Calculating Conductor Temp
Calculating Conductor Temp
(OP)
I have a 225A 277VAC 3 phase lighting panel begin feed by 1 4/0 CU 90C cable per phase. The total measured currect is 168 amps per phase. This is a main breaker panel with 90c lugs.
A resent infrared scan indicated an abolute temp on the cables of 70.1C. The scan photo shows a uniform temp on all three phases.
Two questions:
1. Does this temp at this current alarm anyone?
2. What is the formula to calculate the estimated conductor temp?
Thanks,
Dan
A resent infrared scan indicated an abolute temp on the cables of 70.1C. The scan photo shows a uniform temp on all three phases.
Two questions:
1. Does this temp at this current alarm anyone?
2. What is the formula to calculate the estimated conductor temp?
Thanks,
Dan






RE: Calculating Conductor Temp
RE: Calculating Conductor Temp
70oC seems rather high for conductors carrying only 73% of their rated ampacity. Are the connections all good, tight, and clean?
RE: Calculating Conductor Temp
RE: Calculating Conductor Temp
However, because you stated that this is a lighting panel, I'd also suggest looking into the possibility of harmonic content in the current. If your load is primarily fluorescent lighting harmonic content may be high enough to contribute to your heating problem. Did you take your current measurements with a RMS type meter or an average reading meter?
Every situation is different, but I'd not neglect this possibility.
RE: Calculating Conductor Temp
I will check the current wave form with our Fluke434.
Would the cable be subjected to the same K factor requirements as a transformer?
Dan
RE: Calculating Conductor Temp
quote:
--------------------------------------------------------------------------------
What are the allowable temperature rise limits for circuit breaker parts? Customer is doing thermal imaging.
Answer UL Surface Temperature Rise Limits
Non-metallic 60 deg. C
Metallic 35 deg. C
UL Terminal Temperature Rise
80% Rated CB 50 deg. C
100% Rated CB 60 deg. C
--------------------------------------------------------------------------------
Another question on the same site says that the rise is over a 40 deg. C ambient.
Don
RE: Calculating Conductor Temp
The internal temperature will be a result of the total power dissipated within the enclosure and the ability of that enclosure to dissipate that energy. Harmonics may be a factor, but very often, I find that people do not consider the effect of power dissipated by cables, fuses, circuit breakers etc and commonly find enclosures with very elevated internal temperatures.
If the internal temperature is elevated above 45 degreesC, you will have an accelerated aging of the components within the enclosure and probably premature operation of protection devices.
Best regards,
Mark Empson
http://www.lmphotonics.com