Selecting the correct size and type of cable for any particular job.

This isn't anywhere near as difficult as some people believe. It is also more complicated than other people believe!

Selecting the wrong size or type of cable can result in, at best, a system that does not operate correctly or as desired, a system that is unreliable and subject to continual failures, an instalaltion that fails surveys or safety inspections etc. It can lead to a system that costs far more than it needs to or, as a worst possible scenario, it can result in fires or electrocution.

There are 4 rules that must be followed. Each one is relatively simple but important in it's own right.

Rule 1

The first rule is that the cable must be of the correct type for the voltage. This is related to the insulation breakdown voltage. Cable is specified by the manufacturer (following testing) as being suitable for use up to a certain voltage. This specification will be written on the cable drum. As long as this specification is higher than the system voltage everything is fine. That is to say it is perfectly acceptable to use 1000 volt cable on a 24 volt system. Obviously a cable rated for 24 volts would be totally unsuitable for use on a 1000 volt system.

Rule 2

The 2nd rule is even simpler. This relates to the physical strength and durability of the cable. This one really is down to nothing more than common sense. For instance, cables inside mobile telephones and calculators are tiny. Suppose there is a piece of equipment at the bow of a boat that draws extremely low current but needs powering from the stern. Say a 0.001 amp load. A tiny cable of around 0.1mm² as found in a mobile 'phone will handle the current. But physically this cable is not going to survive very long on a boat due to vibration and chaffing etc. The size of the cable and the physical strength of the insulation must be up to the job.

Further, the cable insulation must be tolerant of any other chemicals it may come into contact with. For instance cables in engine bays should be oil and fuel resistant. Those of us on the UK Inland Waterways are more than aware of the problems with PVC insulation when in contact with expanded polystyrene.

The last 2 rules are slighly more complicated and are related to the actual size of the conductor. This defines how much current the cable can safely carry.

Rule 3

The third rule is that the cable must be able to safely handle the current without overheating the cable and/or it's insulation. This specification can be calculated from the current through the cable and the resistance of the cable (which will show how much heat will be generated). This can then be used with further figures relating to the type and make up of the cable, the ambient air temperature etc to calculate the cable temperature rise. Fortunately this has been made much simpler for us as international standards bodies have drawn up tables which show this in a simple tabulated format. One simply looks up the cable size, the table then shows the maximum safe current for a cable in free air or a cable in a conduit etc. Even more fortunate for us is that the cable suppliers take the worst examples from these tables and specify that as being the current capability for each particular wire.

For instance 2.5mm² cable is usually specified by the standards bodies as being suitable for 30 amps in free air or 20 amps when in a conduit. The manufacturers therefore specify this size cable as being safe for use up to 20 amps.

Any cable you buy should have the current carrying capacity specified on it's packaging. This is sometimes referred to as the cable's "ampacity".

Staying within this specification ensures that the cable will not be overheated.

Rule 4

For volt drop purposes the required cable size in mm²=

18/((volt drop [volts]*1000/current [amps])/length [metres])

= 18/((volts*1000/amps)/metres)

The final rule is one that usually means a much larger cable than that specified by rule 3 must be used. This is almost always the case for low voltage (i.e. 12 or 24 volt) systems.

The reason for this is that rule 3 takes in to account the possibility of overheating the cable only. Rule 4 is regarding the acceptable volt drop down the cable at a certain current. This is usually more of a problem in low voltage systems than it is in higher voltage systems.

An explanation is in order here.

As stated above, 2.5mm² cable is specified as being safe up to 20 amps.

Now suppose we have a 230 volt load drawing 20 amps on the end of 20 metres of this 2.5mm² cable. The resistance of this 40 metres (20 metres each way) of cable is approximately 0.288 ohms. This sounds like nothing. Using Ohm's law (V=I*R : V=volts, I=amps, R=resistance) we can calculate the total volt drop to be 20amps*0.288Ohms = 5.8 volts. So our 230 volt load at the end of the cable will see 224.8 volts instead of 230 volts. This is well within the specification for a 230 volt supply (the acceptable voltage for a 230 volt supply ranges from 216 volts to 253 volts).

So this cable is perfectly acceptable for a 20 amp 230 volt load.

However, irrespective of the voltage the system is running at, a 20 amp load will drop 5.8 volts down a total of 40 metres of this cable. So if our load at the end is a 12 volt load drawing 20 amps then by the time the power gets there, it will now be at 12 - 5.8 = 6.2 volts. Obviously this is no use to us whatsoever! The cable, although being run within it's rating is dropping too much voltge. It was OK at 230 volts, but no use at all at 12 volts.

So this leads to the question of "what size cable should I use?"

The answer is surprisingly simple, and we are somewhat amazed at how often we see the wrong size cable being used.

In fact, the answer is so easy, I'm going to say this again. It's easy!

There are large, complicated tables available that one can carry round showing the resistance of various sized cables, the volt drop per km (or per metre or furlong or whatever) at various current draws etc etc etc. One never seems to have one at hand when it is needed.

Fortunately, since metrication, things have become very simple. This is because the resistance of cable (and therefore the volt drop) is directly, inversely, proportional to the cross sectional area of the cable. And the cross sectional area of cable is now how they are specified and sold.

So. Decide on the acceptable volt drop for the job in hand. For instance a 12 volt light really needs to run on a minimum of 11.0 volts to operate correctly. So in this case the maximum allowable volt drop is 1.0 volt. In a split charge system the cables between various batteries ideally need to drop no more than 0.05 volts in order to allow the system to operate at it's best.

So anyway, decide on the acceptable volt drop in volts.

Multiply this by 1000.

Divide this by the current in amps.

Now divide the result by the actual total cable run length (both positive and negative) in metres.

Now divide 18 by the result of the above. Hey presto, that is the required cable size in mm². Obviously most of the time this will come out to a silly required wire size so you just choose the next up, standard, available, wire size.

Finally choose whichever wire is the largest from rule 3 and rule 4. On low voltage systems rule 4 will almost always dictate the wire size. On high voltage systems rule 3 will usually dictate the wire size.

I told you it was simple. The trick is that the resistance of copper wire is roughly 18/size=Ohms/Km

cable size [mm²]=18/((volt drop*1000/amps)/metres)

This rather messy looking formula was kept in this format because it is easy to work with as the above example shows. However it is much nicer and easier to remember when rearranged thus:-

cable size[mm²]=18*metres*amps/(V*1000)

NOTE - Always build a safety margin in to the cable sizes. i.e. increase the cable size from the calculated size by around 30%. Never try to run cables at their maximum specified current limit.