Feeders length for parallel runs must be within 3% length for both runs of one phase...?

hillbilly1

Senior Member
Location
Atlanta,Ga
I remember taking the amp readings on a larger service after the POCO had trimmed up their ends to make connections at the pole top transformers. Noticeable difference but it sure wasn‘t going to cause a melt down.
I was up at Best Buy corporate headquarters a couple of years ago, and met with their energy management people at a store nearby, I didn’t have my rope ct with me, so I was taking readings on individual service conductors. Most were pretty close, but one was 50 amps lower. Probably a bad connection at the transformer. Never heard back what the utility found.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
If someone is complaining about those parallel feeders I would like to know how they would do them any better.

I really wanna see someone take the cover off the LB under the panel.

No, not the big LB (actually, the LL)... the little one behind the connector...........
Why was that even run that way? Looks like it comes up out of the floor onto an LB then around to another LB into the side of the cabinet. Why not just straight up into the bottom??

Also, I would have straightened, trimmed and dressed those neutral pigtails. Looks like a dreadlocks convention.

-Hal
 

480sparky

Senior Member
Location
Iowegia
Why was that even run that way? Looks like it comes up out of the floor onto an LB then around to another LB into the side of the cabinet. Why not just straight up into the bottom??
Probably it's far enough out and so close you can't bend an offset.
 

kwired

Electron manager
Location
NE Nebraska
The EGCs are not on the neutral lugs; they go to the top of the panel.

The words "main lugs" are printed on the bottom of the enclosure.
I do see now that the EGC's run behind those bigger bodied AFCI or GFCI's.

Threw me off a little because there typically is mounting holes for grounding bars near side walls of cabinet in the same vicinity as neutral bus, or a little lower if longer cabinet is needed for installing a main breaker kit.
 

kwired

Electron manager
Location
NE Nebraska
If someone is complaining about those parallel feeders I would like to know how they would do them any better.



Why was that even run that way? Looks like it comes up out of the floor onto an LB then around to another LB into the side of the cabinet. Why not just straight up into the bottom??

Also, I would have straightened, trimmed and dressed those neutral pigtails. Looks like a dreadlocks convention.

-Hal
My thought though a little hard to tell for certain is the larger conduit is enough in the way to prevent bringing it straight into cabinet, though the solution there might not be that much better. If one didn't want the conductors in the larger raceway inside the cabinet, a junction box or wireway and bring both into it? If low voltage or something that can't be mixed with other conductors then pass the smaller conduit completely through junction box or wireway maybe.
 

480sparky

Senior Member
Location
Iowegia
Call me dumb, but a 3% difference in length changes with the overall length.

Let's say the conductors originate in the cabinet on the right. Assume the average length of 10 feet. 3% of 120 inches is 3.6 inches. That may or may not be an issue.

But suppose the conducutors are 1000 feet long. 3% of 12000 is 360 inches, or 30 feet... certainly a larger potential issue than 3.6 inches. Dropping the difference to a 'legal' 29 feet won't mitigate any issue.
 

paulengr

Senior Member
As said above, all of the conductors in each phase must match. There is no need that all phases match.
More than a 1% voltage difference to a motor means it has to be derated. So if you are close to ampacity this might matter. Same issue with multiple conductors per phase.

If your cables are 10 feet long then differences of a few inches might make a difference. If they are 100 feet long a foot or two really won’t matter.
 

Peter Furrow

Member
Location
Cape canaveral Fl
Occupation
Electrical contractor
Looking at that photo... I was just curious about the sizing of those equipment grounding conductors. The equipment grounding conductors shall not be smaller than shown in table 250.122 based on the rating of the overcurrent protection device.
When running parallel sets you still have to keep the equipment grounding conductor at full size in each conduit.


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LarryFine

Master Electrician Electric Contractor Richmond VA
More than a 1% voltage difference to a motor means it has to be derated. So if you are close to ampacity this might matter. Same issue with multiple conductors per phase.
The motor's imbalance tolerance is a separate discussion, actually.

