What is De-rating?

[vinod]

Can anybody tell me the use of De-rating in electrical equipment and cables?
:?

 

[Dennis Alwon]

Derating is when you must alter the ampacity of the conductors because of various reasons. You may have to derate because of bundling of conductors together, ambient temp., "X" number of conductors in a raceway and basically anywhere the temperature of the conductor can be increased.

 

[iwire]

It is when you have to look at the rating of say, a conductor and reduce the rating due to the conditions of use.

Say you are bundling a lot of wires together, you would have to reduce the rating of each wire.

Say you are running wires across a roof in the sun, because of the high temperatures on the roof you would have to derate the wires.


Another example would be circuit breakers, if they are installed in high temperature areas the rating of the breaker drops.

 

[ggunn]

What they said. I'll just add that the derating of conductor ampacity is for the preservation of the insulation, not for the wires themselves.

 

[iwire]

I'll just add that the derating of conductor ampacity is for the preservation of the insulation, not for the wires themselves.

A great point, if it was not for the insulation and termination ratings the wires themselves could carry much more current.

 

[barclayd]

Transformers also have to be derated for altitude. Some mechanical equipment, too - chillers, cooling towers, etc.
db

 

[pete m.]

Transformers also have to be derated for altitude.

You got me on this one (I'm a flatlander) what is the reasoning? Does it have anything to do with atmospheric pressure at higher elevations in regards to heat dissipation?

Pete

 

[K8MHZ]

You got me on this one (I'm a flatlander) what is the reasoning? Does it have anything to do with atmospheric pressure at higher elevations in regards to heat dissipation?

Pete

I've never heard of that, either.

(We are about 600 feet above sea level)

 

[iwire]

Does it have anything to do with atmospheric pressure at higher elevations in regards to heat dissipation?

I assume it is entirely that.

 

[charlie b]

I would say so too. The KVA rating of any component is based on its ability to reject heat to its surroundings. At higher altitudes, there is less air, and that makes it harder for a transformer to reject heat to the surrounding air.

 

[pete m.]

I guess this is something the NEC looks at as a design consideration because I cannot recall anything in the NEC that even mentions elevations and derating for transformers or other equipment for that matter.

Pete

 

[barclayd]

Yes - heat dissipation. Mostly due to 'thinner air'. It doesn't kick in untill about 3000', and doesn't really matter much untill you get up to about 8000'. In the Denver area, we don't worry about it too much, but the mechanical engineers who spec the chillers & cooling towers always add the altitude to their requirements. I believe diesel generators also have an altitude adjustment.
db

 

[pete m.]

I would say so too. The KVA rating of any component is based on its ability to reject heat to its surroundings. At higher altitudes, there is less air, and that makes it harder for a transformer to reject heat to the surrounding air.

Makes sense. :thumbsup:

Pete

 

[pete m.]

Yes - heat dissipation. Mostly due to 'thinner air'. It doesn't kick in untill about 3000', and doesn't really matter much untill you get up to about 8000'. In the Denver area, we don't worry about it too much, but the mechanical engineers who spec the chillers & cooling towers always add the altitude to their requirements. I believe diesel generators also have an altitude adjustment.
db

Thanks for the info!

Pete

 

[iwire]

I guess this is something the NEC looks at as a design consideration because I cannot recall anything in the NEC that even mentions elevations and derating for transformers or other equipment for that matter.

The NEC tells us to use equipment within it's ratings. The manufacturers determine the ratings.

 

[pete m.]

The NEC tells us to use equipment within it's ratings. The manufacturers determine the ratings.

Good point.

I will have to look closer at the manufacturers instructions on transformer ratings... I just don't recall ever seeing a requirement (even from a manufacturer) for adjusting the kVa rating for a transformer dependent on elevation. Could very well be on every one and I have never paid attention to it due to a mean elevation in Ohio of 850 feet.

Pete

 

[barclayd]

I found some old notes -
Nameplate is good up to 3300 feet.
Derate 0.3% for each additional 300 feet.
I think these are manufacturer's numbers, but I had it written in a little notebook I keep for cheating.
db

 

[iwire]

I will have to look closer at the manufacturers instructions on transformer ratings...

They come with instructions?:jawdrop:

I just don't recall ever seeing a requirement

I can't say I have either.

 

[pete m.]

They come with instructions?:jawdrop:

Ok... I admit it. I walked right into that one. :lol:

Pete

 

[vinod]

Gentlemen,

Thanks a lot for all your response. I receive more than my expectation.. Thanks...