Parallel Conductorology 101

[lile001]

We have an installation where the plans call for 3 sets of 4 600 MCM and 3/0 grounding conductor in 3-3 1/2" conduits. These conduits are installed (in concrete!) and feed a 1200 amp switchboard, also already installed.

However, the switchboard will only accept up to 4 - 500MCM cables. 600MCM is too big for the lugs available, and the manufacturer tells us there is no lug kit that will fit. Running two parallel sets of 4/0's in each raceway (6 total wires per phase) is not an option, because we can only land 4 wires per each phase on the switchboard.

 

[sandsnow]

Re: Parallel Conductorology 101

1. Full size equip grd req. in each raceway
2. Doesn't say anything. 310-4
3. There are "hypress" lug adapters that allow you to terminate a post under the lug that might give you what you need. I haven't seen these used in a while, so I don't know specifically about availability or compatibility.

Start skinning

 

[don_resqcapt19]

Re: Parallel Conductorology 101

I think that you have a violation of 310.4. The physical characteristics of each set of parallel conductors is not the same because you have two sets in one raceway and only single sets in the other two raceways. A change in the 2005 code makes it clear that this istallation is not permitted.
Don

 

[petersonra]

Re: Parallel Conductorology 101

Maybe a big splice box near the switchgear? (3)600MCM to (4)500MCM four times.

I'm not sure how to go about this particular case, but I've seen similar things done coming into an MCC using distribution blocks as splices.

maybe you could make the splice using copper bus bar?

[ November 03, 2004, 10:50 PM: Message edited by: petersonra ]

 

[kiloamp7]

Re: Parallel Conductorology 101

Do not think 310.4 would allow this funny mix in a feeder made up of parallel conductors.

How you gonna pull (8) 500 & (1) 3/0 in a 3-1/2" c. anyway? (4) 600 & (1) 3/0 is tough enough, even if 40% conduit fill rules allow.

As far as lugs, surely with a modern 1200A rated swbd., there is a way to change the lugs to a 3-barrel 600 size or to 6-barrel 4/0 size. Good, clean, tight electrical connections is the bottom line.

If feeder OCP is 1200A, then yes, needs to be a 3/0 EGC in each conduit. Sounds like someone wants a 100% "neutral". If this feeder is serving mixed loads, an Art. 220 calculation may show that a smaller "neutral" is okay.

Sounds like motor loads are involved. So 430.63 may well allow a feeder bkr. with a long-time trip rating that actually exceeds the ampacity of the conductors.

New equipment is bound to be 75 degree rated for terminations, so I'm not sure what all this 320A stuff is about.

Yes, you need some help. But electrical power professionals can do that for you.

 

[bob]

Re: Parallel Conductorology 101

This does not seen to violate 310.4 IMO. The requirements are
1. Same length 2. Same conductor material
3. Same CM area 4 Same insulation 5 Terminated in the same manner.

Unless I am reading Annex C wrong you can not put 8 500 kcm and 3/0 in a 3 1/2" conduit. If this is underground I think you will need a "W" with the insulation type.

 

[don_resqcapt19]

Re: Parallel Conductorology 101

Bob,
You need to read the line after 310.4(5).

Where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics.

If you have two sets in one raceway and only one set in the other raceways, then all of the sets do not have the same physical characteristics.
Don

 

[lile001]

Re: Parallel Conductorology 101

 

[ryan_618]

Re: Parallel Conductorology 101

I agree with Don. I would red-tag this in a second.

 

[sandsnow]

Re: Parallel Conductorology 101

Don,
I disagree with you on this. I believe the Code is silent here, although I'll admit it's an oddball installation. Looking in the Handbook physical characteristics are explained as AL vs. Steel for conduit material due to the difference in impedance. Also The conductors cannot be two AL and two CU in the same conduit.
You guys are right about the fill - no way

lileoo1;
Check out this link. The pin connector described should allow you to stay with 600kcmil and terminate on your switchboard. You will have to confirm with the manufacturer's application engineer that this is a proper use of their product.
http://ecatalog.fciconnect.com/fci/datasheet.asp?PN=AYPO600&FAM=AYPO&P=115196

The power distribution block to make this transition from 3 x 600 to 4 x 500 would be at this link
http://ebusiness.ilsco.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=1&storeId=10001&langId=-1&categoryId=168&from=,%20filter_product%20fp2&where=%20AND%20FP1.FPFVNBR|457 0%20AND%20FP2.FPFVNBR|4597&wherenot=%20AND%20NOT%20fp1.fprfnbr|fp2.fprfnbr%20AND%20fp1.fpprnbr|fp2.fpprnbr


btw, is this a main lug switchboard or circuit breaker you are landing on?

keep skinning

 

[don_resqcapt19]

Re: Parallel Conductorology 101

sandsnow,
This was cleared up in the 2005 code with wording that requires the same number of conductors in each raceway of a parallel installation. The following is part of the panel statement on comment 6-8.

