DIY Shunt Resistor Increases Amp Current Limits of MultiMeter DMM

Ingenuity reins when access to expensive tech is shunned or restricted. You may find this alternative shunt resistor useful when the amps to be measured exceed the capacity of your multimeter. Many digital multimeters (DMM) are designed to handle up to 10 amps. You may need to measure a 30A solar array or a 800A automotive starting or 90A charging system,

When you need to measure amps greater than the maximum capacity of your multimeter or DMM, you need an ammeter or shunt resistor. Shunts need to be able to carry the amps you will be measuring and should be of low resistance so as the circuit is minimally disturbed. The epiphany: Why not use a cable that is already conducting the current as a shunt resistor? Resistance of the cable we can measure or lookup.

A limitation of this po-boy shunt: is that the leads of our meter have to reach exposed ends of a cable that carries the current we want to measure. In addition you will need to know the material of the conductor, as in copper or aluminum, and its length. Then thanks to the 150 year old telegraph and Georg Simon Ohm we can use the millivolt meter of our multimeters to measure amps.

To solve for Ohm’s law, V = I x R, we need to know volts and ohms to shake out the unknown amps. Shunts provide a know constant resistance expressed in ohms. Cable resistance is proportional to the mass, length, and material of the conductor. Thank you PowerStream for this voltage drop calculator, which will tell you what voltage drop you can expect over a run of your cable. The calculator will double the cable run length you enter to account for the return cable, so you have to enter half the length of your alternative shunt resistor. You have to guess at the maximum amps you expect to measure.

For example, we have a solar array we expect to generate 28A @ 24V in full midday sun. We have 4 ft of AWG #6 copper cable between the charge controller and battery bank. So off we go to the online calculator where we enter copper #6  24V 2ft and 30 Amps. The calculator tells us to look for a voltage drop of 49 mV over 4 feet of cable when 30 amps are flowing. Now with the DMM set to 200mV scale and connected to both ends of the 4ft cable, it reads 44mV.

The simplified math uses the ratio (mV  measured / mV expected at full amps) times full amps, or (44 / 49) x 30 = 26.9 amps.

Now maybe you don’t need to sweat the decision whether to buy the $60 dc clamp ammeter or the $200 one.


Measure R Value of Water Heater

If you would rather measure, than be given, the R value of your system, you will need to measure temperature. This method of using standby heat loss to arrive at the empirical R value can be applied to many different systems, but here a tank type water heater is measured.

Measuring resistance to heat transfer (R-value in the US, RSi in Si units) removes uncertainty that arises from theoretical calculations based on lab produced standard conditions, which may have little relation to your local conditions. For example, how much standby energy is really saved by the added insulation of that hot water heater blanket? Following is a method for end users to empirically determine specific R values. This method measures the tank system as a whole, including all nuances such as losses in connecting pipes and temperature pressure valves for example.

Tools required include:

  1. thermometer
  2. clock
  3. tape measure

A method to record readings and the dimensions of the tank to determine the system’s surface area and volume. Before we get to an empirical R-value, we have to determine the actual heat energy transferred from the system (water tank) to the surroundings (air).

Measure Standby Heat Loss Over Time

The concept here is to isolate our test system from energy inputs; so turn off the gas, or the breaker, or the sun.

Surroundings must be large enough that heat transferred from the tank does not measurably raise it’s surrounding’s temperature.

Wireless thermometers with a probe are useful to get next to the tank under the insulation, but the important thing is that you measure the system’s or tank’s temperature and not that of the surroundings. Also keep in mind, it is the change in temperature we want, so measurement technique must be consistent. Measure the temperature the same way each time.

Record the temperature of the system and it’s surroundings after disabling heating energy inputs. As the system cools record the two temperatures every hour for a few hours. Recording exactly on the hour is not critical, but your findings will be more precise. Likewise, a ten hour test will be more precise than a three hour. Overnight testing can be appropriate.

Calculate Standby Heat Loss

One equation to calculate standby heat loss or thermal transfer is:

Heat(power) = V * C * ΔT / t
Where V is volume, C is volumetric heat capacity, ΔT is temperature change, t is time

US units will have to be converted to Si (metric) units of meters, liters, and degrees Celcius. Numbers you provide:

  1. volume of the tank system in liters, or its diameter and height in meters
  2. minutes elapsed during the temp drop test
  3. °C of system temperature change during the test

diameter meters
height meters

Volume Liters
Time minutes
Temp. Drop °Celcius

Energy (kJ) kilojoules
Heat Loss (power) Watts
Heat Loss (power) BTU/h

Calculate R Value (RSi)

Now that we have measured the energy loss, we can calculate the coresponding R value using the following:

R = A (ΔT) / H
Where A is surface area, ΔT is temperature differential across an insulating layer, and H is heat power transferred

Numbers you provide:

  1. surface area of the tank system in square meters, or its diameter and height in meters
  2. Average temperature difference between ithe nside of the tank system and it’s surroundings in degrees Celcius
  3. Heat power transferred from the system to its surroundings in Watts

diameter meters
height meters

Surface Area meters2
ΔTemperature avg. Δ °C

Heat Loss (power) Watts (J/s)

RSi °C·m2 / W
R-value (US) °F·ft2·h / BTU