Cable Limiters for under-rated bus-duct
Cable Limiters for under-rated bus-duct
(OP)
Hi everyone,
I am doing a short circuit analysis on a manufacturing plant in the U.S. and have come across some bus duct runs in the plant that are under-rated for the calculated short circuit currents (22kA rms rated withstand, vs 27kA momentary short circuit, calculated at the end of the 100' run of bus duct). I am of the opinion that a bus duct with a higher rating should replace the one that is presently installed, however would cable limiters be a possible option to reduce the short circuit current (if operating in the first half cycle of course) to ensure the present bus duct remains within its ratings? I am thinking that a swap out with higher rated bus duct may cost more on a material basis, but may be near the same price once engineering assessment and funky installation are considered; not to mention, it is a cut and dry application for a switch out.
My biggest concern is that I understand a P.E must sign off on cable limiter installations and it really doesn't sound like the NEC has provisions for this type of application of cable limiters.
The duct is fed by 2 runs of 350MCM cable per phase in conduit.
Thanks for the help
I am doing a short circuit analysis on a manufacturing plant in the U.S. and have come across some bus duct runs in the plant that are under-rated for the calculated short circuit currents (22kA rms rated withstand, vs 27kA momentary short circuit, calculated at the end of the 100' run of bus duct). I am of the opinion that a bus duct with a higher rating should replace the one that is presently installed, however would cable limiters be a possible option to reduce the short circuit current (if operating in the first half cycle of course) to ensure the present bus duct remains within its ratings? I am thinking that a swap out with higher rated bus duct may cost more on a material basis, but may be near the same price once engineering assessment and funky installation are considered; not to mention, it is a cut and dry application for a switch out.
My biggest concern is that I understand a P.E must sign off on cable limiter installations and it really doesn't sound like the NEC has provisions for this type of application of cable limiters.
The duct is fed by 2 runs of 350MCM cable per phase in conduit.
Thanks for the help






RE: Cable Limiters for under-rated bus-duct
But I don't understand why do you think 350 MCM [CU,XLPE?] could not withstand 27 kA for 1 sec.
If you will use the known formula:
Ik=5.067/10^3*Akcmil*SQRT((TCAP/tc/alpha/ro20oC*LN((Ko+Tm)/(Ko+Ta))
a single 350 MCM copper cable XLPE insulated withstand for 1 sec. 30 kA.
TCAP=3.42;alpha=0.00393;ro20oC=1.72;
Ko=234;Tm=250;Ta=40;tc=1sec.
RE: Cable Limiters for under-rated bus-duct
It is actually the Square D CP bus duct with the 22kA withstand I am worried about; I am not concerned about the cable. That was the issue: can cable limiters be used to protect the bus duct, or would it be better to look at a typical current limiting fuse (i.e. class J, RK, etc.)
RE: Cable Limiters for under-rated bus-duct
eddunnpe
RE: Cable Limiters for under-rated bus-duct
RE: Cable Limiters for under-rated bus-duct
RE: Cable Limiters for under-rated bus-duct
Thank you very much davidbeach. I was thinking that was the ultimate requirement for this type of installation. I am going to stick to what I know with this one!
RE: Cable Limiters for under-rated bus-duct
RE: Cable Limiters for under-rated bus-duct
RE: Cable Limiters for under-rated bus-duct
RE: Cable Limiters for under-rated bus-duct
That is my understanding with series ratings (and applying CLF as well). The idea of increasing the cable impedance may actually be viable. I don't have much knowledge of material and installation costs of bus duct, but what about simply replacing this duct with a higher short-circuit rating one (eg. SQD CP type, 22kA, vs SQD CPH type, 42kA)? Is it worth the cost?