Too much cable, coiled in Switchboard 2

[NSee]

Thought I would post this for general comments. Yesterday I inspected a 4000A, 480/277V, 3ph, 4 wire switchboard installation. The day-before, the cables (13 sets 500kcmil) had been pulled out to a utility company, pad-mount xfrmer and also terminated at the landing lugs of the board.

Problem is, the contractor pulled what looks to be 20 to 25feet of extra cable and just had it all coiled up in the bottom of the switchboard section. 13 sets x all the footage....Its a mess.

Anyone ever see anybody do that? In over 25 years of consulting practice, I've never seen that. Besides some additional voltage drop, it looks like a big AC choke, inductance, not to mention a rat's nest.

I'm writing the report now and directing them to not energize the transformer until this is undone, cut to a reasonable length, assure same distance on all parallels, and then reconnect.

 

[cjhut]

Doesn't sound like a neat and workmanlike manner. I also may inhibit future maintenance.

 

[alehman]

Agree. I'd also be concerned about heat. It probably does not comply with the manufacturer's installation instructions, which must be followed for the ETL listing.

It seems like a huge waste of money on the contractor's part.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[magoo2]

He might have been close on the switchboard rating based on maximum fault current and was trying trying to cover himself (rather than trying to buy a higher rated switchboard). Still it makes for a sloppy installation.

 

[rasevskii]

I agree with alehman. It is a bad practice obviously to leave such an extra length in place, due heating and lack of oversight or ability to easily trace a cable. You should reject the job, IMHO. Also it will be difficult to pull anything new into such a shambles. The contractor may be trying to bill more material than required.

rasevskii

 

[Zogzog]

Have photos?

I am guessing the EC charged for all of this extra cable

 

[rcw retired EE]

If the coils are needed to reduce short circuit current at the switchgear, the contractor should have run out to manhole and back or coiled it in a manhole.

I would be concerned about short circuit forces throwing the loose cables around during a fault. Several switchgear suppliers used to have diagrams on how to tie and anchor the cables to prevent damaging movement during a fault.

 

[waross]

rcwilson expressed it better than I could. We have had to run minimum lengths of cable to limit short circuit currents to the switchboard rating. The extra cable was run straight back in the cable trays in one loop in a neat manner. No extra cable was left in the switch. Neat and workmanlike.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[NSee]

Thanks for the input folks. I put the contractor on notice this AM that this would have to "redone". Most of the reasons posted above were cited in my report. The bottom line is I will not accept it and neither will the Owner.

 

[jraef]

I have had issues with coiled conductors acting as inductors and causing serious voltage drop. But they were hundreds of feet left on a reel (cheap user wanted to return it un-cut when the temp job was over). I don't think 4 or 5 loops in air would add significant inductance.

But to chime in on rcwilson's comment, I have seen some very nasty things happen to cables just under large motor starting stresses, let alone a fault. I'm not sure any strap could adequately secure those looped cables in a serious fault situation, they would act like spaghetti in boiling water...

Here is a cable lashing spec for a project I saw that had just 10kAIC available fault current, although I don't think this was intended to address coiled up extra cable.

"Wrap line cables together and, if provided, tap cables together with nominal 3/8-inch nylon rope or rope having a minimum tensile strength of 2000 pounds at (1) 6 inches and 12 inches from the line terminals with five wraps and (2) every additional 6 inches with five wraps or every 1 inch with one wrap."

Here is a Switchgear manual that discusses cable lashing on page 9.

http://www.sea.siemens.com/us/inter...ocs_EABU docs/SB_Installation_Guide_Final.pdf

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[BJC]

Lets see 13 sets x 4 cables/set x 20 ft x 1.554 x$2.50/ lb= $ 4,040.oo
Make then shorten the cable and put the rabbit in your car.

 

[itsmoked]

That probably is it. They are [green]banking[/green] on you requesting them to shorten it all. Then they'll [green]dispose[/green] of the [green]waste[/green].

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

There has been some concern about added inductance. But you have to remember that these are three-phase cables and coiling them up does not increase inductance. At least not to load currents.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

A common cable length estimate is plus 10 feet plus 10%. Much cheaper than a cable a few inches too short.
The ends are typically trimmed and sold for scrap. The scrap may be stolen or the contractor may collect and sell the scrap. A good solution is for the contractor to sell the scrap but to hold a party for the crew with part of the proceeds. That yields a good return for the contractor and keeps theft to a minimum. The crew will police the scrap and deter others from stealing the scrap instead of looking the other way.
Depending on the arrangement of the cable coils there may be some induction coupling with load currents. There may be greater induction on ground faults but it depends. It doesn't take much cable length to make a noticeable reduction in fault currents but when extra cable is added for fault reduction, typically only the length of cable between switches or cabinets is considered in the calculations. The small lengths of cable inside switches and enclosures is neglected. It would be extremely difficult to calculate the impedance of random coiled cable ends inside an enclosure. If this impedance can not be calculated accurately it should not be used in current limiting calculations. (Unless only resistance is being considered.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Much cheaper than a cable a few inches too short.

Unless you have the proverbial "Left Handed Cable Stretcher" that the old dogs always send the new guy out to find on the other side of the job site...




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

I actually saw that used on some feeders that were 12 inches too short. Many years ago at the start of my career, you don't want the details!! We didn't have the regulation "Left Handed Cable Stretcher". We had to make do with a come-along.
grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

We have Stretch-O-Lin oil in spray cans for cables being too short. The new guys don't buy that joke about "Left Handed Cable Stretcher" any more. But they happily go looking for the Stretch-O-Lin can.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Thanks for the update, Gunnar. I'm always glad to learn about new products and methods. Grin
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

I subscribe to Bill's comment on excess cable.

Especially the part about selling off the scrap for a the crew party.

old field guy

 

[PHovnanian]

Careful with those cable stretchers. Under no circumstances should you exceed middle-C on any one conductor. ;-)