Understanding Wire Gauge Current Rating
The wire most frequently used, and recommended, for electric motor power systems is often just called ‘Silicone Wire‘.
The wire is a flexible, multi-strand wire with a silicone insulation sleeve that gives it its name.
This post is aimed at you understanding wire gauge current rating when connecting Lipo Batteries to Electronic Speed Controllers (ESCs)
Determining Wire Gauge Rating
The size or “gauge” of the power wires between the Lipo Battery and Electronic Speed Controller (ESC) is based on:
1) The application
2) The anticipated Maximum current
3) The length of the wire from the BATTERY TO THE ESC AND BACK TO THE BATTERY ( In other words, it is the total length of the positive and negative leads combined).
This is an important consideration because the resistance of wire is directly proportional to this length and is responsible for reducing the voltage (volt drop) over longer lengths.
Electrical Wire Gauge Chart
In North America and the UK, American Wire Gauge (AWG) is used to identify the wire ‘size’. The table below gives the conversion from AWG to Metric cross sectional area.
|American Wire Gauge
|Cross Sectional Area
You will notice that the smaller the gauge number, the larger the wire diameter.
Large gauge wire (small gauge number) can safely handle more current, over longer distances, with less voltage drop than smaller gauge (large gauge number) wire, but it is heavier. Wire that is capable of ‘handling’ the current (amps) without too much voltage drop also has to be sized for the aircraft.
Selecting Wire Gauge Amp Rating
AWG 12 wire would be useless in an indoor flier requiring only a couple of amps of current as it would be far too heavy. On the other hand, a giant scale model requiring 100 amps at full power would not work with AWG 12 wire. The resistance of the wire would create an unacceptable voltage drop and, depending on the wire’s insulation, it could melt.
Because we are usually running only a couple feet of wire in our applications, we can get away with using much smaller wire than we would if we were installing long cables. The cross sectional area of cables is measured in “Circular mils”.
The ‘Circular Mil’ unit is calculated by taking the diameter of the wire, in thousandths of an inch, and multiplying it by itself. This gives a value that accounts for the cross-sectional area of the wire without involving π (Pi – 3.142). For example, a 20 gauge wire measures 0.032″ in diameter which is 32 thousandths of an inch, also known as 32 mils. If we take 32 x 32 we get 1,024 circular mils.)
Often, in RC applications, we can use 100 “circular mils” for every Amp of current or even 75 “circular mils” per Amp is acceptable in some circumstances.
Based on 100 circular mils per amp, an application requiring a maximum 50 amps needs 5000 circular mils of wire ( 50 x 100), which is equal to a 13 gauge wire. (From the above chart we can see that 13 gauge wire has a diameter of 0.072″ or 72 mils so 72 x 72 = 5184 which is the nearest size to the 5000 we require).
To be on the safe side, I would step that up to a 12 gauge wire which has 6,530 circular mils, and would provide 130.6 circular mils per amp with minimal weight penalty.
The above table gives wire gauges for specific current carrying capacity based on a very conservative 120 circular mils which give a very safe margin for error should larger currents occur in extreme circumstances.
The Power Wire and Power Connector Relationship
The list below shows the maximum wire gauge a given connector will physically accept. The manufacturer or supplier does not specify the current rating of their connectors based on the wire gauge current rating they accept. These ratings are specific to the connector only. It is always acceptable to use a smaller gauge wire with most connectors.
AWG Name Of Connector
4 Progressive RC (PRC) 10mm bullet
6 6.5mm Castle polarized bullet, PRC8 polarized bullet, PRC6 bullet
8 PRC 8mm bullet, PRC6 polarized bullet, 6.5mm Castle bullet, 8mm Castle bullet
10 HXT 6mm, 6mm bullet, EC5, Anderson Power Pole (PP45), 5.5mm Castle bullet
12 Deans Ultra, XT-60, HXT 4mm, 4mm bullet, EC3, PP30
13 4mm Castle polarized bullet
(PRC = Progressive RC) (PP = Anderson Power Poles)
Anderson Power Poles call their connectors PP15, PP30 and PP45 and they are all rated to 55 amps
PP15s are suitable for AWG 20 -16,
PP30s are suitable for AWG 16 – 12,
PP45s are suitable for AWG 14 – 10,
Most ESC suppliers DO NOT state the wire gauge of the power leads. Some Lipo Battery suppliers DO advise the power lead gauge.
I hope this information will prove useful for some of you. If you like this post you will probably like my website: www.rookiercflyer.com especially if you are new to our hobby. Please feel free to share this with anyone you think may find it helpful.