[RE-wrenches] cable trays and battery cable

[Mick Abraham]

Hi, Mechanix~

Kirk Herander asked about cable trays & basing the wire calculations as if
the wires are in "free air". The 2023 Article 100 definition of Free Air is
unchanged from the 2020 NEC:

2023: "Open or ventilated environment that allows for heat dissipation and
air flow around an installed conductor." However at the 2020 sessions for
CMP6 Code Making Panel 6, the original draft language was:

2020 first draft: "Open or ventilated environment that allows for air flow
around an installed conductor with no other conductors closer than one
cable diameter and no insulation or building materials in contact with the
conductor insulation for more than 75 mm (3 in.) for every 3 m (10 ft) of
conductor." This is not the letter of the law since it was changed in the
second draft but it does show the thinking process.

I specified some cable tray recently which was "solid" on the sides &
bottom (unlike some other types that are perforated or ladder style). This
tray was for AC wires & the electricians did support the wires so there's ~
one cable diameter of air space around each insulated conductor. There's no
forced air ventilation on this project; only natural convection via the
open top of the trays. The wires were sized for "free air" conditions &
there was no pushback from the AHJ. As with so many cases, this goes to the
individual AHJ's interpretation, but the air-gaps seem more appropriate
(compared to laying the wires in a heap) if one wants the installation to
be interpreted as "free air", especially when the tray sides & bottom
surface are solid as in this example.

I hope this will be helpful; the Wrench List is the Bomb!

Mick Abraham, Proprietor
www.abrahamsolar.com

Landline: 970-731-4675
Cell phone or for text messaging: 970-946-6584
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