Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?
Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?
(OP)
Sometimes I hate the NEC.
So I know that if I have a ladder or ventilated tray that has a SOLID cover top, I need to de-rate by almost 60% or so.
But the NEC rules don't cover the scenario of a ventilated cable tray used with a VENTILATED tray cover. In this scenario, can I use free-air ratings? Ofcourse I'll be maintaining 1OD separation between cables.
So I know that if I have a ladder or ventilated tray that has a SOLID cover top, I need to de-rate by almost 60% or so.
But the NEC rules don't cover the scenario of a ventilated cable tray used with a VENTILATED tray cover. In this scenario, can I use free-air ratings? Ofcourse I'll be maintaining 1OD separation between cables.
RE: Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?
Converting energy to motion for more than half a century
RE: Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?
ANSI/NEMA WC 51-2009/ICEA P-54-440-2009
AMPACITIES OF CABLES INSTALLED IN CABLE TRAYS
3.4 CORRECTION FACTORS FOR TRAY COVERS
The following table provides ampacity correction factors for trays employing covers. The adjustment factors represent worst case conditions where the air gaps between the cable mass and cover are greater than ½ inch. For an air gap spacing less than ½ inch the thermal resistance of the air gap decreases dramatically resulting in an increase in the multiplier.
From Table 3-3 the derating factor is about 80-85%. Ventilated Flanged may be 82-87% but it depends on many other conditions and NEC is a better source, in my opinion.
RE: Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?
@gr8blu: which standard are you referring to?
@7anoter4: I went through the NEMA standard, it looks like all the equations/etc assume a fully stacked (touching side-by-side) and no adjustment factors for 'spacing' between cables. or am i misreading it?
RE: Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?
If we take 1 copper conductor 600 V/90oC insulation 40oC NEC[full 24" width solid bottom tray] 25 cables, not spaced, according to Art.392.80.2.b,Tb 310.15.B.17, Tb 310.15.B.2 uncovered:414 A;solid unventilated cover: 382 A and ANSI WC51 uncovered:427 A ; covered: 333 A. However, NEC is the law.
RE: Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?
ANSI WC51 consider more than one row but not less.
RE: Ventilated Tray + Ventilated Covers = Free-Air Rating or De-rate?
However, maintaining spaces in solid bottom cable tray is a problem -we need a frame all 3 feet to maintain the distances.