1200A Panels - Inadequate Ground Question?

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tmillard

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I ran into an interesting problem at a facility and thought I would post it here for discussion.

The facility has (3) 1200A 277/480V 3 phase 4 wire switchboards that are all located in different areas of the building. All the switchboards are fed from the same 4000A main switchboard. Each board is fed with (3) individual sets of 4#500, which includes a full size neutral, through 4" PVC conduits in a concrete ductbank. A single 3/0 ground wire originates from the main 4000A switchboard and is run through a separate adjacent conduit in the ductbank and then connected to the 1200A switchboard. It appears that this was done during the original construction in the early '70's.

I have reviewed both the NEC and Mike's Grounding verses Bonding book and believe the only solution is to (1) remove the existing 500 feeders, (2) remove the single 3/0 ground wire, and (4) pull 3 new sets of 4#600, EACH with (1) 3/0 ground.

Am I missing anything here?

Tom
 
assuming these three boards all have 1200 amp OCP at the 400 amp switchboard, I would say you have developed a good plan.
You might want to investigate the need for a full size neutral.
 
Unless you absolutely need 1200A at each MCC, why not just 'adjust' the feeder protective devices to a max of 1140A and leave the cables as they are.
 
tmillard said:
I ran into an interesting problem at a facility and thought I would post it here for discussion.

The facility has (3) 1200A 277/480V 3 phase 4 wire switchboards that are all located in different areas of the building. All the switchboards are fed from the same 4000A main switchboard. Each board is fed with (3) individual sets of 4#500, which includes a full size neutral, through 4" PVC conduits in a concrete ductbank. A single 3/0 ground wire originates from the main 4000A switchboard and is run through a separate adjacent conduit in the ductbank and then connected to the 1200A switchboard. It appears that this was done during the original construction in the early '70's.

I have reviewed both the NEC and Mike's Grounding verses Bonding book and believe the only solution is to (1) remove the existing 500 feeders, (2) remove the single 3/0 ground wire, and (4) pull 3 new sets of 4#600, EACH with (1) 3/0 ground.

Am I missing anything here?

Tom
Click to expand...
Its not the only solution. it might end up being the only code compliant solution.

Is it possible this was installed under a version of the code that permitted this kind of installation?

I don't believe there is any need to remove the existing ground wire. Its not hurting anything to leave it there. By any chance is the conduit with the 3/0 wire in it the same size as the other conduits? Hint!

Rather than pulling the existing wiring it might be cheaper to add another undergound conduit using the flexible plastic stuff.
 
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petersonra said:
I don't believe there is any need to remove the existing ground wire. Its not hurting anything to leave it there. By any chance is the conduit with the 3/0 wire in it the same size as the other conduits? Hint!
Click to expand...

How would that help?
 
infinity said:
How would that help?
Click to expand...

4 conduits
Phase A,B,C, and N.

Phase A/B/C (3) 500s for power and a 500 for ground

If you need a full size neutral the 4th conduit would be (3) 500s for N and a 3/0 for ground. if you don't need a full size neutral...

Three 500s is good for 1140 amps. Chances are 1140 amps is good enough.
 
petersonra said:
4 conduits
Phase A,B,C, and N.

Phase A/B/C (3) 500s for power and a 500 for ground

If you need a full size neutral the 4th conduit would be (3) 500s for N and a 3/0 for ground. if you don't need a full size neutral...

Three 500s is good for 1140 amps. Chances are 1140 amps is good enough.
Click to expand...


Are you saying to run all A phase in one conduit, B in another, C in another, and run all the neutrals and grounds in the last?

Be kind of hard to meet the intent of 300.3(B)(1) if so?
 
hurk27 said:
Are you saying to run all A phase in one conduit, B in another, C in another, and run all the neutrals and grounds in the last?

Be kind of hard to meet the intent of 300.3(B)(1) if so?
Click to expand...
It is my opinion that 300.3(B)(3) would permit the installation proposed by Bob. It is also my opinion that there is no need for 300.3(B)(1) in the code as (B)(3) permits everything that is in (B)(1) without the limitations found in (B)(1).
 
petersonra said:
4 conduits
Phase A,B,C, and N.

Phase A/B/C (3) 500s for power and a 500 for ground

If you need a full size neutral the 4th conduit would be (3) 500s for N and a 3/0 for ground. if you don't need a full size neutral...

Three 500s is good for 1140 amps. Chances are 1140 amps is good enough.
Click to expand...

What about the 3/0 required in the other conduits?
 
4th set of 500s

4th set of 500s

petersonra said:
I don't believe there is any need to remove the existing ground wire. Its not hurting anything to leave it there. By any chance is the conduit with the 3/0 wire in it the same size as the other conduits? Hint!
Click to expand...

Another thought regarding the 4th conduit....if of the same size.

Wouldn't it be cheaper to pull a 4th 'set' of 500s into this 4th conduit...install the necessary 3/0 EGCs throughout...than pulling out all the existing conductors and pulling all new 600s?
 
What's the motive to fix this install? I just took a look at 250.122 and 3/0 is large enough for the 1200 amps. I understand you are supposed to pull a EGC for each raceway but if you had pulled them in one raceway 3/0 would be large enough.

The OCP is only 60 amps over on this if you use the 75 degree column. Maybe your terminations are 90 degrees?
 
wawireguy said:
What's the motive to fix this install? I just took a look at 250.122 and 3/0 is large enough for the 1200 amps. I understand you are supposed to pull a EGC for each raceway but if you had pulled them in one raceway 3/0 would be large enough.

The OCP is only 60 amps over on this if you use the 75 degree column. Maybe your terminations are 90 degrees?
Click to expand...

Motive #1 - 3 x 500 kcmil does not equal 1200 amps [the size of the OCPD]
 
dana1028 said:
Motive #1 - 3 x 500 kcmil does not equal 1200 amps [the size of the OCPD]
Click to expand...

Changing the feeder breaker settings would seem to be a relatively cost effective thing to do over ripping the whole installation apart if the load calcs support doing that.

This kind of thing makes me cringe because its is very hard to convince me there is an actual hazard, and its an almost certainty that spending money to "fix" this kind of problem will reduce the amount of money spend on more urgent problems.
 
petersonra said:
Changing the feeder breaker settings would seem to be a relatively cost effective thing to do over ripping the whole installation apart if the load calcs support doing that.

This kind of thing makes me cringe because its is very hard to convince me there is an actual hazard, and its an almost certainty that spending money to "fix" this kind of problem will reduce the amount of money spend on more urgent problems.
Click to expand...

I agree completely - very unlikely there is any hazard with this installation [other than EGCs].... but we get into these 'code' discussions here where all I's get dotted and T's get crossed.
 
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