FITTING THE RIGHT SIZE FUSE FOR YOUR LED LIGHTING

This advice relates to 12v low voltage equipment only and NOT to 230v or mains powered lights.

The fuses in your 12v boat fuse panel are there to protect the cables. If a fault develops on a circuit, you want the fuse to blow before the cable does, thus avoiding a boat fire that could lead to serious damage to your boat or even loss of life. It always makes sense to fit the smallest fuse possible on a circuit, because this increases the margin of safety protection in the event of a fault developing. However, if you fit a fuse that is too small, it will blow when you turn your equipment on.

If you are moving from incandescent bulbs to led lighting, the power consumption will be far lower (that's why you bought them, right?). In practice, this means that you will be able to move to a lower fuse rating. When the boat was originally wired up the electrician will have sized the fuse according to the number of lights on each circuit and their power consumption.

In order to find out what fuse you need on a circuit, you need to work out the total number of watts with everything turned on. Having done that, you can use a simple formula to convert watts to amps:
Amps = Total watts on circuit divided by 12 volts (assuming you are using a 12v system). Then, you should fit a fuse that is the closest size you can get above that figure (e.g. if you get a figure of 6.5A, you would fit a 7A fuse)

Typically, a boat's cabin lights will be split into 2 circuits, perhaps port and starboard or fore and aft. It makes sense from a safety point of view not to wire a large number of appliances all on the same circuit. Plus, if your lights blow on one side, you can still see what you're doing if you don't have a spare fuse or breaker to hand.

An example:
A boat has 2 wiring circuits, each with 6 cabin lights on them. Originally, the lamps were 15w halogen, now the owner has replaced them with led units each drawing 2.4w.
Originally, each circuit with halogen lights would have been drawing a maximum of 6x15w = 90w.
90w divided by 12v = 7.5A and there will have been an 8A or 10A fuse or breaker fitted.
Now, each circuit will be drawing a maximum of 6x2.4w = 14.4w
14.4 divided by 12 = 1.2A and you should now fit a 2A fuse or breaker.

Two things to remember:
Amps = Watts divided by Volts.
Always work out the total number of watts when everything is turned on on the circuit that the individual fuse protects.

This advice assumes that you already have wiring in place for your lights and you are just switching from one type of bulb to another. If you are wiring up a boat from scratch you will also need to consider something called 'voltage drop' when sizing your circuits which is outside the scope of this article.

Finally, NEVER work on live electrical circuits. ALWAYS isolate your battery (i.e. turn your battery master switch to OFF before commencing work).

This advice was provided by Jim, a qualified Marine Electrical Technician based in New Mills. If you are still stuck you can email him at jim@boatwiring.co.uk
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