220 Volt Electrical Additions

I have previously written a case study on how not to carry out an electrical addition, but now feel the need to update it.

During the week commencing 28th June 2015, I came across another poorly converted American vehicle. It was a 5th wheel that looked like it had been done by someone with no electrical knowledge or understanding of what they were doing. This came to light when I had to remove a cupboard to fit some reverse sensors.

For a start they obviously haven’t ever seen a junction box before and thought a chock block and a bit of duct tape would be sufficient (image ref 2).

For people who know what they are doing, they will know that the transformer that is used in this picture, is not a “soft start” and relies on a device that is seen at the top of the picture with (MS35) on it (image ref 1).

The circular device acts as a soft start and these are well known to catch fire if not wired into the R.C.D. (residual current device).

Soft start device

Image ref 1

The 5th wheel has two cables chock blocked just after the 16 amp input socket.

The 5th wheel has two cables chock blocked

Image ref 2

One cable goes to a consumer box and goes through a R.C.D. and 16 amp trip.

consumer box

Looks professional until you start to look closer!

The other cable goes straight to the transformer and then to the 5th wheels distribution board. The voltage enters the transformer at 220 volts and exits at 110 volts. If you half the voltage then double the ampage, the cable should have the capability of carrying 32 amps as the supply socket is 16 amp. As the cable used is only 2.5 mm it should only carry 24 amps not 32.


This 5th wheel has been wired so that the power supply to the transformer does not go through a trip or R.C.D. I would take an educated guess that the transformer would trip the R.C.D. and also to buy a good quality soft start transformer without the addition start device would be about £10.00 more expensive. As they say a little knowledge of electrics in this case could be lethal.

If you would like help with your motorhome electronics, give us a call on 01604 861999.