Sizing Bonding Jumper for Service Entrance Conduits?

[tx2step]

I'm under the 2002 code.

I'm trying to properly size the bonding jumpers for the service entrance conduits (2 ea. parallel 4" RMC.

The service is 480/277V 3ph, 4 wire, feeding an 800A fusible service disconnect switch.

All wire is Cu, THHN.

The service is 2 ea. parallel sets of 4 ea #600 Kcmil. The drawings also call for a #1/0 Cu. ground wire in each service conduit (in addition to the neutral), to be run out to the utility pad mt. transformer.

Why is the 1/0 ground being run within each service conduit? How did they get 1/0 as the correct size?

For the service conduit bonding jumpers, I think that 250.102(C) would apply, right? If so, then...

600+600 = 1200 Kcmil would be the largest service phase conductor? So 1200 x 12.5% = 150 Kcmil, minimum size, right?

So the jumper to each 4" service conduit would need to be #3/0 cu (=167 Kcmil), right?

If #3/0 is correct, can a single 3/0 be run from the ground bus to the first 4" conduit, then looped to the second 4"? Or do you have to run an individual 3/0 from the ground bus to each 4" conduit? Grounding bushings will be used on the conduit ends.

What size does the main bonding jumper (from neutral to ground bus) have to be? The GEC is shown as 3/0 Cu ran from the ground bus to the building steel and to the UG water pipe. I think the main bonding jumper should be 3/0, but I'm not sure where to find that in the code?

Thanks!

 

[augie47]

Since you say "service" I assume this is a utility transformer (the rules for customer Separately Derived Systems are not the same as Service transformers).
There is no reason for a bonding jumper to be installed from a utility transformer to the service.
The transformer case is bonded at the transformer and the grounded conductor serves as the ground path from the gear to the transformer.
Your grounding electrode conductors to building steel and water should be sized per 250.66 with the largest required conductor being a 3/0 which is applicable in your situation.
The bonding conductor at the service is normally supplied as part of the listed service gear. If you are installing a field jumper it would be sized at 12-1/2% of the sum of your service conductor cir mil. (this is covered by 250.28)which would still be a 3/0 as your math indicates.

 

[tx2step]

Since you say "service" I assume this is a utility transformer (the rules for customer Separately Derived Systems are not the same as Service transformers).
There is no reason for a bonding jumper to be installed from a utility transformer to the service.
The transformer case is bonded at the transformer and the grounded conductor serves as the ground path from the gear to the transformer.
Your grounding electrode conductors to building steel and water should be sized per 250.66 with the largest required conductor being a 3/0 which is applicable in your situation.
The bonding conductor at the service is normally supplied as part of the listed service gear. If you are installing a field jumper it would be sized at 12-1/2% of the sum of your service conductor cir mil. (this is covered by 250.28)which would still be a 3/0 as your math indicates.

Sorry I didn't describe everything better. This is a project that's already well underway - I was sent out to double check the Service Equipment (an 800A fusible disconnect) and Service-Entrance to make sure it is all ready for the utility company to energize. I saw a few things that I wondered about. I am having to go back and look stuff up in the 2002 code, and it's a little confusing.

The UG Service-Entrance from the utility company pad mt. transformer to the Service Equipment is 2 ea. parallel 4" RMC (underground), with each conduit containing 4 ea. #600 Cu Service-Entrance Conductors + 1 ea. #1/0 ground. The ground is called for by the AE building design drawings. Those two ea. #1/0 grounds are terminated to the ground bus in the Service Equipment (an 800A fusible disconnect). I don't know what the design engineer was trying to accomplish with those grounds, or what the utility company will do with them in their transformer. Do they service any useful purpose?

There is a separate ground bus and Grounded Conductor (Neutral) bus in the Service Equipment (an 800A fusible disconnect). There is no factory "main bonding jumper" installed between the ground bus and the Grounded Conductor (neutral) bus -- I don't know if it didn't come with one, or if it was lost or discarded. Right now, there is a #2/0 Cu main bonding jumper installed between them. I think (but not entirely sure) that this jumper should be larger - I don't know how they arrived at the 2/0 size, and I don't know where to find it in the 2002 code. I think it should be at least a 3/0, but might need to be larger? What 2002 code article applies?

There is a 3/0 Cu GEC run from the Service Equipment ground bus to the building steel. There is a 3/0 Cu (bonding jumper?) run from the UG metal water pipe where it emerges from the slab, up to the building steel. That part looks OK to me. There is also a ground rod installed by the Service Disconnect with a #2/0 Cu ran from it to the ground bus in the Service Equipment (the disconnect). That's oversized, but is OK.

In the Service Equipment (disconnect), there are #3/0 Cu bonding jumpers installed from the ground bus to grounding bushings on each of the two 4" UG service conduits. I think that 250.102(C) would apply to those bonding jumpers, right? If so, then they should be 12.5% of 1200 Kcmil = 3/0 Cu, right?
Does each 4" service conduit need it's own separate 3/0 bonding jumper, or can one 3/0 be run to the first 4" and then looped to the second 4"?

I hope this makes sense -- I have 2 main questions:

1. The required size of the Main Bonding Jumper between the ground bus and neutral bus in the Service Equipment, and what 2002 code section that comes from.

2. The required size of the bonding jumpers between the ground bus and the two ea. 4" UG RMC Service-Entrance conduits, and what 2002 code section that comes from.

Thanks for the help!!!

