What Does “Cable Size in mm2” Mean?

Andre – Australia

QUESTION:

I am connecting a 200w solar panel to a Projecta 12VDC 3-stage controller. My cable run will be around 5 metres and the wiring diagram is showing 8mm² cable size.

What does 8mm² cable size mean – the copper wire diameter is 8mm thick when the insulation is removed (e.g. as thick as an 8mm drill bit)? Or does it mean something else?

ANSWER:

Great question, it’s a confusing issue.

Short answer – 8mm² cable size refers to the cross-sectional area of the copper core, not the diameter.

To explain…

If you cut cleanly through a cable, you’ll see the outer insulation and the inner copper bundle.

The face of the cut is the cross-sectional area of the entire cable.

The 8mm² cable size refers to the cross-sectional area of all the individual copper strands making up the copper bundle.

Cross-sectional area is not the same as the diameter of the copper bundle. This diagram shows the difference between cross-sectional area and diameter.

Obviously, one affects the other. For example, as diameter increases, so too will cross-sectional area.They’re related by the following formula:

Cross-sectional area = (Diameter²) x (𝜋/4)

We’ll re-arrange this formula, so we can calculate the diameter when we already know the cross-sectional area:

Diameter = 2 x √(Cross-Sectional Area/𝜋)

Now, let’s go back to your 8mm² cable size. You want to know what the diameter of the copper core. Use the above formula:

Diameter = 2 x √(8/𝜋) = 3.2mm

So the copper bundle (or core) in an 8mm² cable (insulation removed) will be about as thick as a 3.5mm drill bit, or a 1/8” drill bit.

Looking at the Cable Size Chart here, you’ll want to use 8 AWG, 8 B&S or 10 ISO cable.

Hope this helps.

Regards,

Andrew

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