Cable tray fill rate NFPA70

According to NFPA 70 Clause 392.9, the following definition can be read ;
- 392.9 Clause (B) Multiconductor Control and/or Signal Calbes only- a ladder or ventilated trough cable tray ----> cable fill rate of such tray is defined as less than 50%
- 392.9 Clause (D) Solid bottom cable tray- Multiconductor control and/or Signal cables only ---> cable fill rate of such tray is defined as less than 40%

In my thinking, solid bottom type ( such as instrument Duct ) is capable of having a little higher fill rate than ladder tray case which have 'rungs' with another occupancy inside ladder tray.

Could anyone please let me know why a ladder type tray fill rate is higher than solid bottom case ?
Thanks a lot in advance.
Jasoncha. . .
This argument has gone on for years. I've attached a paper by Newton Instrument Co. suggesting that Article 392.9(B) not be applied. You can tell by its date no one is listening ! I believe the dispute goes on because many Government and Manufacturing organizations like OSHA, FEMA, NEMA, NEC, to name a few, are involved.
Regards, Phil Corso


My two cents:
The solid bottom tray does not allow for the same level of conductor cooling thus the less than 40% rating.
Seems odd to me also, but I'm not the rulemaker.
I've seen several tray fires, often caused by way over filled trays, and not pretty.
But, Clauses 392.9(B) and 392.9(D) refer to Multiconductor Control and/or Signal Cables only ! Not conductors or wires carrying significant heat-producing currents.

Regarding your experience, I never witnessed a cable-tray fire, but I did investigate a situation where many thermal-couple wires melted in a 4" steel conduit. No, not because of current, but because it was too close to a 12" steam-line that had no insulation. The plant's designers were sure they met the "it doesn't meet the need for costly insulation" requirement !

Yep, same LNG plant I talked about earlier on this forum! I'm sure many of you have heard the phrase M.T.B.F., Mean-Time Before-Failure . Well, in a report I actually wrote, M.B.T.F. stood for Mean-Time-Before-Flare !

You all might find this next tidbit interesting. I was assigned to this plant for a 3-month start-up. Knew it would take 5 months... but was there for 7-years, yes, years! !
Phil Corso
BTW, if any of you younger forum participants want to learn about real plant experiences I urge you to search this forum's Archives for "BITE Stories", which stands for, "Believe It, True Event".