Kuldging an Ammeter Shunt?

[hellosailor]

If you're in a situation where you need (ok, WANT) to measure amperage and you don't have a shunt to install, or a good heavy duty DC ammeter, would it be realistic to just tap into a primary power cable and use it as a shunt?

For instance, using nominal resistance values for 8 AWG (0.000739 ohms per foot) and 6 AWG (0.000465 ohms per foot) copper cables, numbers show roughly that at 12 volts with a 100A load, the 8 AWG cable could be tapped 0.775 feet apart (236mm) and the 6 AWG cable could be tapped 1.23 feet apart (290mm) and in both cases, a load of 100A would produce a neat 100mV drop in the cable.

Granted, the cables will vary, the voltage will vary, the distances won't be exact to the fractional millimeter...

But shouldn't it be possible to use a DC voltmeter, on the millivolt scale, and with some basic calculations like this come up with a fairly close reading for the amperage the cable is carrying?

Not good enough to resolve 60-vs-62, but good enough to resolve within a couple of amps of what's really going on?

[brak]

seems plausible. However, I think that if you were to actually measure your cable, you'd find very high variation from the nominal resistance.

On the other hand, you could probably just remove the cable, measure it's total resistance, reinstall and use the real number? That should get you a result that's as precise as your meter permits.

[Bene505]

Absolutely. And I second what Brak said.

If you use a longer cable, you'll be measuring a larger a voltage drop, which would likely improve the accuracy, considering the type of meter you are using. I'd probably measure the voltage drop from the panel breaker to the light/fan/whatever itself, taking advantage of the full length of it's powered wire. Then disconnect the wire and measure it's resistance on the ohm scale.

I=V/R (in amps, volts and ohms)

Regards,
Brad

[hellosailor]

"remove the cable, measure it's total resistance,"
In an ideal world with a lab grade ohm meter, sure. But any ohm meter under a hundred bucks is unlikely to accurately measure anything less than ten ohms, and absolutely nothing under one ohm. They're just not designed for that. Most of them have an accuracy of 1/2 to 1%, and then a 'float' of 2-5 digits on the rightmost digit. So on a 10-20 ohm scale...it might well be off by 1/10th of an ohm every time.
I also was shooting for lengths near one foot, since that's a space fairly simple to access, if you want to push two probes or needles through a cable.

Taking advantage of the whole length of a cable is a good idea though. Assuming you can get leads all the way from here to there, and not wind up getting them caught up in moving pieces.

[Bene505]

Originally Posted by hellosailor

"remove the cable, measure it's total resistance,"
In an ideal world with a lab grade ohm meter, sure. But any ohm meter under a hundred bucks is unlikely to accurately measure anything less than ten ohms, and absolutely nothing under one ohm. They're just not designed for that. Most of them have an accuracy of 1/2 to 1%, and then a 'float' of 2-5 digits on the rightmost digit. So on a 10-20 ohm scale...it might well be off by 1/10th of an ohm every time.
I also was shooting for lengths near one foot, since that's a space fairly simple to access, if you want to push two probes or needles through a cable.

Taking advantage of the whole length of a cable is a good idea though. Assuming you can get leads all the way from here to there, and not wind up getting them caught up in moving pieces.

Good point on the ohm meter. Go with your measured length of known cable. Assuming this is a temporary setup, use a long-ish cable for greater accuracy, but not so long (or thin) that it adds too much resistance and drives-down the voltage to the device.

Regards,
Brad