Voltage drop calculators

[wirebender]

I notice that with voltage drop calculators (Southwire's for Iphone and one I found here on my computer) use the 60 degree rating for the wire. Is this necessary?
If it calls for #8 @40amps, can I use #10 instead?

Also, if you have three equal 120 volt loads, can you use 3 phase in your calcutlation?

Example:

3 LED lighting loads @ 15 amps 300 feet.

If I use 3 phase 208 @ 5% drop it calls for #10 @30 amps. Wouldn't #12 work?

If I use single phase 120 @ 5% drop it calls for #6 @55 amps.

What is the proper method?

 

[steve66]

I notice that with voltage drop calculators (Southwire's for Iphone and one I found here on my computer) use the 60 degree rating for the wire. Is this necessary?

It depends on what the wire is rated for. I believe NM cable is rated for 60 deg., so if you are running NM cable, yes, you should use the 60 deg rating.

If it calls for #8 @40amps, can I use #10 instead?

#10 is generally limited to 30 amps, sometimes 35 amps. Not 40, even before you consider voltage drop.

Also, if you have three equal 120 volt loads, can you use 3 phase in your calcutlation?

Example:

3 LED lighting loads @ 15 amps 300 feet.

If I use 3 phase 208 @ 5% drop it calls for #10 @30 amps. Wouldn't #12 work?

If I use single phase 120 @ 5% drop it calls for #6 @55 amps.

What is the proper method?

It depends if you are sharing the neutral wire, and how long the run is before the loads combine.

Voltage drop depends on distances. You didn't list any distances, so we can't tell you what size wires you need.

But #12 can NOT be used on a 30 amp circuit.

 

[don_resqcapt19]

I notice that with voltage drop calculators (Southwire's for Iphone and one I found here on my computer) use the 60 degree rating for the wire. Is this necessary?
If it calls for #8 @40amps, can I use #10 instead?

Also, if you have three equal 120 volt loads, can you use 3 phase in your calcutlation?

Example:

3 LED lighting loads @ 15 amps 300 feet.

If I use 3 phase 208 @ 5% drop it calls for #10 @30 amps. Wouldn't #12 work?

The calculator I use shows that 4 wire, 120/208Y circuit will give you 6.7% drop using #12 and a 4.1% drop using #10. This is assuming that all of the loads are at the same location. If they are not, you need to do additional calculations starting at the last point where all 3 circuits are together.

If I use single phase 120 @ 5% drop it calls for #6 @55 amps.

The calculator I use shows a drop of 5.1% using #8 on a two wire 120 volt circuit for this load, or 3.2% if you use #6.

What is the proper method?

If you are using the 4 wire circuit then use that calculation and if you are using a two wire circuit use that calculation. Don't forget about the rule in 250.122 that will require you to also increase the size of the Equipment Grounding Conductor for this application. If the circuit rating is 30 amps or less the EGC will be required to be the same size as the circuit conductors.

I don't really see where the temperature rating of the conductor enters into the voltage drop calculation. That calculation is based only on the impedance of the conductive path and the temperature rating of the conductor does not even enter into that calculation. You only have to look at the temperature rating of the conductor for the current flow through that conductor.

 

[wirebender]

I don't really see where the temperature rating of the conductor enters into the voltage drop calculation. That calculation is based only on the impedance of the conductive path and the temperature rating of the conductor does not even enter into that calculation. You only have to look at the temperature rating of the conductor for the current flow through that conductor.

That's what i thought.


Thanks, everyone.

 

[ggunn]

I don't really see where the temperature rating of the conductor enters into the voltage drop calculation. That calculation is based only on the impedance of the conductive path and the temperature rating of the conductor does not even enter into that calculation. You only have to look at the temperature rating of the conductor for the current flow through that conductor.

Right. Calculate wire size based on ampacity (which is all about temperature rating) and again using the acceptable voltage drop, and then use the larger of the two.

 

[Transportation Guy]

Don't forget about 240.4 (D) protection of small conductors. Just because 310.16 shows ampacities, there are exceptions and further limitations. (example - #10 THHN copper is good for 40 AMPS but 240.4 (D) states you cannot protect it with more than a 30AMP OCPD)