Voltage Drop when using the 90 vs 75 Column

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e-medic

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When running more amperage on a circuit as allowed using the 90 degree column on NEC table 310.16, the voltage drop formula would require that I run the wire a shorter distance than if I sized using the 75 degree column. Is this correct?

Thanks,
Preston
 
You are not allowed to use the 90? column unless your terminations are also rated for 90?, which they likely aren't. I would recommend using the 75? column for the voltage drop calculation.
 
e-medic said:
When running more amperage on a circuit as allowed using the 90 degree column on NEC table 310.16, the voltage drop formula would require that I run the wire a shorter distance than if I sized using the 75 degree column. Is this correct?

Thanks,
Preston
Click to expand...

I don't believe so. Voltage drop in its simple form is just a calculation of the resistance in the wire to amperage.
 
Resistance rises with temperature, unless operating within thermal limits of a superconductor. I won't ask for that 90c superconductor.
 
Drop It

Drop It

Single phase voltage drop:

Vd = 2K x L x I / Cm
For both examples: Vd = voltage drop, K= 12.9 for resistance for a copper conductor, L = length of circuit in ft., I = current or amps of the load, Cm = area of the conductor in Circular Mills.

Three phase voltage drop:

Vd = 1.73K x L x I / Cm
The first examples give you the voltage drop if you have the length of the conductor, the current draw of the load in amps and the size of the wire in Circular Mils.
 
Dennis Alwon said:
Single phase voltage drop:

Vd = 2K x L x I / Cm
For both examples: Vd = voltage drop, K= 12.9 for resistance for a copper conductor
Click to expand...

In the above equation, 'K' is the resistance of a copper wire 1 foot long and 1 circular mil in cross section.

This resistance changes with temperature.

For different temperatures, the value of K for copper is:

20C -> 10.1
40C -> 11.5
60C -> 12.9
75C -> 13.9
90C -> 14.9

(For reference, I used the values for copper resistance from http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/rstiv.html#c1 scaled for circular mills and feet rather than meters)

-Jon
 
winnie said:
In the above equation, 'K' is the resistance of a copper wire 1 foot long and 1 circular mil in cross section.

This resistance changes with temperature.

For different temperatures, the value of K for copper is:

20C -> 10.1
40C -> 11.5
60C -> 12.9
75C -> 13.9
90C -> 14.9

(For reference, I used the values for copper resistance from http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/rstiv.html#c1 scaled for circular mills and feet rather than meters)

-Jon
Click to expand...
This is true but that fact that you use 90C insulation on the conductor does not change the VD formula. I guess that was the point I was trying to make. The ambient temp. may affect it but not the insulation itself.
 
I think this is a bit of a trick question. The question boils down to "If I have more current in a wire of constant size, is the voltage drop affected?" The answer is "Yes, because voltage drop is directly proportional to current."
 
Dennis Alwon said:
This is true but that fact that you use 90C insulation on the conductor does not change the VD formula. I guess that was the point I was trying to make. The ambient temp. may affect it but not the insulation itself.
Click to expand...

It is quite true that the temperature rating of the _insulation_ will not change the VD that you calculate

But what will change the VD that you calculate is the _actual_ temperature of the conductor.

In general, the more current that you push through the conductor, the hotter it will get, and this self heating, as well as heating caused by adjacent conductors, _and_ ambient temperature, will have a bearing on the temperature of the conductor.

For very long runs, where you've upsized the conductors to reduce voltage drop, there will be minimal self heating, and the conductor temperature will pretty much be ambient. The K=12.9 number will overestimate VD.

For 'value engineering' where you are pushing the most current through a conductor possible, say using 100% rated breakers and 90C conductors, there will be lots of self heating and the K=12.9 might underestimate VD.

-Jon
 
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