Voltage drop vs derating

[rojodo3030]

So something i am having trouble grasping...

When sizing wire do I need to calculate for voltage drop, AND derating factors? Here is my thought process: derating is so the wire won't heat up beyond its insulation capabilities. Voltage drop is so the wire carries enough power without losing voltage. However, if you size a wire to compensate for voltage drop, it will still have the same resistance value than a smaller, but also shorter wire run of the same ampacity. This will cause the same heating effect no? In which case it seems logical to calculate your voltage drop, then calculate your derating factors for your final wire size.

 

[infinity]

You do both calculations and then choose the conductors based on whichever method gives you the larger conductors. Just a note the derating calculation is one required by the NEC, VD compensation generally is not.

 

[rojodo3030]

You do both calculations and then choose the conductors based on whichever method gives you the larger conductors. Just a note the derating calculation is one required by the NEC, VD compensation generally is not.

I appreciate your input. Now, another question, will a larger wire generate the same amount of heat as a smaller wire if they both had the same resistance (due to the larger one being longer). The reason I am asking is because if so, it would make sense to factor in both calculations.

 

[wwhitney]

I appreciate your input. Now, another question, will a larger wire generate the same amount of heat as a smaller wire if they both had the same resistance (due to the larger one being longer).

Total heat (per unit time)? Yes. But what matters is heat per unit length (per unit time), as the wire's ability to reject heat will depend on its length as well. And then the larger wire will generate less heat per unit length (per unit time) for a given current.

So upsizing the wire can simultaneously satisfy derating requirements and voltage drop requirements, and there is no reason to try to "stack" two different upsizing requirements. Just calculate the minimum size for each effect separately, and as infinity says, pick the larger of the two sizes.

Cheers, Wayne

 

[rojodo3030]

Total heat (per unit time)? Yes. But what matters is heat per unit length (per unit time), as the wire's ability to reject heat will depend on its length as well. And then the larger wire will generate less heat per unit length (per unit time) for a given current.

So upsizing the wire can simultaneously satisfy derating requirements and voltage drop requirements, and there is no reason to try to "stack" two different upsizing requirements. Just calculate the minimum size for each effect separately, and as infinity says, pick the larger of the two sizes.

Cheers, Wayne

This makes sense. Thank you.

 

[ggunn]

I appreciate your input. Now, another question, will a larger wire generate the same amount of heat as a smaller wire if they both had the same resistance (due to the larger one being longer). The reason I am asking is because if so, it would make sense to factor in both calculations.

First I use derating and terminal temperature to establish the minimum size wire that protects the insulation and terminals from overheating, and then I calculate the voltage drop to see if the wire needs upsizing; it can never be smaller. All three of these calculations set a lower bound on the wire size; the largest minimum wire size is the one to use.

 

[wwhitney]

All three of these calculations set a lower bound on the wire size; the largest minimum wire size is the one to use.

There's one more check that occasionally controls for continuous loads, namely 240.4.

Cheers, Wayne