Motor Control Center 3 wire with 4 wire load

[loosewire64]

I have a large chiller project that has 4 motor control centers. This is an add-on project and we have added an additional section to an existing MCC. This is done as a subsection rated at 500amps and fed from a 500amp existing breaker in the existing MCC. The new section was specified as 3phase 4wire and the existing sections which are SE rated are 3phase 3wire (2000amp MCB). In the existing MCC there is a distribution panel built in that has 5-1P/20amp breakers feeding the interior lighting. We have a new 100a 3phase panel being fed from the new section for additional interior and exterior lighting. The exisiting MCC has 6-500MCM conductors per phase and 6-250MCM conductors connected to the ground buss from the utility transformer. We have verified the 250's are connected to XO in the utility transformer. I have contacted the design engineer for the project and requested guidance on how he wishes to connect the neutral conductors. The existing distribution section is getting it's nuetral from the ground buss. The engineer is directing us to connect our 2-250MCM nuetrals to the ground buss to act as our nuetral for the new section. He also wants us to connect our 2-#2 equipment ground conductors to the same bar. My concern is we will then be connecting a labeled 4wire MCC to a labeled 3wire MCC and getting our neutral from the ground buss which is only 1" wide. I have told the engineer I believe by code we cannot connect to anything that is not properly labled for the connection. He say's since it is an industrial application it is allowed. Does anybody have opinion or fact based thought on this?

 

[tom baker]

Hmm thats a long post and its hard to follow. I bet you typed it as you thought about it, see if you can simplify.

 

[loosewire64]

Basically by code can you connect a 3 phase 4 wire subpanel or sub-MCC section with neutral loads to a 3 phase 3 wire source that is using the ground bar for nuetral connections?

 

[weressl]

I have a large chiller project that has 4 motor control centers. This is an add-on project and we have added an additional section to an existing MCC. This is done as a subsection rated at 500amps and fed from a 500amp existing breaker in the existing MCC. The new section was specified as 3phase 4wire and the existing sections which are SE rated are 3phase 3wire (2000amp MCB). In the existing MCC there is a distribution panel built in that has 5-1P/20amp breakers feeding the interior lighting. We have a new 100a 3phase panel being fed from the new section for additional interior and exterior lighting. The exisiting MCC has 6-500MCM conductors per phase and 6-250MCM conductors connected to the ground buss from the utility transformer. We have verified the 250's are connected to XO in the utility transformer. I have contacted the design engineer for the project and requested guidance on how he wishes to connect the neutral conductors. The existing distribution section is getting it's nuetral from the ground buss. The engineer is directing us to connect our 2-250MCM nuetrals to the ground buss to act as our nuetral for the new section. He also wants us to connect our 2-#2 equipment ground conductors to the same bar. My concern is we will then be connecting a labeled 4wire MCC to a labeled 3wire MCC and getting our neutral from the ground buss which is only 1" wide. I have told the engineer I believe by code we cannot connect to anything that is not properly labled for the connection. He say's since it is an industrial application it is allowed. Does anybody have opinion or fact based thought on this?

The Neutral and ground should only be bonded at the transfomer, all other systems should be 3 phase, 5 wire. Neutral bus should never be used as a ground bus - or vice-versa - neither in MCC's nor in switchgears.

The main reason for double bonding at the transformer(utility) and at the panel(customer) in the case of service is that both parties would want to LEGALY assure that the service is grounded.

 

[petersonra]

Basically by code can you connect a 3 phase 4 wire subpanel or sub-MCC section with neutral loads to a 3 phase 3 wire source that is using the ground bar for nuetral connections?

If the ground bar you are talking about is at the service point the answer is yes.

If it is downstream of the service point the answer is no.

 

[loosewire64]

I agree at point of service there is no difference in the ground and nuetral bar. My concern is the sizing of the bar in place. Typically a nuetral in a MCC is at minimum to be 12.5% of the rating of the MCC (250amps). This MCC has a 1" bar that is 1/4" thick. I have no way of rating this bar and with the two panels being fed from it (1-100amp & 1-250amp) it does not seem correct.

 

[erickench]

If the service is 3 wire 3 phase then it would be a delta connected transformer and no wye configuration. Are you connecting the neutral to the midpoint of a transformer secondary?

 

[loosewire64]

the transformer is 480/277 Y and the XO connection is being used. I believe when the project was originally built they ordered the wrong MCC using 3phase 3wire and it should have been 4wire. They went ahead and connected it up on the ground bar and nobody noticed. Now it is 10years later and we discover it. My recommendation to the engineer is to re-terminate the 6-250mcm conductors on a rated buss bar and extend nuetral conductors from there to our section. I believe he realizes he has a mistake and wants the easy way out. My idea is relatively inexpensive and would correct the issue for the future.

 

[erickench]

But what specifically is connected to the ground bar on the supply side? Is it a service neutral? Is it the transformer neutral?

