How to install main bonding jumper(s) for a dual utility fed service entrance ATS

[belkymd]

I have a dual utility fed service entrance ATS in which both services have unground neutrals pulled into the switch. Being that both utility feeds are service entrance, should their be a common neutral and ground bus with one main bonding jumper (neutral-to-ground)? Being that it is an ATS, only one service will be active at a time, and therefore there should not be unwanted circulating ground currents right? I also have to include that both ATS utility mains have ground fault protection. Does this affect this design?

 

[ron]

Are the two sources ungrounded for a reason, or just not considered yet?

If you want accurate GF protection, you will want to consider the application carefully. Does the xfer switch transfer the neutral as well as the phases?

There are lots to consider here, it is not so straight forward.

First think about why both sources have ungrounded neutrals, then whether they should be, then go from there. Think about GF current and where it will go based on grounding and transferring the neutral at the xfer switch.

 

[belkymd]

The utility feeds are coming directly off a delta to wye padmount utility transformer. So the services are 3ph, 4w without ground. Since the ATS will be service entrance, I will need to install a main bonding jumper. Will a common ground and neutral buss with one main bonding jumper suffice? The ATS will be 3 pole open transition. The two service entrance mains will have ground fault protection.

 

[ron]

I'm confused, where are the two service entrance mains located in the distribution path relative to the padmount utility transformer and the SE Xfer Sw? Is the service entrance mains between those two?

 

[WIMaster]

I'm confused, where are the two service entrance mains located in the distribution path relative to the padmount utility transformer and the SE Xfer Sw? Is the service entrance mains between those two?

You are not alone there Ron.

Also a 3 pole XFER Sw. with gound fault mians usually spells trouble. But we are missing something here.

 

[belkymd]

Hey guys, thanks for your responses! This is a custom built transfer switch utilizing power circuit breakers and an ATS controller. The power circuit breakers combine to be the open transition from utility to utility to a common load bus. It just so happens they are service entrance rated and are 480/277V. They are also 3000A with requires ground fault protection. I am concerned with how to effectively bond both SE sources. Your thoughts are appreciated! Hope this make more sense.

 

[SG-1]

The two services cannot be ungrounded & have ground fault protection. Either the ground fault protection will not work or it will defeat the purpose of being ungrounded. I am thinking system is to be grounded. You are trying to figure out how & where.

In this case the neutrals ( grounded conductors ) will be connected/bonded together with or through the GEC.
Is there a provision in the transfer switch for this ?

There should not be any circulating current between the two sources. You need two conductors to complete a circuit path. The current from either source is only interested in returning to the transformer that it came from.

 

[ron]

The utility wouldn't require you to bond the neutral?

 

[SG-1]

Ron, I think the easiest place to bond the neutrals would be inside the transfer switch enclosure, because it also contains the service disconnects. I am wondering why there is not an obvious place to do it.

Was that transfer switch designed for use with an ungrounded system ?

 

[ron]

If the N-G bond occurs at the transfer switch, then you need to be sure the GFP will work considering where the current will be flowing. If there is no N-G bond, all bets are off.

 

[SG-1]

Agreed, no N-G bond and the fault may not be cleared as intended. It is of vital importance that no neutral (grounded conductor) is accidentally or purposely re-grounded downstream.

 

[iwire]

Ron, I think the easiest place to bond the neutrals would be inside the transfer switch enclosure, because it also contains the service disconnects.

The NEC requires the bond be in the service disconnect enclosure so there is little choice.

 

[SG-1]

The NEC requires the bond be in the service disconnect enclosure so there is little choice.

As I understand 250.24(A)(1) through (A)(5) the bond shall be made at any accessible point between the load end of the service drop or lateral to the service disconnect. In between may lay a CT enclosure, pull & junction boxes, auxiliary gutters...

I will agree that the service disconnect is the most common. My POCO has mine in the meter compartment... not code.

 

[Smart $]

As I understand 250.24(A)(1) through (A)(5) the bond shall be made at any accessible point between the load end of the service drop or lateral to the service disconnect. In between may lay a CT enclosure, pull & junction boxes, auxiliary gutters...

I will agree that the service disconnect is the most common. My POCO has mine in the meter compartment... not code.

250.24(A) is GEC to grounded conductor connection. Main bonding jumper is 250.24(B).

(B) Main Bonding Jumper. For a grounded system, an
unspliced main bonding jumper shall be used to connect the
equipment grounding conductor(s) and the servicedisconnect
enclosure to the grounded conductor within the
enclosure for each service disconnect in accordance with
250.28.
Exception No. 1: Where more than one service disconnecting
means is located in an assembly listed for use as service
equipment, an unspliced main bonding jumper shall
bond the grounded conductor(s) to the assembly enclosure.
Exception No. 2: Impedance grounded neutral systems
shall be permitted to be connected as provided in 250.36
and 250.186.

 

[iwire]

I will agree that the service disconnect is the most common. My POCO has mine in the meter compartment... not code.

Even with one in the meter compartment / enclosure the NEC will require a bond in the service disconnect enclosure. Smart posted the section.

 

[SG-1]

Thanks guys for clearing that up for me. I was thinking, for some reason, if it were bonded between point of service & the service disconnect it should not be be bonded again. :slaphead:

 

[iwire]

if it were bonded between point of service & the service disconnect it should not be be bonded again. :slaphead:

I think in a perfect world that would be true, but the interface between the power companies muitligrounded neutral and the NECs grounded and grounding conductors often has some snags.

 

[Smart $]

Thanks guys for clearing that up for me. I was thinking, for some reason, if it were bonded between point of service & the service disconnect it should not be be bonded again. :slaphead:

You have to remember there is no EGC on the line side of the service disconnect. The main bonding jumper establishes connection of the EGC to grounded conductor on the system's load side. So if you bond the GEC at the meter, in effect, you are not bonding an EGC to the grounded conductor. What you need to consider in this case is 250.6 and mitigate neutral current on non-current carrying metal parts.

 

[SG-1]

Thanks again. I have been looking at 250.142 & that helps to make this clearer. Exhibit 250.10 is also very good.

A few years ago I could not find any ground rods & bad things were happening during severe thunder storms. I followed a bare conductor into the meter base, then I called my POCO to reseal it. I also had a bare conductor in the panelboard. Both ends of the same conductor ? I drove two rods & bonded them all together. My service is about 40 years old.

 

[belkymd]

Guys, this is a custom ATS that has not been built yet. I agree that we need a neutral to ground bond within the ATS. However, will this create a circulating neutral current loop between mains thus causing a ground fault sensing issue?