Measurement of the Cable Cross Section

When buying the inverter for my motorhome, cables were included. Unfortunately, there was no information about the cross section of these cables. Since the inverter can draw up to a maximum of 200 A current, I would like to make sure that the cables can withstand the current and that the motorhome does not burn down.

Figure 1: Supplied cable of unknown cross-section (Image Source: Michael Marwell)

How do I know if the supplied wires can carry that current?

The maximum current carrying capacity and the permitted maximum fuse protection depends on the type of installation, ambient temperature, and other parameters for different cross-sections and is determined by norms. So, I have to determine the cross-section of the cable.

How do I measure the cross-section of the cable?

It is not possible to measure a cross-section directly. However, I can determine the cross-section of a cable in various ways.

If I have different wire end ferrules, I can try out which wire end ferrule just fits on the cable to determine the cross-section. If the cable does not fit into the ferrule, the cable cross-section is larger, and I have to choose a larger wire ferrule. If the cable is too loose in the ferrule, I have to use a smaller ferrule. Of course, only the copper material has to be inserted into the ferrule.

Another possibility would be to measure the diameter of the cable with a micrometer and calculate the cross-section from that. To do this, the diameter of the copper conductor must be measured.  This is very reliable for single-wire conductors. With flexible multi-wire cables, however, it is important not to compress these fine wires too tightly and thus get a wrong measurement - but also to pay attention to the fact that these wires have a certain amount of air between the individual strands. It is therefore advisable to measure directly next to the insulation.

Figure 2: Measuring the diameter (Image Source: Michael Marwell)

The cross-section A is calculated according to the formula "Diameter² x Pi / 4".

I test this measurement on a cable whose cross-section I know. I measure a diameter of 3.1 mm. According to the calculation, the cross-section is 7.5 mm². In Europe the cables are manufactured in certain cross-sections, e.g. 4 mm², 6 mm², 10 mm², and 16 mm². Of course, thicker and thinner cables are also manufactured. Typically, however, no intermediate dimensions. The next two values would be either 6 mm² or 10 mm². The 6 mm² cable should have a diameter of 2.8 mm and the 10 mm² cable 3.6 mm. Since I certainly didn't compress the strands by 0.5 mm during measurement, but rather not tight enough, the 6 mm² cross-section is closer, and also complies with the value of the cross-section known to me.

The origin of the cable should also be considered here. In Europe, the cable size is typically given in mm² of the cross-section, but AWG (American Wire Gauge) is also often shown. However, these values should be checked either in a table or in the data sheet of the cable manufacturer.

It may also be possible for the cross-section to be printed directly on the cable. This would of course be the most reliable way to determine the cross-section.

With the present cables the cross-section is unfortunately not printed, and without damaging the cables the copper material is not measurable either. So, what else can I do?

Can I determine the cross-section via the diameter of the external insulation?

This may be possible as well, but it is a rather unreliable possibility to determine the cross-section. My cables have a total diameter of approximately 7 mm. I was able to find a table with the imprint on the cable showing the thickness of the insulation, the diameter and the cross section of the cable. In the table I see that 7.3 mm diameter corresponds to a 10 mm² cable cross-section.

Figure 3: Measuring the outer diameter (Image Source: Michael Marwell)

Because I could find the table with the values, I am sure that I have determined this cable correctly. Without this table, this kind of measurement is extremely inaccurate. If I subtracted the insulation thickness, 2 * 1 mm, from the measured 7 mm, the copper conductor would have a diameter of 5 mm, resulting in a cross-section of 19.6 mm². 10 mm² cable corresponds to the diameter of 3.6 mm.

For comparison, the previously measured 6 mm² cable has an outer diameter of 5.3 mm and the insulation is 2 * 0.8 mm, so I would calculate a copper core diameter of 3.7 mm or a cross-section of 10.7 mm².

These calculations show that the cross-section of a cable can only be reliably determined via the outer diameter if the cable can be clearly identified via the manufacturer's data sheet.

After all of this, I decided I will not use the supplied cables for my RV because I found out the cross-section is too small for the maximum possible 200 A. Additionally, the supplied cables have been bent by the manufacturer more than the minimum bending radius would allow.

About this author

Image of Michael Marwell

Michael Marwell is Manager, Digital Technical Marketing for Digi-Key in EMEA. He started his career after his German Dipl.-Ing. (FH) as a Hardware Development Engineer for Access Control Systems, followed by different Application Engineering Roles for Suppliers and Distributors. In his free time, he likes photography and tinkering with electronic devices.

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