one conduit in parallel set was damaged - replace one vs replace all?

Status
Not open for further replies.
M

malachi constant

Senior Member
Location
Minneapolis
Hi all,

Have an existing 1000A feeder (three parallel sets of what I assume is #500 CU). Underground, exterior, in PVC, about 15 years old. [EDIT: magnitude of length is on the order of 175'-200'.] Someone hit one of the conduits this morning damaging one set of feeders. The question has come up as to whether all feeders should be replaced or just the damaged set.

The argument for replacing all sets is when pulling new parallel feeders one gets them all from the same manufacturer, cuts them the exact same length, they have the same resistance, etc. So to only replace one set runs risk of slight differences in lengths and resistance.

I found this thread on the forum...still reading through it, but I want to get the question out to you all ASAP.
http://forums.mikeholt.com/showthread.php?t=52259

Any thoughts? Thanks in advance!
 
Also: There is likely closer to max 450A (probably closer to 300A max) on the 1000A feeder. (Historic max utility loads are 850A, and the portion fed by this damaged feeder does not include the chiller and more than half the building square footage.) We could theoretically abandon the damaged cable and downfuse at the source. But it is preferable to restore to original design - voltage drop, ~precise~ original calcs or actual loads are unknown, etc. FWIW system is 480V, so voltage drop isn't that big a concern.

I'm not advocating this solution - I started to add this information only because I thought it would be helpful to know we are not talking about a situation where there is 990A on a 1000A feeder, where having cables off by 2% length could push one set past the calculated loads. We are well under 1000A. But then I got distracted with possibilities that aren't going to be politically acceptable. :)
 
Considering everything you posted, I'd replace just the one feeder, as long as the same conductor/insulation type is still available.
 
Based on the lengths in question I think we are fine with only replacing the one, unless there is a concern with dissimilar manufacturers.

Let's assume 175' and it was the outside feeder that got hit. Maybe they have trouble estimating the replacement length and they end up 10' longer.

Using the formulas:

LE = 1/((1/L1)+(1/L2)+(1/L3)) = 1/((1/175)+(1/175)+(1/185)) = 59.4'

Ik = Itotal*LE/L1
If you ridiculously assume the entire max historical utility load (Ik = 850A) went on this feeder, that works out to 289A on L1 and L2, and 273A on L3.
If you realistically assume Ik = 450A (still generous), that works out to 153A on L1 & L2, and 144A on L3.
If you assume they can get the length down to within 5' those ampacities change to 151A, 151A, and 147A.

On 380A feeders this is of no concern, correct?
 
You will never have the same length in a parallel feeder just too many variables. Replace the one set.
 
We'd replace the one set too. Even if the cable length is off just a tiny bit, you will be fine.

You can always amp clamp across all the sets after you've finished just for your own curiosity.
 
sameguy said:
You will never have the same length in a parallel feeder just too many variables. Replace the one set.
Click to expand...

I agree too. The same length thing is impossible and somewhat irrelevant with a 175'-200' feeder. Even if you ordered three of them to be cut exactly the same they wouldn't be.
 
malachi constant said:
Hi all,

Have an existing 1000A feeder (three parallel sets of what I assume is #500 CU). Underground, exterior, in PVC, about 15 years old. [EDIT: magnitude of length is on the order of 175'-200'.] Someone hit one of the conduits this morning damaging one set of feeders. The question has come up as to whether all feeders should be replaced or just the damaged set.

The argument for replacing all sets is when pulling new parallel feeders one gets them all from the same manufacturer, cuts them the exact same length, they have the same resistance, etc. So to only replace one set runs risk of slight differences in lengths and resistance.

I found this thread on the forum...still reading through it, but I want to get the question out to you all ASAP.
http://forums.mikeholt.com/showthread.php?t=52259

Any thoughts? Thanks in advance!
Click to expand...

why do you need to replace the one set at all? I might be inclined to install a handhole for access and a junction box underground and patch it back together.
 
I see GD's point but it's a stretch to say that the characteristics of the parallel conductors have changed due to a splice in one set.
 
infinity said:
I see GD's point but it's a stretch to say that the characteristics of the parallel conductors have changed due to a splice in one set.
Click to expand...
The electrical characteristics may not have changed significantly (if the splice is high quality), but the mechanical characteristics and termination geometry certainly have. An AHJ call, I guess.

Sent from my XT1585 using Tapatalk
 
GoldDigger said:
The electrical characteristics may not have changed significantly (if the splice is high quality), but the mechanical characteristics and termination geometry certainly have. An AHJ call, I guess.

Sent from my XT1585 using Tapatalk
Click to expand...

A splice is NOT the same thing as a termination.
 
petersonra said:
why do you need to replace the one set at all? I might be inclined to install a handhole for access and a junction box underground and patch it back together.
Click to expand...

petersonra said:
A splice is NOT the same thing as a termination.
Click to expand...

You've been on this forum for a long time, long enough to know the intent of the code pertaining to installing parallel feeders in a similar fashion with a similar termination characteristics. If you want to talk semantics over whether a splice is a termination or not, fine.

But, if you think cutting into just one conduit to install a handhole with conductors brought up and spliced together meets the intent of the code to keep the installation characteristics similar, then I don't think there is any point in trying to tell you otherwise.
 
infinity said:
I see GD's point but it's a stretch to say that the characteristics of the parallel conductors have changed due to a splice in one set.
Click to expand...
That splice is going to have different resistance than what was originally there. May only be similar resistance to adding a foot, 5 feet, 10 feet of conductor to the entire run, but is going to cause some change in characteristics of that element of the parallel set, may or may not be significant in a long run to cause significant imbalance of current though.

Cow said:
You've been on this forum for a long time, long enough to know the intent of the code pertaining to installing parallel feeders in a similar fashion with a similar termination characteristics. If you want to talk semantics over whether a splice is a termination or not, fine.

But, if you think cutting into just one conduit to install a handhole with conductors brought up and spliced together meets the intent of the code to keep the installation characteristics similar, then I don't think there is any point in trying to tell you otherwise.
Click to expand...
:thumbsup:
 
ptonsparky said:
One of the sales pitches were that splices made with a compression tool had less resistance than the wire itself. The one time I measured and compared the voltage drop on a section wire, they were correct.
Click to expand...
I find that hard to believe, unless the connector made of a material of less resistance than the conductor being spliced, but even then won't lessen overall resistance of the completed assembly if it is only a small section involved in the splicing method.

Splice 10 feet of gold into a length of copper, and you first have a very expensive splice, but may also noticeably reduce the resistance of the overall length.
 
Status
Not open for further replies.
Top