How to Improve Caravan Electric Brake Wiring

The concept of how to fix caravan electric brake wiring for many owners is near impossible. Although I have written many, many articles on this same subject, the concept of voltage drop remains elusive.

Some trailers and caravans are very long and therefore incur more voltage drop than usual. Voltage drop is measured from the car battery to the last electric magnet.

Voltage drop does not concern me as I do not have braking problems.

Fair enough but do yourself a favour and get under your rig and measure your magnet volts. Your wife can push the override on your brake controller rather than pull the breakaway pin.

What voltage should I see at my electric magnets?

The minimum voltage you should see is 12vdc. If lower then your cable is too small. (Run your car as the battery may be under 12v, with the car running  the battery voltage should be in excess of 13v)

If your manufacturer has run a cable across the axle, then it is almost impossible to get identical voltages at each magnet.

How do I get even voltages?

Run a cable down each chassis rail so that the same length of cable feeds each magnet.

What size is my electric cable?

Herein is the problem, there are two calculations for cable sizes.

  • Most automotive outlets use American wire gauge.
  • Electrical Outlets use the cross-Sectional area calculation method.

Narva_CableThis comparison chart below removes a lot of confusion.

If you start by measuring the cable size (copper size) of the electric magnet, it is 1.15mm diameter or close to it.

Narva advises the 3mm Size of this cable in this photo is a stock number only. The 3mm is not actually a size. The 1.13mm nominated is the actual cross-sectional area size of this cable.

So please ignore this 3mm size.

Two sizes are identified in the chart below

  • 17 AWG or
  • 1mm Cross-Sectional Area


caravan cable size comparison

Is 17 AWG or 1mm cable sufficient size for a single axle trailer?

A 17AWG almost gets it right with correct voltages. Upgrade the cable to 15AWG and feel your electric magnets come back to life.

Is  15AWG  of sufficient size for my tandem trailer?

Sorry to say, No. Use 14 or 13 AWG for the ultimate braking experience.

Voltage Drop

For those of you that are technically minded

To calculate voltage drop in a copper wire, use the following formula:

Volts= (Length x Current x 0.017) /   Area

  • Volts= Voltage drop
  • Length= Total Length of wire in metres (including any earth return wire)
  • Current= Current (amps) through wire
  • Area= Cross-sectional area of copper in square millimetres

What if changing the cable does not work?

You are right; changing cable may not work. There is a little know phenomenon known as drum polarisation.

I will write more on this subject in a few weeks, but here is a brief overview.

Electric Drum Polarisation

The connection of 2 wires determines if the magnet surface has a North pole or a South pole. By looking at the wires, one cannot tell the correct polarisation.

The Drum face can become polarised accidentally if the brake magnet is engaged by say a breakaway switch for a period of more than 15minutes.

In this case, the drum face will be magnetised to the same polarity as the magnet.

When the magnet re-engages, it will repel instead of attracting onto the drum face.

The remedy in the past was to change the polarised drum for a brand new drum. As a result, braking returned to normal.

Finally, an alternative is to interchange the wires, to reverse the magnet polarity. Initially, a robust connection will occur when a north polarity magnet is attracted to a south polarity drum or visa versa.

©Steve Wotherspoon 25th April 2020. Images and text may be copied, provided a link back to the copied page and the source is displayed where the content or image is located.

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