Too much cable, coiled in Switchboard
Too much cable, coiled in Switchboard
(OP)
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.
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.






RE: Too much cable, coiled in Switchboard
RE: Too much cable, coiled in Switchboard
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.
RE: Too much cable, coiled in Switchboard
RE: Too much cable, coiled in Switchboard
rasevskii
RE: Too much cable, coiled in Switchboard
I am guessing the EC charged for all of this extra cable
RE: Too much cable, coiled in Switchboard
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.
RE: Too much cable, coiled in Switchboard
Bill
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"Why not the best?"
Jimmy Carter
RE: Too much cable, coiled in Switchboard
RE: Too much cable, coiled in Switchboard
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.
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RE: Too much cable, coiled in Switchboard
Make then shorten the cable and put the rabbit in your car.
RE: Too much cable, coiled in Switchboard
Keith Cress
kcress - http://www.flaminsystems.com
RE: Too much cable, coiled in Switchboard
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Too much cable, coiled in Switchboard
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
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"Why not the best?"
Jimmy Carter
RE: Too much cable, coiled in Switchboard
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
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RE: Too much cable, coiled in Switchboard
grin
Bill
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"Why not the best?"
Jimmy Carter
RE: Too much cable, coiled in Switchboard
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Too much cable, coiled in Switchboard
Yours
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Too much cable, coiled in Switchboard
Especially the part about selling off the scrap for a the crew party.
old field guy
RE: Too much cable, coiled in Switchboard
RE: Too much cable, coiled in Switchboard
Be very careful..
On some of today's newer jobs, you have to specify the "Metric Left Handed Cable Stretcher".
(Always be sure to double-check the calibration date on any cable stretcher before you issue it to the apprentice. They go out of date faster than you'd think. Just like those calibrated Metric Counter-clockwise Crescent Hammers the millwrights need to use to quickly round off bolt heads.)
RE: Too much cable, coiled in Switchboard
There are no fixed limits for when to use and not to use. It all depends on total cable length, if level differences are present, ambient temperature - especially if there's load and/or temperature cycling - and several other factors that an experienced cable team intuitively knows about.
In general, though, it is wise not to stretch more than about one foot. We agree there.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Too much cable, coiled in Switchboard
a little inductance during normal operation is not a problem, but under fault conditions it could delay the opening of the circuit protectors, and allow a dangerous situation to linger a few moments longer.