 

[Joe Villani]

Sorry I didn't describe everything better. This is a project that's already well underway - I was sent out to double check the Service Equipment (an 800A fusible disconnect) and Service-Entrance to make sure it is all ready for the utility company to energize. I saw a few things that I wondered about. I am having to go back and look stuff up in the 2002 code, and it's a little confusing.

The UG Service-Entrance from the utility company pad mt. transformer to the Service Equipment is 2 ea. parallel 4" RMC (underground), with each conduit containing 4 ea. #600 Cu Service-Entrance Conductors + 1 ea. #1/0 ground. The ground is called for by the AE building design drawings. Those two ea. #1/0 grounds are terminated to the ground bus in the Service Equipment (an 800A fusible disconnect). I don't know what the design engineer was trying to accomplish with those grounds, or what the utility company will do with them in their transformer. Do they service any useful purpose?

There is a separate ground bus and Grounded Conductor (Neutral) bus in the Service Equipment (an 800A fusible disconnect). There is no factory "main bonding jumper" installed between the ground bus and the Grounded Conductor (neutral) bus -- I don't know if it didn't come with one, or if it was lost or discarded. Right now, there is a #2/0 Cu main bonding jumper installed between them. I think (but not entirely sure) that this jumper should be larger - I don't know how they arrived at the 2/0 size, and I don't know where to find it in the 2002 code. I think it should be at least a 3/0, but might need to be larger? What 2002 code article applies?

There is a 3/0 Cu GEC run from the Service Equipment ground bus to the building steel. There is a 3/0 Cu (bonding jumper?) run from the UG metal water pipe where it emerges from the slab, up to the building steel. That part looks OK to me. There is also a ground rod installed by the Service Disconnect with a #2/0 Cu ran from it to the ground bus in the Service Equipment (the disconnect). That's oversized, but is OK.

In the Service Equipment (disconnect), there are #3/0 Cu bonding jumpers installed from the ground bus to grounding bushings on each of the two 4" UG service conduits. I think that 250.102(C) would apply to those bonding jumpers, right? If so, then they should be 12.5% of 1200 Kcmil = 3/0 Cu, right?
Does each 4" service conduit need it's own separate 3/0 bonding jumper, or can one 3/0 be run to the first 4" and then looped to the second 4"?

I hope this makes sense -- I have 2 main questions:

1. The required size of the Main Bonding Jumper between the ground bus and neutral bus in the Service Equipment, and what 2002 code section that comes from.

2. The required size of the bonding jumpers between the ground bus and the two ea. 4" UG RMC Service-Entrance conduits, and what 2002 code section that comes from.

Thanks for the help!!!

1. 250.28(D), Chapter 9 Table 8. I came up with 3/0

2. Finish reading 250.102(C) if there is a separate 3/0 to each conduit it is fine as it. Just leave it. 1/0 minimum.

 

[tx2step]

1. 250.28(D), Chapter 9 Table 8. I came up with 3/0

2. Finish reading 250.102(C) if there is a separate 3/0 to each conduit it is fine as it. Just leave it. 1/0 minimum.

1. Excellent! That was the code section I was having trouble finding. Thanks for the help!!!

2. I did read that, but was not clear about what "...where routed with the raceways or cables..." was intended to mean, so I didn't know if that last part applies. These bonding jumpers are installed entirely within the Service Equipment (discoonnect), and just jump off the ground bus to 2 each 4" grounding bushings - they're not "routed with" anything. So I didn't know if they should be 1/0 or 3/0. Thanks for the help!!!

 

[kwired]

The UG Service-Entrance from the utility company pad mt. transformer to the Service Equipment is 2 ea. parallel 4" RMC (underground), with each conduit containing 4 ea. #600 Cu Service-Entrance Conductors + 1 ea. #1/0 ground. The ground is called for by the AE building design drawings. Those two ea. #1/0 grounds are terminated to the ground bus in the Service Equipment (an 800A fusible disconnect). I don't know what the design engineer was trying to accomplish with those grounds, or what the utility company will do with them in their transformer. Do they service any useful purpose?

That separate conductor will end up carrying neutral current unless you only connect to neutral at the transfomer, land them on an equipment ground bus at the service equipment and do not install a bonding jumper between the neutral and equipment ground at the service equipment. The only thing it really would do for you though is eliminate what little voltage drop the service neutral has on it from being imposed on the equipment grounding system. In 99.99 percent of installations is nothing but a waste of $$ on the extra conductor IMO.

Also is since is effectively creating a parallel path for the neutral one may say we have violations of 310.10(H) as parallel conductors need to be same size, length and type so that they have equal resistance and current will divide evenly across them.

 

[George Stolz]

That separate conductor will end up carrying neutral current unless you only connect to neutral at the transfomer, land them on an equipment ground bus at the service equipment and do not install a bonding jumper between the neutral and equipment ground at the service equipment.

That would be a code violation. 250.24(B) states that every service disconnect will feature a main bonding jumper.

The OP describes a common engineering mistake in my area, the solution is to RFI deleting the unnecessary conductor from the one-line.

 

[kwired]

That would be a code violation. 250.24(B) states that every service disconnect will feature a main bonding jumper.

The OP describes a common engineering mistake in my area, the solution is to RFI deleting the unnecessary conductor from the one-line.

Correct, try to minimize one problem and you create others. Bottom line is only one grounded conductor is necessary from the utility up to service disconnecting means. Using additional conductors other then for purposes of increasing the overall conductor ampacity (intentional parallel sets), creates problems and any compromises to those usually leads to other problems.