metropower
Electrical
does the table in 310.16 referencing conductors installed in free air apply to the interior of an unventilated motor starter panel, if the conductors are not installed in wiring duct.
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industrial control panel wire ampacity 1
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davidbeach
Electrical
Probably not, doesn't sound like free air does it?
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metropower
Electrical
thanks for reply, the obvious nature of this question was apparent to myself but not to a supposedly ul approved mfg who built said panel. if any proof to this interpretation please let me know.
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From UL508A - Industrial Control Panels
Internal wiring of a power circuit shall not be smaller than 14 AWG and shall be determined by .....(lots of stuff left out here)...UL table 28.1.
As it happens, table 28.1 is very similar, but not identical to, NEC table 310.15(B)(16), which is for conductors in a raceway, not free air.
If the panel is unventilated, you also have the issue to derating due to temperature.
EE
Internal wiring of a power circuit shall not be smaller than 14 AWG and shall be determined by .....(lots of stuff left out here)...UL table 28.1.
As it happens, table 28.1 is very similar, but not identical to, NEC table 310.15(B)(16), which is for conductors in a raceway, not free air.
If the panel is unventilated, you also have the issue to derating due to temperature.
EE
I agree with eeprom. You have to appreciate the inner air temperature.
According to ABB Switchgear Manual ch. "Temperature rise in enclosed switchgear cubicles" dT=Pv/a/Am where : dT= temperature difference above the ambient [oC]
Pv=power loss with allowance for load factor
[allowance factor= 0.5-0.8 usually]
a= Heat transfer coefficient max 6 min 3 w/m^2/K
Am=heat-dissipating surface area of the cubicle.
The lateral all-around surface and the roof surface[m^2].
See also:
For heat loss from different electrical devices see:
According to ABB Switchgear Manual ch. "Temperature rise in enclosed switchgear cubicles" dT=Pv/a/Am where : dT= temperature difference above the ambient [oC]
Pv=power loss with allowance for load factor
[allowance factor= 0.5-0.8 usually]
a= Heat transfer coefficient max 6 min 3 w/m^2/K
Am=heat-dissipating surface area of the cubicle.
The lateral all-around surface and the roof surface[m^2].
See also:
For heat loss from different electrical devices see:
There has always been a running conflict with the tables in UL508A and the NEC. I believe UL realigned their numbers a little while back, it used to be a lot worse when I started in this business 30 years ago. The real common issue came up regarding cables used inside of a control panel or MCC bucket being smaller than those required by the contractor to connect to the motor. When I worked for an MCC mfr, invariably once or twice a year a local inspector would red-flag an MCC saying the cables were too small, and we would have to pull out the UL845 charts (actually the UL508 charts) to prove to him that we were OK and the contractor was not responsible for how we built the gear internally.
Another issue in the difference is that (IIRC) UL508A requires that the wires be rated 90degC and can thus be sized that way, whereas most of the common tables used in the NEC for installations are for 75degC.
But you have to remember, the NEC is for an INSTALLATION, UL508A is for an ASSEMBLY. If you have a UL508A listed panel, the wiring inside of it has ALREADY been approved by UL (theoretically) and from an NEC standpoint, the contractor installing it need not be concerned. If you were assembling your own panel in the field without the benefit of getting a UL508A listing however, then the NEC rules would apply.
But I think we have gone back and forth on this in this forum several times and I don't think there is a consensus. The only real consensus is that the AHJ is the final authority and can, if he insists, require the internal wiring to be changed even after being shown that it's acceptable under the different rules. It happens.
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Another issue in the difference is that (IIRC) UL508A requires that the wires be rated 90degC and can thus be sized that way, whereas most of the common tables used in the NEC for installations are for 75degC.
But you have to remember, the NEC is for an INSTALLATION, UL508A is for an ASSEMBLY. If you have a UL508A listed panel, the wiring inside of it has ALREADY been approved by UL (theoretically) and from an NEC standpoint, the contractor installing it need not be concerned. If you were assembling your own panel in the field without the benefit of getting a UL508A listing however, then the NEC rules would apply.
But I think we have gone back and forth on this in this forum several times and I don't think there is a consensus. The only real consensus is that the AHJ is the final authority and can, if he insists, require the internal wiring to be changed even after being shown that it's acceptable under the different rules. It happens.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376
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