 

[loosewire64]

They brought 6-500 per phase and 6-250's for the neutral. The issue is they did not have a neutral bar in the MCC so they landed them on the ground buss. They then bonded building steel, cold water, and a counterpoise system. This is point of service for the building and there are neutral loads existing from this equipment.

 

[erickench]

Okay, it's the wrong type of MCC. Tell me is it possible to install a neutral terminal bar into the existing MCC thereby converting it from a 3 wire to a 4 wire?

 

[loosewire64]

That is my opinion but the EE is wanting us to just connect into the ground buss with our neutrals and grounds from the new section. I do not agree with that and was looking for any reason he could be basing his position on leaving it as is and adding to it.

 

[Cold Fusion]

...looking for any reason he could be basing his position on leaving it as is and adding to it.

Well, how about looking at Bob's post 5 and your post 10. If I am understanding you correctly, the existing MCC is the service equipment and the neutral and ground are susposed to be connected there.

cf

 

[loosewire64]

Yes, I agree with that but the ground buss is not rated for the potential load. It seems to me they pulled a 50% neutral in from the transformer and landed it, at best on a 10% rated ground buss. To me it is no different than using 250volt equipment on 277volt applications. It will work and may never be a problem but it is not correct.

 

[Cold Fusion]

Yes, I agree with that but the ground buss is not rated for the potential load. It seems to me they pulled a 50% neutral in from the transformer and landed it, at best on a 10% rated ground buss. ...

... It will work and may never be a problem but it is not correct.

I agree with that.

You did actually look at the existing MCC specs and know this for a fact - right?

cf

 

[erickench]

If it's a separately derived system then you can bond the transformer neutral to the case that encloses it. Then run a supply side bonding jumper from the transformer casing to the MCC ground terminal bar. The SSBJ would be sized according to 250.66.

 

[weressl]

Okay, it's the wrong type of MCC. Tell me is it possible to install a neutral terminal bar into the existing MCC thereby converting it from a 3 wire to a 4 wire?

It is possible, but would require the complete disassembly of the MCC.

But I think you have more trouble than that since the switchegar would required to have a neutral bus too, an isolated bus or even just a termination block where the neutral can land and continue with the service wire. Then the question of feeders sizing would come. What size from the TR to the SWGR to the MCC to the individual branch OCPD. Modifying the listed egar is anotehr issue you would need to look and get manufacturers advise and consent.

 

[erickench]

I don't see why you have to disassemble the MCC. You're just running a jumper from the transformer casing to the MCC ground terminal bar. The MCC stay's as a three wire. I think this engineer is looking at an old schematic from a past year NEC Handbook that show's the neutral running from the transformer to the MCC.

 

[petersonra]

Okay, it's the wrong type of MCC. Tell me is it possible to install a neutral terminal bar into the existing MCC thereby converting it from a 3 wire to a 4 wire?

I have bought a few MCCs rated as service equipment for 4 wire. None of them actually used the neutral for anything, but for some reason that was all the Poco would give the customer. In every case, the neutral was just a terminal labeled neutral for the incoming neutral to land on.

It should not be real hard to find out from the manufacturer.

In any case since it is not service equipment, I don't see why you could not just field add a neutral distribution block to the incoming power section.

 

[Smart $]

Well, how about looking at Bob's post 5 and your post 10. If I am understanding you correctly, the existing MCC is the service equipment and the neutral and ground are susposed to be connected there.

cf

Yes, I agree with that but the ground buss is not rated for the potential load. It seems to me they pulled a 50% neutral in from the transformer and landed it, at best on a 10% rated ground buss. To me it is no different than using 250volt equipment on 277volt applications. It will work and may never be a problem but it is not correct.

They may have pulled a "50%" neutral for future reasons. The neutral could be as small as 1/0 (minimum paralleled size) then 3/0 or its busbar equivalent in the gear where not paralleled. Since there is no conductor ampacity rating by way of 250.24(C) and 250.66, we have to compare cross sectional area and ampacities based on 1000A per in? of busbar.

3/0 Cu conductor = 0.132in? (85.01mm?)... ? 1000A/in? = 131A
1/4" x 1" Cu busbar = 0.250in?@250A​

The busbar has greater ampacity than the required minimum neutral and is certainly adequate for handling 5 1P20A lighting circuits, which if balanced would be a potential maximum unbalanced neutral load of 32A. In the new section is going to be a 100A panel which would be a potential maximum unbalanced neutral load of 80A. Therefore, the possible total maximum unbalanced neutral load would be 32A + 80A = 112A, and well within the ampacity of the grounding busbar.

Yes there is a bit of assumption here, but without all the details on all line-to-neutral loads, I say the system will be compliant as the EE designed it.

PS: FWIW, I'd land the new section's neutral jumpers as close as physically possible to the service neutrals... to minimize circulating currents on the bus and enclosure.