Parallel 750MCM Conductors

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KarlS

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I have a project where (4) per phase 750MCM conductors were used in parallel to connect a switchgear main to external bushings for a utility connection. The distance is less than 20'. The inspector on the project has a problem with the cables not being the same length. He is using NFPA 310.4.B as the reason. Keeping the 750MCM cables the same length and staying in the cable tray is not Practical.
Has anyone faced this same problem?
 
He is correct, how much is the difference in length ?
 
KarlS said:
I have a project where (4) per phase 750MCM conductors were used in parallel to connect a switchgear main to external bushings for a utility connection. The distance is less than 20'. The inspector on the project has a problem with the cables not being the same length. He is using NFPA 310.4.B as the reason. Keeping the 750MCM cables the same length and staying in the cable tray is not Practical.
Has anyone faced this same problem?
Click to expand...

Get a new inspector. First ask him where you could purchase a 3000 mcm connector then ask him where you could get a 3000 mcm hypress and dies to match. If he persists on this avenue you may have to go with smaller cable that is more pliable the same length in higher numbers so termination can be a headache for you. Put in more cable tray to loose the excess cable on the shorter run. Sounds like the cable is already installed tho.......
 
Try contacting the manufacturer, and backdooring this to 110.3(B)?

Do you have an EE on board for approving this per 90.4?

What, in particular is it, that the inspector doesn't like?

Will he allow energizing and checking per conductor, the amperage draw?

If it's ugly, does he have a supervisor, who will overide this poor decision he is rendering?
 
quogueelectric said:
Get a new inspector. First ask him where you could purchase a 3000 mcm connector then ask him where you could get a 3000 mcm hypress and dies to match. If he persists on this avenue you may have to go with smaller cable that is more pliable the same length in higher numbers so termination can be a headache for you. Put in more cable tray to loose the excess cable on the shorter run. Sounds like the cable is already installed tho.......
Click to expand...

"Get a new inspector" Thats a good one, do you call the building dept. and say " I just failed an inspection, could you please send out another inspector that won't fail me?" :D
I think you may be hard pressed to find a code exception that says "unless it is to difficult to do". That is why I ask how much of a difference is it in cable length, if it is negligible I don't know why he would be hard nosed about it, but if it is a lot he is correct.
 
The inspector on the project has a problem with the cables not being the same length. He is using NFPA 310.4.B as the reason. Keeping the 750MCM cables the same length and staying in the cable tray is not Practical.
Click to expand...

310.4 requires that the parallel conductors:

1) Be the same length
2) Have the same conductor material
3) Be the same size in circular mil area
4) Have the same insulation type
5) Be terminated in the same manner

So I agree with the inspector.

Chris
 
I can think of two related things that are absolutely impossible. One is getting two conductors exactly the same size. The other is getting the Code Making Panel to agree on a number for "how close is close enough."

I can also think of three things that are possible. One is getting an engineer to sign and seal a letter that states that the conductor lengths are close enough to being the same, and that there will be no adverse impact of the differences in length. A second is getting the inspector to accept the installation on the basis of the engineer's letter. A third is that the engineer might conclude, after reviewing the installation and performing a calculation, that the conductor lengths are, in fact, not close enough to each other.

That's my take on the situation. Sorry it couldn't be more helpful.
 
Karl,

How much out of spec are you? Is this petty, or is the inspector out of his league? Just trying to get the full picture.
 
It is rare - if not impossible - for paralled conductors to be of the same exact length.


As the others have asked: Just how long are they out of sync?
 
It is interesting to me how some people immediately assume the inspector is wrong even without more detailed information, and some immediately assume he is right. Be careful, your pre-dispositions are showing. :D

We need more information here, as many have pointed out.


edit: It is amazing how many of my posts end up being the first one on the second page...
 
drbond24 said:
edit: It is amazing how many of my posts end up being the first one on the second page...
Click to expand...

There must be a setting that I can't find.
I am still on page 1.

I see comments about the pages, but I always have many less than others.
 
20 feet of total length for the conductors???

This could be a problem, as it will not take much of a difference in length difference to create a problem. Do the math, it is easy - ohms law.
Put a clamp-on tester on these if they are installed, and you will be able to see the difference of the current on each.

Also:
If the load is not much now, it is not an issue. As they add loads, it will become more of an issue.



CODE is CODE
 
Which cables aren't the same length the 4 cables per phase?

What I'm getting at is if the 4 "A" phase cables are 20' and the 4 "B" phase cables are 19' and the 4 "C" phase cables are 18' there is no violation.
 
Are all the pipes run together in the same ditch or overhead? I think you can get by with about 10%. I had a problem like that once with an inspector an that's what the engineers said and they had to approve it. Mine was about 30 feet. Thats only 2 foot in your case. Steve
 
We had 6 parallel runs of 750 I don't think there is any way to make them the exact same length. Can you? And make it it fit in a 20" by 24"?
Steve
 
It's impossible to have the cables exact. The inspector is right however. Maybe a little too right but he is. I mean if you're talking 5 or more feet on a 20 foot run then thats too much the current difference will be a lot worse. Although with a cable as big as 750 MCM it isn't easy to pipe and terminate it without struggling to make sure the cables are identical.
 
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