Voltage drop field measurements?

[JoeNorm]

Hi All,

I have a simple question. On a long run of wire can you measure VD without a load? For instance if you measured voltage at a transformer and then measure voltage 400 feet away is the voltage difference actual VD or do you need to apply a load first to see anything significant?

thanks

 

[Rock86]

Well... you will have lose from the resistivity of the conductors, but it may be so low that you think its no big deal. Once you apply the load however, you will notice a greater difference. Play around with some calculations and see what you get in theory.

Just as a quick example from the 2020 NEC; Using VD calculation Example #1 from Table 9 (Ch.9),
No load would give you a VD of .024V at 150ft (L to N), but add a 100A load, and now you have a VD of 2.4V.

 

[Strathead]

E=I*R with 0 amps, and anywhere from .0000000000000000001 to 1,000,000 ohms of resistance, you have E=0*resistance, so no since anything times zero is zero. The reason I explained this to you is because voltage drop is 100% dependent on the load. The bigger the load, the bigger the voltage drop, with all other factors remaining the same.

 

[LarryFine]

The only voltage drop would be that caused by the load current the meter itself.

Voltmeters are usually designed to have minimal effect on the circuit under test.

 

[JoeNorm]

thanks. Thats what I thought just wanted to be sure. Sometimes I measure unexpected voltage differences even when no load is applied. I'm curious why this happens at certain locations.

 

[LarryFine]

thanks. Thats what I thought just wanted to be sure. Sometimes I measure unexpected voltage differences even when no load is applied. I'm curious why this happens at certain locations.

It could be that one conductor is actually energized, and another only has phantom voltage, which even a high-impedance meter can load enough to cause a lower reading. That's why I use a solenoid tester for troubleshooting.