October 9

Understanding Wire Gauge Current Rating

The wire most frequently used, and recommended, for electric motor power systems is often just called Silicone Wire.

8 awg 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
(AWG)
Diameter
(inches)
Diameter
(mm)
Cross Sectional Area
(mm2)
0000 0.46 11.68 107.16
000 0.4096 10.40 84.97
00 0.3648 9.27 67.40
0 0.3249 8.25 53.46
1 0.2893 7.35 42.39
2 0.2576 6.54 33.61
3 0.2294 5.83 26.65
4 0.2043 5.19 21.14
5 0.1819 4.62 16.76
6 0.162 4.11 13.29
7 0.1443 3.67 10.55
8 0.1285 3.26 8.36
9 0.1144 2.91 6.63
10 0.1019 2.59 5.26
11 0.0907 2.30 4.17
12 0.0808 2.05 3.31
13 0.072 1.83 2.63
14 0.0641 1.63 2.08
15 0.0571 1.45 1.65
16 0.0508 1.29 1.31
17 0.0453 1.15 1.04
18 0.0403 1.02 0.82
19 0.0359 0.91 0.65
20 0.032 0.81 0.52
21 0.0285 0.72 0.41
22 0.0254 0.65 0.33
23 0.0226 0.57 0.26
24 0.0201 0.51 0.20
25 0.0179 0.45 0.16
26 0.0159 0.40 0.13

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.

Wire Gauge/Current Rating

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
16            PP15
20            JST-RCY
(PRC = Progressive RC)       (PP = Anderson Power Poles)

Additional Information

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.



Copyright © 2014. All rights reserved.

Posted October 9, 2015 by Colin Bedson in category "Electric Flight

4 COMMENTS :

  1. By Jim on

    Great information. I can certainly see how using a lighter wire may not be enough for a voltage. But I bet a lot of people don’t think about a heavier wire being too much for the voltage needed and also not working as desired.

    This is something that comes up in my industry as an audio engineer. Length of cable is also a big deal if you want your audio signal to full force when it reaches it’s destination.

    I have always found it a little confusing though that a smaller gauge is a heavier wire and larger gauge is a smaller wire.

    Reply
    1. By Colin on

      Hi Jim, Thanks for your comment. I’m glad you got something useful from it.

      Colin

      Reply
  2. By JeffWA on

    Hi Colin,

    At your site, I just read a very informative and well-written article about understanding how to interpret wire gauge ratings. It certainly is something that I was clueless about before reading your terrific article. It was my late dad who was more into that type of stuff as when I was a kid he partially turned the basement of our house, (along with the help of others) into this awesome rec room that we practically lived in.

    This included having to do some re-wiring of certain aspects regarding electricity as our then biggest television set was located down in the rec room. He also had installed a mini-bar complete with refrigerator – again something that required a knowledge of electricity.

    This indeed was a great article that you wrote Colin; served to educate people interested in finding out more about dealing with this ideal.

    I also see that you’re into RC planes, judged by other articles seen on your website. I have always been fascinated myself with planes/jets. What I do however and have done so for the last 15 years is participate in Microsoft’s jet flight simulations available as computer software.

    As I’ll never in real life be the pilot on a Boeing 747 jumbo jet as I’m now in my late 50’s, at least on my computer I can enjoy doing so from the “cold and dark” stage, to programming an actual flight. correct starting procedures, navigating the flight once in air, and getting that baby safely back down on the runway at my destination airport. To me, the game is the greatest one ever invented for a computer. Those war simulation games, because of their violence just don’t interest me.

    Anyway, great article Colin,

    Sincerely,

    Jeff

    Reply
    1. By Colin on

      Thanks for your great comments Jeff.

      This article or post is part of a series of posts following on from my website tutorials at http://www.rookiercflyer.com.

      RC flyers, especially electric plane flyers need to understand the way their electric power systems work and this post gives them a more in depth explanation of the way wire affects power in the system and is designed to help them understand the way electricity works in their planes.

      It sounds like your late father was a great DIYer and that you were able to benefit from his skills.

      Glad to read that you are interested in aviation and enjoy your computer simulator games.

      Colin

      Reply

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