How do you explain to someone that conductor amapacity adjustment is based on the insulation rating of the conductor and not the temperature limitation of a terminal? Conductor ampacity and terminal limitations are different but tough to explain to someone for some reason.
Temperature limitation of terminations and conductor ampacity
- Thread starter cppoly
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Almost all chemical reactions go faster with increasing temperature and probably the aging of materials is considered to be a chemical reaction.
Approx. one hundred years ago some guy found that the life of motor insulation halves with each 8C to 10C increase in temperature, so Romex lasting 50 years at 30C will only last ~25 years at 40C.
Approx. one hundred years ago some guy found that the life of motor insulation halves with each 8C to 10C increase in temperature, so Romex lasting 50 years at 30C will only last ~25 years at 40C.
Speedskater
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- retired broadcast, audio and industrial R&D engineering
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From the NEC:
This is what I understand it as:
I would first start with if the conductors where within the same ambient temperature as the OCPD them the simple answer would be that the wire is sized per the 75deg column of table 310-16 unless 60deg applies.
Should that wire run through an are that is exposed to a higher ambient temperature then the wire rating must be adjusted to compensate accordingly either by increasing the wire size or a higher insulation class selected. The objective is to assure that the wire's temperature never exceeds its insulation rating. It is also very common for wirers to be derated because of the number that there are in a raceway because of the same reasoning.
As such you start with the OCPD terminal rating but derating the wire may also be required due to where and how it is run.
Along that same line, conductors get bundled in raceways, but when they hit terminals the conductors are no longer bundled. So the bundling derating factor goes away at terminations but not forther down in the raceway. And as templdl said, there could be sections of raceways in a high heat area, such as a roof, that doesn't esxist at the terminations.
So there are two separate issues to check and correct for -- termination rating, and thermal derating due to bundling and ambient temps.
So there are two separate issues to check and correct for -- termination rating, and thermal derating due to bundling and ambient temps.
That should be all the evidence I need.
Sorry, this is what I meant to quote. How come I can't edit my post?
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This info is for all, there is a time limit to edits, this is for everyone's benefit.
If there comes a time anyone really need an edit done you can send a PM to any moderator and ask them to make the edits.
We do ampacity adjustments to make sure the capacity of the insulation to withstand heat is not exceeded. Obviously these adjustments should start with the rating of the insulation. The temperature limitation of the terminals is there to make sure that the temperature of the conductors does not exceed the capacity of the equipment to withstand that heat. The rating of the insulation doesn't affect the ability of the equipment to withstand heat, and the rating of the terminals does not affect the ability of the insulation to withstand heat. These are two separate considerations. We do our ampacity adjustments to see how much current we can run and not damage the insulation, then we check the rating of the terminals and take the lesser of the the two results so that neither the equipment nor the insulation will be damaged.
kwired
Electron manager
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- NE Nebraska
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I like this answer the best. Other replies give us some supporting evidence to this answer.
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