392 Cable Tray Fill & ampacity

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yoopersup

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392.12 & 392.13 High Voltage.
392.12 gives Impression cable tray must be sized by dia. od conductors laying side by side.
750 unshielded copper 5 kv=1.62 Dia. 18x1.62=29.16 = min 30 inch tray.
Ampacitys of cable (single conductor) 392.13(b)
#1 = Side by side no spacing 310.69 uncovered 75%.
Covered over 6 feet 70% of the table.

#2 Single row one conductor space between Ampacity Table 310.69 100%.

#3 Cables Bundled 3 in a bundle (a-b-c)and a space between of 2.15x conductor size.= 100% of table 310.67.

Question can bundles be put side by side?? if so ampacity??? If so can tray be Sized by width of cables side by side. 1.62x2=3.24x6(bundles)=19.44
say a 24 inch tray.

I feel min tray size 30 inches(392.12). Cables must be installed per 392.13(B) 1 ,2, or 3.

Any Imput >???
 
Re: 392 Cable Tray Fill & ampacity

I think 392.13(B)(3) covers your question. For 6 bundles of 3 cables each bundle in a triangular configuration, at 750 MCM with an outside diameter (OD) of 1.62 in, and a spacing of 2.15*OD between bundles I calculate the required width to be:

1.62*2*6 = 19.44 in across (just the bundles)
plus
2.15*1.62*5 = 17.415 in across (just the spacing)

for a total of 36.855 inches minimum wide tray.
 
Re: 392 Cable Tray Fill & ampacity

WireNut 1980

That I know. Question Is Can Bundles be Put side by side per code??? And What would ampacity be IF they could, And Could tray be a 24 inch If they could. I know how to cal. all by code BUT thoses questions are not addressed in the code directly.
 
Re: 392 Cable Tray Fill & ampacity

Originally posted by yoopersup:
...Question can bundles be put side by side?? if so ampacity??? If so can tray be Sized by width of cables side by side. 1.62x2=3.24x6(bundles)=19.44
say a 24 inch tray.

...
Click to expand...
Subsection 392.13 only speaks to the ampacity of the conductors after they have been installed per 392.12.

Subsection 392.12 is very clear that the computaion of the tray width is determined by the diameters of the 1/C cables as if they were laid in a single layer. Therefore, while the bundles could theoretically be "side-by-side" the tray width cannot be reduced.

Subsection 392.13(B)(1) is always the "fallback" condition for determining 1/C, MV ampacities:

Uncovered - 75% of 310.69 or 310.70 as applicable
Covered - 70% of 310.69 or 310.70 as applicable

Ampacities per 392.13(B)(2) or (3)may only be applied if the installation is fully compliant with the Subsection.

Therefore, if the bundles were side-by-side their ampacities would be determined by 392.13(B)(1) and a tray width less than that computed by 392.12 would be noncompliant.

[ March 24, 2005, 01:29 PM: Message edited by: rbalex ]
 
Re: 392 Cable Tray Fill & ampacity

Rablex
Answered Most of My questions. I think the cables If bundles were put side by side there would have to be additional derating or are you saying the 75 % would be apply?
Next question since I got you here Theses are Parallel conductors! and 140Ft of them run thru a Ductbank 3 wires in a pipe 6 pipes. 310.60 Detail 3 would then apply refereing you to table310.77 for ampacity of conductors , (some in duckbank some in tray. The weakest link(ampacity) would then be the ductbank.750 mcm terms (460 amps x 6 runs=2760 amps .
where in the tray bundled and spaced would be 310.67 750 mcm terms 750 amps x 6=4500amps.
Am i correct?
Underground run is 140 ft of a 450 ft run unbroken.
Tray = 4500 amps
ductbank 2760 amps
So conductors for run would be rated for lowest ampacity per310.60 (B)1
 
Re: 392 Cable Tray Fill & ampacity

Other than additional ambient condition considerations that may be imposed by 310.60 on the Tables, the 75% factor from 392.13(B) (1) applies.

Ampacity is defined term:

Ampacity. The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
Click to expand...
The ?conditions of use? affects selection of ampacities for conductors if they are not in uniform conditions throughout their length.

From 310.15(B) (2):

Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used

Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.

FPN: See 110.14(C) for conductor temperature limitations due to termination provisions.
Click to expand...
The ?ampacity? for each ?condition of use? including the terminations must be evaluated and ??the lowest value shall be used.?

Note: The exception in 310.15(B) (2) applies only to adjacent conductor portions - not to the terminations, but for MV cables they are unlikely to be the limiting factor.

Edit Add: Table 310.77 for "310.60 Figure 3" 750kcmil has an ampacity of 365A

[ March 24, 2005, 03:53 PM: Message edited by: rbalex ]
 
Re: 392 Cable Tray Fill & ampacity

Rbalex
Opps yes I looked at 3 circuit Detail 2 meant 3.
Getting old I guess, its 365amps.
Thanks for your imput ,sometimes you kinda know what to do but nice to have someone looking over your shoulder.
The pleasures been all mine .
Yoopersup
 
Re: 392 Cable Tray Fill & ampacity

Rich Buschart, the Institute's Tech Director is one of the finest men I know - a true gentleman. He also happens to be quite sharp and will help you with many of the finer application questions.
 
Re: 392 Cable Tray Fill & ampacity

Why don't you look into 5 KV busway for this amount of capacity? Some makes are listed for field cutting/customizing so it should not be that hard to build. Parallel cables are a pain in a certain body part for this amount of capacity. 5 KV busway can also be equipped with strip heaters to keep down condensation when the load is low.

I would think very carefully before using unshielded single conductors in a cable tray. If this is for wire management in an outdoor substation or inside a transformer room or vault that is 1 thing. Same for cable tray that is 20 feet off of the ground between buildings. Other areas I would not do this.

Have you thought about using 3 conductor type MC 5,000 volt cable? The individual conductors are unshielded but the corrugated sheath serves as an overall shield. This would be a safer overall installation. If you like to clean each wire strand with #220 silicon carbide paper and then goop them up with joint compound as recommended by the Consumer Product Safety Commission you will probably have fewer wire strands to scrub. In a 750 KCM copper cable that has not had the wire strands cleaned and greased only about 1/2 of the wire strands are going to be conducting after a few years. There are some other details such as using a hose clamp to prevent insulation distortion and cutting off the sharp tips before cleaning the strands so that you do not cut your fingertips.

One problem is that National Electrical Code requires a thoroughly good connection but does not define it. The Old Signal Corps definition was that you had to remover all copper oxide with sandpaper or diagonal cutters for both soldered and solderless connections. The CPSC definition is to clean all wire strands with silicon carbide paper and goop up with joint compound. I have found that garnet paper works if semiconductive silicon carbide particles will contaminate insulators.

The best and fastest way to make a connection of this size is to WELD. You can get aluminum lugs that you can tungsten-inert-gas weld onto the end of 1000 KCM and up aluminum transmission line conductors. These are NOT tim plated as the heat would destroy the plating so you have to use #220 silicon carbide abrasive paper and Ilsco Deox(R) to connect to busbar.
 
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