Derating MC for Garage Temp Use

Status
Not open for further replies.
E

euclid43

Senior Member
Wondering if I can accept a ratio of higher than 20 amp (#12 gauge) for a 240 v heater. According to 310.60 it appears that I get 1.18 multiplier. The heater will only be in operation in cold temperatures, but will eventually reach 70 degree F. The heat unit has 20.75 as nameplate.
 
Does derating apply to cold temp (garage) installs? There's a list of MC cable rules, but I'm not sure if all, some, or one stipulation makes derating not applicable. For some reason, I imagined a "big" duct heat unit heater as the space heater definition, not a blowing infra-red unit with dissipating fan and thermostat.
 
The only way you would get away with a larger breaker on heating equipment with fans is if the nameplate calls for max HACR circuit breaker size let’s say of 30a but minimum ampacity is listed on the nameplate as under 20.

Most derating occurs with 8awg or bigger wire. Just run #10 and be done with it.


Sent from my iPhone using Tapatalk
 
For the same reason we have to derate with rooftop installs, I wonder if we can derate with cold environments, that's all.
 
blkmagik21 said:
Most derating occurs with 8awg or bigger wire. Just run #10 and be done with it.
Click to expand...
:huh:

Ampacity adjustments apply to all size conductors. Number of conductors in a raceway or bundle of cables and ambient temperature are what determines when adjustments apply.
 
euclid43 said:
Well, just knowing that the only time the heater operates is during cold times.
Click to expand...
Cold ambient can allow adjustments that actually increase ampacity. But as has been mentioned 240.4(D)(5) still puts a 20 amp limit on 12 AWG, and also as mentioned your 20.75 amp nameplate likely needs 125% factor applied which would give you a result of 26 amp conductor being minimum needed. If that 20.75 is actually marked MCA and there are also motors or compressors supplied then 240.4(G) sends you to 422, 430 or 440 for overcurrent protection and you may be able to use 12 AWG for any of those applications.
 
How would you apply a cold ambient factor (to increase conductor ampacity) to a circuit?
 
infinity said:
How would you apply a cold ambient factor (to increase conductor ampacity) to a circuit?
Click to expand...
Same way you reduce conductor ampacity if the ambient temp is high. T310.15(B)(16) is based on 30C ambient. If temp is higher or lower adjust accordingly with 310.15(B)(2)(a). Many forget that for temp below 30C there is adjustment factors greater than 1.0.
 
Status
Not open for further replies.
Top