Cable Tray Fill question

deadshort

Member
Location
Nor Cal
Hi all,

So we are designing a commercial parking structure PV array. I am working on sourcing cable tray but have a few nagging questions I can't seem to find anywhere so was hoping someone here could help.

From what I understand, all the literature seems to say that the "tray" will not be filled past the 50% capacity of it's area (Other literature recommends no more than 40%).

So, If using 1000V PV wire @ 0.261 O.D. x 42 conductors (most in any one length of tray) = 10.962 in

Which means I would need a tray with at least TWICE that fill capacity correct?

That sucks because instead of the 6"W x 2"D tray I was looking at with a fill capacity of 12.09 in. I will need to use the 12"W x 2"D tray with a fill capacity of 24.18 in. unless there is something I am missing.

Does this sound right to you guys?

Thanks in advance.
 

Smart $

Esteemed Member
Location
Ohio
Are your conductors cable tray rated?

(NOTE TO MODERATOR: Consider moving this and the preceding post to a new thread in the PV forum)
 

deadshort

Member
Location
Nor Cal
Are your conductors cable tray rated?

(NOTE TO MODERATOR: Consider moving this and the preceding post to a new thread in the PV forum)
Well, it's PV wire. I don't think it has to be especially rated for cable tray. It's UV resistant and outdoor rated.
 

Smart $

Esteemed Member
Location
Ohio
Well, it's PV wire. I don't think it has to be especially rated for cable tray. It's UV resistant and outdoor rated.
I think you are correct but I can't find it in 690 of the 2011 NEC.

The reason I brought it up is the sizing requirements of 392 do not go below 1/0 for single conductors. Perhaps I conjured up the wrong picture of your installation in my mind. Is your PV wire multiconductor?
 

SolarPro

Senior Member
Location
Austin, TX
Correct. The Code-Making Panel added language on NEC 2014 to clarify this point:

690.31(C)(2) Cable Tray. PV source circuits and PV output circuits using single-conductor cable listed and labeled as photovoltaic (PV) wire of all sizes, with or without a cable tray marking/rating, shall be permitted in cable trays installed in outdoor locations, provided that the cables are supported at intervals not to exceed 300 mm (12 in.) and secured at intervals not to exceed 1.4 m (4.5 ft).

Informational Note: Photovoltaic wire and PV cable have a nonstandard outer diameter.
 

Carultch

Senior Member
Location
Massachusetts
Is there single conductor PV wire smaller than 1/0? If so, how does one size the tray for single conductors smaller than 1/0?
Most PV wire in common use is smaller than 1/0. There is no formal sizing algorithm or ampacity algorithm for cable trays with <1/0 PV wire, so by default it is as much as you can physically fit. In a practical sense, 50% fill will appear to fill every usable square millimeter, even though you can attempt to draw it to scale and think you can fit a lot more. You'd need to be extremely meticulous to pack the wires in a 91% fill hexagonal close pack lattice at every cross section, which is not going to happen.
 
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deadshort

Member
Location
Nor Cal
Is there single conductor PV wire smaller than 1/0? If so, how does one size the tray for single conductors smaller than 1/0?
We're using #10 PV wire. Cross sectional area is 0.261 in. Talked to the cable tray engineer, for 42 conductors @ 10.96 in sq, I would need to use the 12" W tray with a fill capacity of 24 in sq to comply with the 50% fill rule.
 

Carultch

Senior Member
Location
Massachusetts
We're using #10 PV wire. Cross sectional area is 0.261 in. Talked to the cable tray engineer, for 42 conductors @ 10.96 in sq, I would need to use the 12" W tray with a fill capacity of 24 in sq to comply with the 50% fill rule.
That sounds like the nominal outer diameter, not the cross sectional area.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Looks like 690.31(C)(2) is a bit more restrictive as compared to 392.30 as 690 requires the cables be secured to the tray, even for hortizontal runs and 392 does not.

Are there any derating rules that will apply to this installation with that many conductors in the tray?
 

Smart $

Esteemed Member
Location
Ohio
...

Are there any derating rules that will apply to this installation with that many conductors in the tray?
I do not see anything in 690 which overrides 392 and 310 ampacity determinations... so it appears we have an ampacity by size and installation method problem. Perhaps we can throw a bundling problem in there as well.
:D
 

Carultch

Senior Member
Location
Massachusetts
I do not see anything in 690 which overrides 392 and 310 ampacity determinations... so it appears we have an ampacity by size and installation method problem. Perhaps we can throw a bundling problem in there as well.
:D
Given that there is no specific algorithm for calculating the ampacity of conductors in a cable tray when allowed for listed PV wire of all sizes, our most defensible strategy is to default to the method for calculating ampacity of the conductors as if they were in a standard closed raceway. Unfortunately, we don't get to take credit for the fact that the cable tray is open to the air, and therefore might make one think that the conductors can have a higher ampacity. Snaketray mentions this in their promotional videos (long before the rule was added to the NEC about listed PV wire being in cable tray), but doesn't go in to details on how it is code compliant to do that calculation.
 
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