Parellel service conductors (feeders)

[pdesai_pe]

I have a situation where we are running 8 sets of 4 # 500 MCM underground feeders for 3000Amp service switchboard. Conductors are installed in place and the inspector wants the verification and proof of these conductors are cut exectly in equl length as per NEC 310.4.B . His understanding is that, for sutible landing on panel lugs, contractor may have cut the cable may be 4" or 6" or 12" short in length. To dertermine the length, we proposed to mesure the conductor impedence.

Q.1. Is this correct method to measure conductor length?
Q.2 Is there any other method to mesure the length for the installed cable?
Q.3 Is there any tolerance any one know of that can be applied?

 

[wasasparky]

Approximately how long are the conductors?

 

[LarryFine]

I'd start by asking him what he would consider to be adequate proof.

Approximately how long are the conductors?

In other words, what percentage of the entire lengths are the 4", 6", and 12" differences? If the wires are 6' long, that's a big difference.

It also matters how close to the adjusted ampacity the actual load is. Slight variations in termination resistances can also be a factor.

 

[Smart $]

The NEC does not specify the means for measuring length, and neither exactness nor a tolerance. You will have to ask your inspector if he will accept resistance measure to length conversion, and any conditions placed thereon.

Only the inspector can answer your questions definitively, no matter what we say here.

 

[Sierrasparky]

just curious are those feeders before or after the meter? sometimes people mix up feeder and service entrance conductors.

 

[infinity]

Like Larry said the real significant question si the length. You will never get them the exact same length. A 12" difference over a 200' run won't matter much on a 20' run maybe.

 

[wasasparky]

You can also measure how the current divides between each paralleled conductor...

Did the contractor land conductors on both ends? I've heard that some utilities making their own terminations may not have "equal length" on their mind...

 

[brian john]

Even in cases where they are all exact length the current sharing will be different based upon connection resistance and routing of conductors.

I have seen parallel feeders with 10 feet difference in length and the percentages of load was minimally different between conductors.

I always STRIVE for as equal as possible, but it will seldom insure equal loading.

 

[Sierrasparky]

The reason I asked is if these were Service entrance why is the EC installing them. And why is the inspector concerned.
Out here the POCO is resposible for UG service . We don't touch them. The POCO even has wire with their name on it.

 

[steve66]

Do the conduits pretty much follow the same route, and are they pretty much ran side by side?

Some differences are to be expected when you do your terminations. Otherwise, if the conduits are the same length, wouldn't everyone agree the conductors are the same length?

It's not like you could hide an extra 20' of conductor inside a conduit somewhere.

Steve

 

[Sierrasparky]

Fortunatly were I do work I know the inspector. If he were trying to pull that crap on me I'd kindly tell him to go pound sand. I'd like to see that a couple of inches from a 500MCM will ever make a Significant difference.

I'd show him that it is far more important to make sure all connections are good. That way is decreases the occurance of a connection at the lug going bad and the remainder of the conductors going bad.

I have seen too many times that the installer is not carefull in securing the whole wire in the lug and causing a future failure.

 

[dbuckley]

The best way is to demonstrate current balance on a constant load. After all, that is the intent behind the same blah blah blah stuff. A decent clamp should be all you need, or a flexiclamp if the access is a bit tight which is entirely possible.

 

[pdesai_pe]

parellel service conductors

parellel service conductors

These conductors are approximately 20 to 30 feet from service switchboard. These conductors from main service disconnect to utility transformer and installed by electrical contractor and termination has to be coordinated with the local utility company.

 

[ohmhead]

I have a situation where we are running 8 sets of 4 # 500 MCM underground feeders for 3000Amp service switchboard. Conductors are installed in place and the inspector wants the verification and proof of these conductors are cut exectly in equl length as per NEC 310.4.B . His understanding is that, for sutible landing on panel lugs, contractor may have cut the cable may be 4" or 6" or 12" short in length. To dertermine the length, we proposed to mesure the conductor impedence.

Q.1. Is this correct method to measure conductor length?
Q.2 Is there any other method to mesure the length for the installed cable?
Q.3 Is there any tolerance any one know of that can be applied?

Well i think your inspector is out of line a little ! Greenlee makes a cable length meter it works fine we use it for left over wire feeders .

But that said we do parallel feeders on every job for over 38 years and ive never heard of any inspector question a length yet.

 

[Leestarkweather]

It's a mystery to me

It's a mystery to me

After being quizzed 20+ years ago by an inspector, our SOP is to pull in a thin measure tape (like a Dottie type) in each individual pull if it's over 20 ft. long.
They are non-flammable and I've had several inspectors look and see them, then say--"it looks like you are on top of the equal length rule" and not even look at the footages. We don't do it to fake it--we do it to have the proof installed.

Per centage of overall length is the most critical.

I recently had a 1600A ATS on a 2000A feeder rebuilt, on an off-hours schedule. We installed it 7 years ago. The connection from a 2000A CB to the ATS was 5 feet of 12x12 wireway, with (6) 400MCM per phase and (3) 400MCM on the neutral. I and my foreman actually laid out the 6 conductors for each phase and cut them equal within 1/2", and terminated them without any further cutting. With 900+/- amps flowing per phase upon startup, we were getting Amprobe (RMS) readings of +/- 20% from one parallel conductor to the next on the same phase. Of course, it is impossible to arrange the geometry of the conductors into 2-A, 2-B, 2-C and 1-N like it would be in a group of 3 discrete conduits, since they were all in the 12 x 12 wireway. But why would they vary so much? Gives a lot of pause to the equal length rule.

 

[ohmhead]

well the meaning of length the same raceway or distance is never going to be exactly the same underground conduit can not ever be the same length so the conductors in real life are not the same length ever .


No one ever in this electrical trade would ever get a perfect measurement on a multi pull parrallel run to a main switchboard try making this exactly the same length http://i611.photobucket.com/albums/tt195/stringking/IMG_0630.jpg

 

[nakulak]

I try to get equal length, but there are times when the geometry of the installation makes it so impractical that its (not done). I find it interesting that there are inspectors that are trying to enforce this (where is their liability ? ) Over the last few months several areas around here have been downsizing their staff of inspectors, and I have noticed that inspectors are looking at more and more things (more time on their hands or justifying their jobs ?) , especially in the areas with the worst cuts.

(just my perceptions, I could be mistaken)

 

[msteiner]

My understanding is that the NEC "equal length" rule is aimed at ensuring equal impedance on all paralleled conductors. So verifying the length is really an indirect method of ensuring equal impedance. Therefore, if you measure the actual impedance, you're verifying compliance with the INTENT of the NEC, and should therefore satisfy the inspector.

 

[iwire]

My understanding is that the NEC "equal length" rule is aimed at ensuring equal impedance on all paralleled conductors. So verifying the length is really an indirect method of ensuring equal impedance. Therefore, if you measure the actual impedance, you're verifying compliance with the INTENT of the NEC, and should therefore satisfy the inspector.

Strictly speaking it is not enough to meet the intent of the NEC we must meet the words contained in the adopted code. Unfortunately the wording of the NEC in this case is virtually imposable to meet.

 

[Smart $]

My understanding is that the NEC "equal length" rule is aimed at ensuring equal impedance on all paralleled conductors. So verifying the length is really an indirect method of ensuring equal impedance. Therefore, if you measure the actual impedance, you're verifying compliance with the INTENT of the NEC, and should therefore satisfy the inspector.

While I agree with you, the NEC is not actually written that way. The inspector is not obligated to accept such measure.