Double Derating
- Thread starter blues
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hardworkingstiff
Senior Member
- Location
- Wilmington, NC
Just calculate the voltage drop with the up-sized conductors, then make your decision.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
You never "derate" for voltage drop. The word "derate" does not apply to voltage drop considerations.
To "derate" means that you have to change the numerical value of the ampacity of a conductor. With three #8 THW conductors in a conduit, for example, and at a 25C ambient, each conductor has an ampacity of 50 amps. Put four #8 THW current-carrying conductors in the same conduit in the same 25C ambient, and each conductor will have an ampacity of 40 amps. You "derated" the conductor itself. The maximum amount of current it is allow to carry has been reduced. That has nothing to do with the amount of current you actually plan to put through it, nor with the impact of that current on the voltage at the receiving end.
If you decide to use a larger size conductor because you are concerned with voltage drop, you are not changing the amount of current any given conductor is allowed to carry. So you are not "derating" the conductor.
To "derate" means that you have to change the numerical value of the ampacity of a conductor. With three #8 THW conductors in a conduit, for example, and at a 25C ambient, each conductor has an ampacity of 50 amps. Put four #8 THW current-carrying conductors in the same conduit in the same 25C ambient, and each conductor will have an ampacity of 40 amps. You "derated" the conductor itself. The maximum amount of current it is allow to carry has been reduced. That has nothing to do with the amount of current you actually plan to put through it, nor with the impact of that current on the voltage at the receiving end.
If you decide to use a larger size conductor because you are concerned with voltage drop, you are not changing the amount of current any given conductor is allowed to carry. So you are not "derating" the conductor.
bphgravity
Senior Member
- Location
- Florida
I like Charlie's explanation.
I hear the term "double derating" a lot from students and contractors. I'm not sure why this misconception exists and persists. They must assume if you make a correction factor to a conductor it also covers any adjustments you may needed to make and the other way around. I guess I can see why one would make that determination without working a real life problem through.
I hear the term "double derating" a lot from students and contractors. I'm not sure why this misconception exists and persists. They must assume if you make a correction factor to a conductor it also covers any adjustments you may needed to make and the other way around. I guess I can see why one would make that determination without working a real life problem through.
jjs
Member
- Location
- Puryear, TN, USA
When you do your voltage drop calculations, you should use the resistance of the conductor being installed in the conditions being installed. So if you are derating for heat of space, then when you do your voltage drop calculations, you should be using the resistance of the conductor you chose above at the heat you chose above.
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