# Signal boost for 20AWG wire to voltmeter

#### dinotom

Joined Apr 26, 2016
2
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#### crutschow

Joined Mar 14, 2008
25,247
Amperage is a measure of current. The maximum current you are measuring and the voltage drop you can tolerate determines the wire size required for the ammeter.

The voltmeter draws only a tiny current so virtually any wire size will work for that.

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#### dinotom

Joined Apr 26, 2016
2
Maybe I wasn't clear, I am not asking about wire size. I want to ENSURE that I am getting accurate readings for amps. Does the signal wire need a booster, and is that even doable, so I can ensure accurate amperage readings?

#### Papabravo

Joined Feb 24, 2006
14,205
You measure current with a series circuit. As long as the measurement apparatus has much less resistance than the load you will be just fine. Suppose you have a load of 10Ω and a source of 10 Volts. You would expect the current to be 1 Ampere. Now suppose we know the resistance of the ammeter and the probes is 1 mΩ. How does that change the problem? We still have a 10V source but the total load has gone from 10Ω to 10.001Ω. What is the actual current? It will be 0.99990001 Amperes and a lab quality instrument of 8 digits will measure that value. A handheld device of three digits will show 1.00 Amperes. Is that the information you were looking for?

Voltage measurements are made in parallel with a meter that has a high input impedance. In the case above we have our 10V source driving a 10Ω load and we want to measure the voltage across the load. So first of all the source has some source resistance and the meter might have an input impedance of 10 MΩ. So first question, what is 10Ω || 10MΩ. Second question if the source resistance is 0.2Ω, what is that resistance in series with the result of 10Ω || 10MΩ. Using the voltage divider theorem what will the voltmeter read across the load?

The answer to your question is: no, signal wires don't need boosters to make measurements.

Piece of Advice: Put the instrument where you want to make the measurement.

Extra Credit: What is the resistance of 40' of #20 AWG stranded wire? My guess is about half an Ohm.

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#### crutschow

Joined Mar 14, 2008
25,247
Maybe I wasn't clear, I am not asking about wire size. I want to ENSURE that I am getting accurate readings for amps. Does the signal wire need a booster, and is that even doable, so I can ensure accurate amperage readings?
You were clear.
Apparently I was not. Accurate is a relative word.
How "accurate" do you need to be?
The current reading is as accurate as your meter, independent of wire length and wire size.
The wire length and size however, does affect the voltage drop (reduction in voltage) to the load that will occur in your circuit due to that wire resistance.
And, if it's not clear, you need two separate pairs of wires, one pair to measure current that's in series with the load, and one pair of wires across the load to measure voltage.
Thus to ENSURE an accurate answer to you question, you need to answer mine in post #2.

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#### Nykolas

Joined Aug 27, 2013
115
Get a 0.1 (or 0.01) Ohm low tolerance resistor and place it as close as you can at the load. Measure the voltage across it at any distance away and calculate I=V/R. E