The panel agrees that the installation described in the proposal could cause unequal division of current.

Don

 

[sandsnow]

Re: Parallel Conductorology 101

Don
Thanks.
So this relates to a question by Pierre about using a clarification in a later edition to enforce your present code. With the code I am on I would have to allow it if backed up to the wall.

 

[rbalex]

Re: Parallel Conductorology 101

Larry,

As a fellow Californian, I think you would be well within your authority to reject such an installation (even on the '99 Code) and for exactly the reason Don gave; i.e., the conductors are obviously being installed in a manner where they do not have the same "physical" characteristics.

Where I would question the CMP is whether it constitutes an automatic safety hazard. An uneven current divider is not necessarily unsafe - but it is definitely questionable. Depending on several issues, especially the overcurrent protection method(e.g.,its possible to monitor each conductor separately) there may be an "acceptable" engineered solution.

My biggest problem with most of the CMPs is that, despite 90.1(C) the Code is becoming more and more a "design specification."

 

[petersonra]

Re: Parallel Conductorology 101

I am not opposed to allowing the code to take on the role of design book, especially for more commonly encountered situations. It just makes sense to have one place you can go and get commonly encountered problems dealt with so redesign is not required except where you run across unusual situations.

If you look at the code in total, there is an amazing amount of design material embedded in it. In my mind this is a good thing as there are many ways you could do something safely, yet there is a lot to be said for also doing things in similar ways across many installations. I'd be inclined to error in the direction of making things more similar.

 

[bob]

Re: Parallel Conductorology 101

Don

Where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics.

In this quote the cables are not conductors. The cables are somethibg like Type MC. I assume that the conduits are of the same type material and are the same size.

Ryan

I agree with Don. I would red-tag this in a second

Why?

rbalex

the conductors are obviously being installed in a manner where they do not have the same "physical" characteristics.

Bob again these are conductors and not cables. The conductors have the same physical characteristics such as 1. Same length 2. Same conductor material
3. Same CM area 4 Same insulation 5 Terminated in the same manner.
I would agree this is a bad installation.

[ November 04, 2004, 02:34 PM: Message edited by: bob ]

 

[don_resqcapt19]

Re: Parallel Conductorology 101

Bob,
The basis for the code change is the fact that the cables no longer have the same ampacity.
Don

 

[rbalex]

Re: Parallel Conductorology 101

Oddly enough, I don't think ampacity is the immediately relevant issue here; current division is.

Say we have a 4x4 ductbank (A to D, left to right; 1 to 4, top to bottom.) If we used the lower left quadrant (A3,B3,A4,B4) 3/c per duct, for a 4 conductor feeder, NONE of the individual conductors per phase would have the same ampacity. For example B3, would have a significantly lower actual ampacity than A4. However with the impedences still relatively comparable, we can make the reasonable assumption that the current divides equally among the 4 conductors and we can still directly protect the feeder. We simply protect for the 4 x lowest single conductor ampacity (B3).

However, if there is a significant current divider between the conductors, even if the ampacity per conductor were the same, we have no direct idea how to protect the feeder as a whole.

 

[ryan_618]

Re: Parallel Conductorology 101

Other Bob: I would red tag it because, as Don said, they must have the same physical charecterstics. What about 2 sets, one installed in 3" IMC, the other installed in 4" IMC? I would red-tag it for the same reason.

 

[sandsnow]

Re: Parallel Conductorology 101

Hmmm

I'll have to look at a 2005.
Bob,
It wasn't obvious to me before and it's still a little fuzzy. When the Code describes phys. characteristics it does not mention number of conductors.
Don
Please tell me which proposal. I would like to look it up. Thanks

 

[don_resqcapt19]

Re: Parallel Conductorology 101

Larry,
Proposal 6-10 and comment 6-8.

Don