Multiple conductors of any given phase will be forced to have the same voltage at both ends by the terminals. Current imbalance is the concern, and the closer to ampacity they happened to have been sized, or rather loaded, the more that matters.

If your cables are 10 feet long then differences of a few inches might make a difference. If they are 100 feet long a foot or two really won’t matter.
Assume the average length of 10 feet. 3% of 120 inches is 3.6 inches. That may or may not be an issue.

But suppose the conducutors are 1000 feet long. 3% of 12000 is 360 inches, or 30 feet... certainly a larger potential issue than 3.6 inches.
You two seem to be contradicting each other.
 

480sparky

Senior Member
Location
Iowegia
.......
You two seem to be contradicting each other.
If making the lengths of the various conductors 'of equal length', then introducing a percentage of differences will cause long runs to compound whatever issues the requirement is attempting to mitigate. I dunno.. induction heating, out-of-phase, whatever.

Whatever the requirement is trying to avoid, if one conductor is 120 inches and the other 121, then electrically it should be the same if one conductor is 1000020 inches and 1000021 inches.
 

wwhitney

Senior Member
Location
Berkeley, CA
If making the lengths of the various conductors 'of equal length', then introducing a percentage of differences will cause long runs to compound whatever issues the requirement is attempting to mitigate.
Pretty sure the issue is entirely covered by differential resistance, and that percentage difference precisely captures that. So indeed, 3.6" difference on a 10 foot conductor is comparable to a 30 foot difference on a 1000 foot conductor.

Cheers, Wayne
 

Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
I'm pretty sure I've heard many times that paralleled feeders must be within 6" of their corresponding phase conductor.
 

jusme123

Senior Member
Location
NY
I was told to change how I ran my wire for parralel runs of feeders. What's the code requirement for running parralel runs for feeders does the length need to match within 3%?


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Lay all 10 conductors on floor, cut to same length, install wire than strip ends for termination without cutting wire again. Try to use up extra slack in a creative but workmanlike manner!
 

jusme123

Senior Member
Location
NY
I'm pretty sure I've heard many times that paralleled feeders must be within 6" of their corresponding phase conductor.
NEC states they must be equal, anything less doesnt meet code, regardless of who says it.

With that being said, I would wager 95% of parallel runs installed are not exactly equal lengths.
 

Speedskater

Senior Member
Location
Cleveland, Ohio
Shouldn't the real concern be about end-to-end resistance of the conductors?
The two conductors may come off 2 different reels, made on different dates and maybe with different machines. No reason to think that the cross-section area will be exactly the same.
 

Carultch

Senior Member
Location
Massachusetts
If making the lengths of the various conductors 'of equal length', then introducing a percentage of differences will cause long runs to compound whatever issues the requirement is attempting to mitigate. I dunno.. induction heating, out-of-phase, whatever.

Whatever the requirement is trying to avoid, if one conductor is 120 inches and the other 121, then electrically it should be the same if one conductor is 1000020 inches and 1000021 inches.
Neglecting terminal contact resistance, the wire resistance is proportional to length, and current will divide inversely proportional to the resistances. It is the percentage difference that matters, and not the incremental difference. The same incremental difference has a lot greater impact on shorter length circuits, than it does on long distance circuits.

In situation 1, set 1 being 120 inches long, and set 2 being 121 inches long. Suppose it is a 400A circuit. This means the 120 inch conductors will carry 200.83A, and the 121 inch conductors will carry 199.17A. This is what your calculation will look like: 400*(1/120)/(1/121 + 1/120).

In situation 2, the percentage difference is a lot less. The 1000020 inch long conductors would carry 200.0001A, and the 1000021 inch long conductors would carry 199.9999A. At that point, manufacturing tolerances will have a much greater effect, than the fact that the conductors are an inch different in length.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
There's also the calculated sizing and actual loading to consider. Some installations have wire sizes that will never be fully loaded, and others have wire sizes that are often fully loaded. The closer to capacity, the more critical the mismatch.
 
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