Voltage Drop And Cable Lengths

Being able to separate the battery from the motor can be a key component and requirement of your electric outboard installation.

However, due to the often high amperage drawn by the motors (especially true if using a lower voltage eg 12v system), we need to be aware of some key issues and then understand how to avoid them.

What Is Voltage Drop?

multimeter leads

Voltage drop occurs when the voltage at the end of a run of cable is lower than at the beginning.

Any length or size of wires will have some resistance, and running a current through this DC resistance will cause the voltage to drop. As the length of the cable increases, so its resistance and reactance increase in proportion.

Therefore, Voltage Drop is particularly a problem with long cable runs.

Electrical cables carrying current always present inherent resistance, or impedance, to the flow of current. Voltage Drop is measured as the amount of voltage loss which occurs through all or part of a circuit due to cable “impedance” in volts.

Why Is Voltage Drop Important?

Too much Voltage Drop in a cable cross sectional area can cause lights to flicker or burn dimly, heaters to heat poorly and most importantly for us, motors to run hotter than normal and burn out.

This causes the load to work harder with less voltage pushing the current.

Why Is Cable Length Important To Us?

Cable length as we now know affects Voltage Drop, but why is that a concern for us?

Surely the battery is just next to the motor? So who cares?

Well, for a lot of smaller and lighter boats particularly, weight distribution can make a big difference to performance – too much weight at the back will make the stern sit down in the water and slow you down. Sitting forward can counteract this but if using large Lead Acid batteries you can find it hard to counteract the weight imbalance sufficiently.

You also may not want the battery at the back of the boat and close to the motor due to simple layout limitations to your boat.

Therefore we need to have flexibility of placement but we cannot simply assume that we can use the same cable size if we move the battery forward a significant distance (this can mean as little as a few 10s of centimetres in lower voltage, high current systems)

How Do We Reduce Voltage Drop?

To decrease the Voltage Drop in a circuit, you need to increase the size (cross section) of your conductors – this is done to lower the overall resistance of the cable length.

However, larger copper cable sizes increase cost and weight, so it is important to calculate the Voltage Drop accurately and find the optimum voltage wires size that will reduce the Voltage Drop to safe levels while remaining cost-effective.

Calculate Voltage Drop For Your System

We have produced this handy calculator to help you understand the voltage drop in your system and so be able to size your cabling accordingly.

Simply enter your system data in the green boxes and the system will tell you both your absolute and % voltage drop for copper cabling.

We use 2% voltage drop as a general rule of thumb but always round up to the next cable size to match available cabling.

Testing Voltage Drop

You can of course test your voltage drop directly using a multimeter at the battery terminals and then compare the voltage drop at the motor after the length of connecting cable.

using the multimeter

Leave a comment