Considering Voltage Drop When Sizing DC Cables

Quite often DC cables are selected based on their current carrying capabilities, however in many applications voltage drop can be the deciding factor on selecting the right cable for the application.

All cables have resistance, meaning that the longer cable run the more resistance.  The effect of resistance over longer runs is the drop in voltage at the end of the cable run.  While this doesn't come into play on low current draw loads such as LED lighting, the effect of voltage drop on circuits such as pumps, fridges, stereos, chargers and inverters can be quite dramatic.  First off think about a bilge pump, the rated flow capacity is based on 12V, if you are only getting 11V at the pump the flow will be reduced. When it comes to items such as inverters or fridges the effect of voltage drop can quite often cause low voltage alarms or the equipment to shut off due to low voltage, even if the battery voltage is within the normal range.  Another critical effect on voltage drop is with battery chargers, a common issue experienced is battery chargers going into float mode when the batteries are not fully charged.  This is due to the lower voltage at the battery compared to the higher voltage on the charger's output which results in the charger thinking the batteries are at a higher voltage and triggering it to go into float mode.  It is worth noting that all the issues can also be caused by loose or poor terminations.

The Standards

Australian Standard AS/NZS 3004:2 states that for DC supplied equipment the voltage drop shall not exceed 10% and for critical circuits such as navigation equipment, communication equipment, winches and engine starting it is to be determined by the equipment manufacturer.  ABYC standard, E-11 goes a bit further and states a maximum of 3% voltage drop for critical circuits.  Furthermore, AS/NZS 3004:2 recommends a maximum of 1% voltage drop for charging circuits.

Fault Finding

So how do you test for voltage drop in the field?  The easiest way is using a voltmeter with the equipment turned on an in an operating mode (bilge pump actually pumping water).  Firstly, measure the voltage at the battery terminals, then measure the voltage at the equipment.  The difference in voltage is your drop, this will also take into account any voltage drop due to poor connections.  Another way which is especially useful when trying to identify where the drop is occurring in the circuit is to use a voltmeter but this time measure along a cable run, i.e. one probe on the positive battery terminal and the other end on the positive at the equipment.  This will show you the voltage drop just for that run.  To take it further you can connect just from the positive output of the controlling switch to the positive at the equipment.  This will show voltage drop for just that length of cable.  Similar if you suspect a faulty switch you can measure across the terminals to see if you are getting considerable drop, if so then a failed switch is the issue.

Calculating Before Installation

How to calculate voltage drop for selecting a cable.  There are many charts available that have been published by ABYC, or there are formulas.  To make it even easier we have created a useful voltage drop calculator.  Enter the circuit information and maximum voltage drop and you will get a mm² cable size or enter your chosen cable size to get the voltage drop percentage.

DC CABLE VOLTAGE DROP CALCULATOR