What is Volt Drop?

Very simply, volt drop is caused by resistance in a circuit. Anything that gets in the way of the flow can be considered a resistor, so that includes the wire conductor itself and the size of it.

Why is it Important in Narrow Boat Installations?

This is especially important to consider on narrow boat installations due to the lengths of cable involved.

With all earth/negative sides of the cable runs returning to the battery the current has a long way to travel to make a circuit.  As it passes through an appliance, say a 12v DC pump, it has to have enough voltage left to actually power the appliance. Any restrictions (resistance) along the way can reduce the voltage, causing the appliance either not to run at all or to run outside it’s ideal specifications thus shortening the life of the appliance at best & at worse causing fire. Why fire? Well in the circumstances of excessively high resistance, the lost voltage has to go somewhere & the energy is converted to heat.

Some appliances deal with volt drop better than others. Lighting may still be able to function with a drop of 1V over the cable run with but compressor fridges, pumps, inverters or anything that has an element the circuit must be designed correctly to avoid serious consequences.

Avoiding Volt Drop

So how do we avoid volt drop? The simplest method is to use the correct sized cables in order to minimise the resistance over the required distance. It is possible using Ohms Law to work out volt drop in an existing circuit, but it seems more logical to specify the wire at the start of the process.

To do this you need to decide on a few parameters…

Firstly, what is the maximum load, in amps, on the wiring when all appliances in the circuit are used at once? How long are the cable runs, in both directions from the fuse board and back to the battery/earth bar? What is the maximum volt drop allowable on the circuit? We’ll make this easy by saying that for best practice allow no more than 0.5V for volt drop.

See manufacturers specifications or speak to a qualified and respected marine electrician if you are in any way unsure what to allow for your particular appliance.

For example, a well known water pump manufacturer supplies a chart for wiring sizes over distance based on minimum wire size for a 10% volt drop. This chart is really a recommendation of a minimum size & there is no harm in going up a gauge for extra security.

The Simplest of Calculations

If you know what volt drop you can work with then the formula for calculating it is:

18 x cable length in m x amps/(volt drop x1000)= mm² cable

Where 18 is a constant, the resistance of copper wire. The “cable length” is the run to the appliance AND back to the battery negative of bus bar. Amps is the maximum draw of the appliance under load. Volt drop is the manufacturers spec or if unsure go for 0.5.

A Working Example...

So here we work an example for a narrowboat water pump drawing 5 amps under full load located 7.5 m away from our battery:

18 x 15 x 5 / (.5×1000) = mm² cable size (x)
18 x 15 x 5 / 500 = x
1350 / 500 = x

2.7 = cable size in mm²

This is quite close to standard 3mm² so we go to the next size up and order 15m of 4mm² tinned marine grade cable for a proper job!

Remember to fuse to protect the wire. In this case, although the wire is rated higher, at around 39amps probably, we would fit a fuse closely rated to the appliances maximum. So in the example above, with a 5 amp pump, an 8 amp fuse is more than adequate and the wiring is well protected.

Stack calculations for economy but never exceed the rating of the cable, always go to the next size up and future proof it by making sure you can add to it if necessary.