometimes it's easier to add disconnects to where all the main bonds are grouped.

this is what i see as the likely code violation

in the O.P.s scenario with two main bonds in different areas(not very close to each-other) served by the same transformer you would have neutral current on your grounded equipment if you have any neutral load, your grounded equipment is a parallel neutral path.

The fire pump will be three phase delta wired and in the absence of a fault will not produce any neutral current. It's neutral conductor to the Service Equipment Rated controller from the wye connected transformer will be sized as if it were an Equipment Grounding Conductor for the size of wire used in the service lateral.

Think of a strip center which has separate services for each occupancy as the code allows. Would we worry about having all of those services using the structural steel as a grounding electrode and also as the pathway to the Concrete Encased Electrode and the Ground Ring.

Many water utilities will not except non metallic water service laterals because they were burned by the polybutylene pipe debacle. Entire neighborhoods are bonded together through the utility's metal underground water piping and yes some neutral current does travel on that piping to other buildings to reach a service drop or lateral neutral which is closer to the transformer and has a shorter run. The water laterals and mains simply have a lower impedance than the 2/0 AWG of the service drop and multi grounded neutral out on the poles or between the pad mounted transformers. Why aren't we excited about those? These two services are no different electrically.

 

[hornetd]

Is it just one service though? I thought code states you can’t have 2 services for exception for special conditions?

A fire pump is certainly a compliant special condition!

 

[kwired]

The fire pump will be three phase delta wired and in the absence of a fault will not produce any neutral current. It's neutral conductor to the Service Equipment Rated controller from the wye connected transformer will be sized as if it were an Equipment Grounding Conductor for the size of wire used in the service lateral.

Per 230.42(C) and a couple references to other articles you eventually end up needing to size it per Table 250.102(C)(1), which typically is larger than what the EGC after the service disconnect needs to be for a minimum size.

 

[Wire-Smith]

Gals and Guys keep in mind that the Fire Marshal gets to stick an oar in this pond as well. In many cases the Fire Pump Controller must not be exposed to the effects of the failure of any other equipment. In large area and High Rise buildings the fire pump many be required to be in a Fire resistive room or enclosure having a fire exposure resistance of up to 4 hours containing only Lighting, a servicing outlet, and the fire pump. You never place a disconnect which is not required by the NEC ahead of a Service Rated fire pump controller. The idea is to minimize the number of points of failure. Fire pump installations are designed so that the motor will destroy itself before it stops pumping water to the fire protection systems it supplies. That is why we size the supply to them by the locked rotor current. That is also why we use rigid conduit right up out of the pad to the controller which only becomes schedule 80 PVC below the slab or floor so it is outside the building.

You could put the controller inside an extra enclosure, I think that is a way to keep the disconect remote as well if you read the whole code section. I gave several suggestions to mitigate current, I personally would not design a system where a transformer with a double bond has neutral current which often means another transformer downstream before any neutral load, it easily solves the problem.

 

[Wire-Smith]

The fire pump will be three phase delta wired and in the absence of a fault will not produce any neutral current. It's neutral conductor to the Service Equipment Rated controller from the wye connected transformer will be sized as if it were an Equipment Grounding Conductor for the size of wire used in the service lateral.

Think of a strip center which has separate services for each occupancy as the code allows. Would we worry about having all of those services using the structural steel as a grounding electrode and also as the pathway to the Concrete Encased Electrode and the Ground Ring.

Many water utilities will not except non metallic water service laterals because they were burned by the polybutylene pipe debacle. Entire neighborhoods are bonded together through the utility's metal underground water piping and yes some neutral current does travel on that piping to other buildings to reach a service drop or lateral neutral which is closer to the transformer and has a shorter run. The water laterals and mains simply have a lower impedance than the 2/0 AWG of the service drop and multi grounded neutral out on the poles or between the pad mounted transformers. Why aren't we excited about those? These two services are no different electrically.

It doesn't matter if pump has no neutral current if the other service does.

Yes the strip mall scenario you suggest is often code violation, often a safety hazard, often a fire hazard and the kicker, easily preventable. Like I posted earlier I first hand witnessed a similar situation where there would have been a fire if it wasn't corrected as soon as it was, let alone electrocute someone.

About water lines, yes those are a concern. Don't forget about gas lines either, when the plumber does a hot swap on a valve and it arcs when he goes to put it back together. The current your talking about is often hunted by people with pool shock problems even when there entire service is deenergized, yeah equipptential bonding masks the problem but everything fails including every connection, so yeah its a concern. And yeah it's easily fixed. There are utilities that only bond at transformer and drop egc(supply side bonding jumper) with service conductors.

 

[kwired]

It doesn't matter if pump has no neutral current if the other service does.

Yes the strip mall scenario you suggest is often code violation, often a safety hazard, often a fire hazard and the kicker, easily preventable. Like I posted earlier I first hand witnessed a similar situation where there would have been a fire if it wasn't corrected as soon as it was, let alone electrocute someone.

About water lines, yes those are a concern. Don't forget about gas lines either, when the plumber does a hot swap on a valve and it arcs when he goes to put it back together. The current your talking about is often hunted by people with pool shock problems even when there entire service is deenergized, yeah equipptential bonding masks the problem but everything fails including every connection, so yeah its a concern. And yeah it's easily fixed. There are utilities that only bond at transformer and drop egc(supply side bonding jumper) with service conductors.[/QUOTE]

If their medium voltage distribution has a grounded neutral, those efforts only minimize stray currents from the secondary, primary neutral still has voltage drop on it and will be imposed on anything bonded to it, and will show up on a premises even if the service disconnect switch is opened. This is a common cause of stray voltages/electrocutions at boat docks and similar situations.

 

[Wire-Smith]

It doesn't matter if pump has no neutral current if the other service does.

Yes the strip mall scenario you suggest is often code violation, often a safety hazard, often a fire hazard and the kicker, easily preventable. Like I posted earlier I first hand witnessed a similar situation where there would have been a fire if it wasn't corrected as soon as it was, let alone electrocute someone.

About water lines, yes those are a concern. Don't forget about gas lines either, when the plumber does a hot swap on a valve and it arcs when he goes to put it back together. The current your talking about is often hunted by people with pool shock problems even when there entire service is deenergized, yeah equipptential bonding masks the problem but everything fails including every connection, so yeah its a concern. And yeah it's easily fixed. There are utilities that only bond at transformer and drop egc(supply side bonding jumper) with service conductors.[/QUOTE]

If their medium voltage distribution has a grounded neutral, those efforts only minimize stray currents from the secondary, primary neutral still has voltage drop on it and will be imposed on anything bonded to it, and will show up on a premises even if the service disconnect switch is opened. This is a common cause of stray voltages/electrocutions at boat docks and similar situations.

I agree what we are talking about does not rid the world of stray currents, your neighbor's on both side of the building could be double bonded and the current run right through your building, I agree. But I don't see how that means you are allowed to add to the problem. What you describe causes problems in some scenarios and not in others and what this thread has discussed has caused problems in some scenarios and not others. I look at it similar to" everybody else drives like an idiot, I may as well to, they're just as likely to run me over I as I am to wreck." doesn't make sense. I know you're not saying that, I know you're just providing related information and I appreciate it.