Testing voltage drop

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Stevenfyeager

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United States, Indiana
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electrical contractor
Can I test voltage drop when no load is on the circuit? I have a 20 amp 120 volt circuit from a utility pole on a 120 ft 10-2 UF then continuing on with a 100 ft #12 extension cord for a new house construction. I tested that 220 ft length with my volt meter and showed no voltage drop compared to the utility pole. What will happen when the framers start using their compressor, etc? Thank you
 
No-
Ohm's law: E = I R

You can't have the "E" (voltage drop) in this case, without the "I" (current). You can, however, use one of the many online voltage drop calculators; they'll give you a decent reading.

200' of 12g at 10 amps will have a drop of around 8 volts.
 
zbang said:
No-
Ohm's law: E = I R

You can't have the "E" (voltage drop) in this case, without the "I" (current). You can, however, use one of the many online voltage drop calculators; they'll give you a decent reading.

200' of 12g at 10 amps will have a drop of around 8 volts.
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That's 6 %. I haven't gone to one of the charts yet. Do you think my #10 will help any ?
 
180928-1311 EDT

Stevenfyeager:

To answer your question. Yes.

How to do it. Get a Kill-A-Watt EZ, and a 1500 W space heater. Plug in the Kill-A-Watt at the cable end, and read the unloaded voltage. Next connect the heater to the Kill-A-Watt, load the cable with the heater on maximum heat (no thermostat function), and read voltage and current. If the heater has an on-off switch, then the heater can be plugged into the Kill-A-Watt before starting the test.

If you want the voltage drop at some other current level, then scale as needed. A typical 1500 W heater will typically be around 10 to 12 A at 120 V.

This technique can be used as a general test method to look for, and isolate the location of bad connections in branch circuits.

.
 
gar said:
180928-1311 EDT

Stevenfyeager:

To answer your question. Yes.

How to do it. Get a Kill-A-Watt EZ, and a 1500 W space heater. Plug in the Kill-A-Watt at the cable end, and read the unloaded voltage. Next connect the heater to the Kill-A-Watt, load the cable with the heater on maximum heat (no thermostat function), and read voltage and current. If the heater has an on-off switch, then the heater can be plugged into the Kill-A-Watt before starting the test.

If you want the voltage drop at some other current level, then scale as needed. A typical 1500 W heater will typically be around 10 to 12 A at 120 V.

This technique can be used as a general test method to look for, and isolate the location of bad connections in branch circuits.

.
Click to expand...

Do you have access to the nameplate on the motor of the compressor. It will tell you the starting amps. Use that number for your voltage drop calc.
 
180928-2012 EDT

zbang:

In the original post the statement was
Can I test voltage drop when no load is on the circuit?
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I interpreted this to mean the normal or expected load. Rather than it excluding any load.

But this does not matter. I will rearrange the circuit slightly.

Instead disconnect the hot wire at the breaker, short hot and neutral wires at the destination point, apply a known current to this wire loop, and measure the voltage drop across the shorted loop.

How do I get that current? Use my above heater. Really the same circuit as before, just rearranged.

If you want to call my far end short a load, then I will create a different circuit.

All the suggestions on how to calculate the resistance is probably an easier approach.

.
 
gar said:
180928-1311 EDT

Stevenfyeager:

To answer your question. Yes.

How to do it. Get a Kill-A-Watt EZ, and a 1500 W space heater. Plug in the Kill-A-Watt at the cable end, and read the unloaded voltage. Next connect the heater to the Kill-A-Watt, load the cable with the heater on maximum heat (no thermostat function), and read voltage and current. If the heater has an on-off switch, then the heater can be plugged into the Kill-A-Watt before starting the test.

If you want the voltage drop at some other current level, then scale as needed. A typical 1500 W heater will typically be around 10 to 12 A at 120 V.

This technique can be used as a general test method to look for, and isolate the location of bad connections in branch circuits.

.
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He asked if he can test voltage drop with no load on the circuit,

You said yes, but then told him to put a load on the circuit.
 
kwired said:
He asked if he can test voltage drop with no load on the circuit,

You said yes, but then told him to put a load on the circuit.
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Sorta. Gar is describing a self-made test meter. They make meters for testing voltage drop but they are also loads.

Any means to test voltage drop will require a load of some size. Gar's tester is just bigger, simple, and readily available.
 
I had assumed the OP meant without the expected load on the circuit. The test then becomes simulating the load, whether it be a tester that does this, or a procedure...

Or just math...
 
MAC702 said:
I had assumed the OP meant without the expected load on the circuit. The test then becomes simulating the load, whether it be a tester that does this, or a procedure...

Or just math...
Click to expand...
The math will do it...and quite well if you model the load correctly and use the exact formula.
 
Simply put, there is no voltage drop without a load. It can be calculated, which is done all the time.
 
Stevenfyeager said:
Can I test voltage drop when no load is on the circuit? I have a 20 amp 120 volt circuit from a utility pole on a 120 ft 10-2 UF then continuing on with a 100 ft #12 extension cord for a new house construction. I tested that 220 ft length with my volt meter and showed no voltage drop compared to the utility pole. What will happen when the framers start using their compressor, etc? Thank you
Click to expand...

Sure, stick a voltage meter at both ends, subtract one from the other, that's the no-load voltage drop.

2nd method is to remove the line wire from OCPD and short it to N bar, then go out to end of your BC and measure DC ohms, then you'll have exact ckt ohms # that you can dump into an Excel equation and graph voltage vs amps, etc. If the wiring is all the same size then the voltage drop at any point on the BC is linear w/ distance (per foot or inch or mm or cm, etc). If you have say outlets across the whole BC and your voltage testing with a load applied does not show a linear voltage drop, then it shows there are some junctions in the wiring that are introducing some ohms but should be near zero (knowing what the actual distance an outlet is on a BC is not ez). These specific problem points can cause hot spots in the wiring, etc. BC wiring is infinite amount of very small resistors, if any one of them is bigger than the rest then it will be hotter, etc.
 
If you want to eliminate any offset error, you should swap the two meters and measure again.
If the voltage is stable enough you can make both measurements with one meter.

Sent from my XT1585 using Tapatalk
 
kwired said:
True in that it is the no load voltage drop. If there is no load there should be no drop though.
Click to expand...

i was making that same point using other words :thumbsup:
the math and test is still valid ;)
 
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