please read the installation manual

please read the installation manual

There are occasions when I get a message and photo that causes me to drop everything and get in the car and drive. The attached photo, received a while ago, was one such occasion. There are some that will say they know the problem and I could have dealt with it by messenger but things are not always as simple as they first seem.

The symptoms were the fuse holder getting hot to the extent that the plastic melted, cables getting over warm and the insulation starting to deform and melt. They were feeding a 3kVA inverter from the boat’s 12V batteries.

Any cable carrying a current has a resistance and that resistance causes heat in the cable and the higher the current the more heat that has to be dissipated. In this case the stud on the fuse and the cable were getting hot, not just warm, but such that the insulation had started to deform and going soft. The query in my mind was, was this a simple case that the connection on the stud was causing the problem or was there something more going on and if I got it wrong then there was a possibility of a fire.

So yes the obvious fault is the nut between the cable lug and the fuse. It should not be there, the cable lug should be straight onto the fuse lug, nothing between them.

The stud on the fuse holder is a 8mm stud and the inverter has two 8mm studs on each of the DC connection pads, positive & negative. A total of four studs why?

The inverter a 3kVA inverter with two 8mm studs on each of the DC connection plates. In the back of my mind I knew there was a problem with this sort of connection; particularly when the cables were being expected to run for more than a very short time at full power, for that inverter a current approaching 250A. The current is within the stated current carrying capacity of the cable being used 70mmsq (485A); but it is over the continuous current carrying capacity of a 8mm stud.

Checking the manufacturer’s manual, it states that for the DC connections, for runs up to 2 metres that two runs of 50mmsq should be used for both the positive and negative, why?

​The area of a 70mmsq 8mm hole cable termination lug including the hole is 313.5mmsq.

The area of a 50mmsq 8mm hole cable termination lug including hole is 261mmsq and for 2 is 522mmsq.

This means that by using two 50mmsq cables and two fuses the contact area of the surface contact of the lug to the connection plate is increased by 1.6 times and the current through the fuses and their studs is halved. Therefore there is a better and easier current path, less resistance etc. Hence why the manufacture has specified twin cables and twin fuses.

Just for those that are thinking the boater should have used 95mmsq cable the area of a 95mmsq 8mm hole cable termination lug has an area of 498.15mmsq, but it is still over the continuous capacity of the 8mm stud and the 50mmsq cable solution is still better and deals with all the problems.

Please please read the installation instruction carefully. If the manufacturer has provided twin studs to connect an inverter please read the manual and if the manufacturer has specified/recommended a similar arrangement follow it. They are doing it for good reasons to stop you ending up with a fire from overheated cables etc.


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About Graham Mills

I have owned boats since I was 10 and have helped other boaters with their electrics for almost sixty years. I sold my last boat in October 2016 as I can no longer singlehand a boat. The result is more time to help others and that has expanded. I have never charged for my help, although now a days as a pensioner I ask boaters to pay my expenses to get to them which can be anywhere in the UK.