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Old Jun 22, 2012, 09:37 AM
Acacia II Lunar Lander
RCRod's Avatar
United States, CA, LADERA RANCH
Joined Feb 2011
640 Posts
Question
Wire Gauge for Digital Servos

I'm starting a re-wiring project on my current F3F racer (Acacia II), and am preparing for a new build (Willow). I've come across a question I'd like some help with: wire gauge for running plumbing to the digital wing servos.

What gauge do you use?

I know the bigger seems better, but I'm finding that running 22 AWG through a single connector on my Acacia II's center panel is feeling very crowded. I have an 8-pin multiplex connector (like the green 6-pin ones we all know and use, but with an extra two pins) through which I'm trying to connect all four servos. With the fat 22 AWG stuff, it's pretty tough to get 12 leads crammed together around the little pins. I'm trying an experiment wherein I'm only running power (plus/minus) from two channels, and just signal wires from the other two into the fuse side of the connector. I'll branch the power on the wing side a little bit downstream from the connector, so I have room.

Is that silly? Am I over-killing it with the fat 22 AWG? Should I scrap the single 8-pin connector and go to two 6-pin connectors, so they're not as crowded?

Any experience-driven opinions would be appreciated!

Cheers,
Rod
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Old Jun 22, 2012, 01:38 PM
Hot Dawg Glider Pilot
schrederman's Avatar
United States, TX, Weatherford
Joined Nov 2002
7,744 Posts
I run 22awg for power and 28awg for the signal wire to save a bit of weight. I tax my wing servos pretty heavily during launches.

Experience:

I manage a large number of various types of radios that average 60 watts output, (railroad). Recently we changed the input wiring in a series of facilities from #12 to #8. These 100 watt radios were drawing enough current that the voltage was dropping below the minimum threshold. The change in input wiring guage solved the problem entirely.
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Old Jun 22, 2012, 01:54 PM
F3B
satinet's Avatar
Warwickshire, England
Joined Sep 2006
5,488 Posts
I wouldn't worry about the gauge too much. The stall current on the servos will be a lot less than the rating of even pretty thin wire.

I've haven't heard of or experienced having problems (f3b etc) running 6 digis off 1 servo plug in to the receiver, which powers the 6 servos. Unless you bypass the receiver an power your servos direct from the battery you are only running all your servos through a pair (or maybe two pairs) of servo pins going in to the rx. (Usually I put two leads in to the RX to be on the safe side. )
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Old Jun 22, 2012, 02:28 PM
Registered User
Joined Oct 2005
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I use 24 guage teflon coated for power and 26 for signal. Expensive wire but using stock 22 guage servo wire adds an extra ounce or more to a model.
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Old Jun 22, 2012, 03:26 PM
Acacia II Lunar Lander
RCRod's Avatar
United States, CA, LADERA RANCH
Joined Feb 2011
640 Posts
All good info, guys. Thanks a bunch!

I think I'll step down to a smaller gauge. Stuffing the crimped ends of the 22 AWG wires into the connector housings is a pain in the butt.

Satinet, you touched on a point I was considering, as well. It hardly seemed necessary to run so many power leads to the wing, when the entire system is pulling juice through only the wires from my BEC.
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Old Jun 22, 2012, 03:43 PM
F3B
satinet's Avatar
Warwickshire, England
Joined Sep 2006
5,488 Posts
well my point is merely that if you are running all your servos through one power lead that is the same rating as each individual servo wire, your problem is not your servo wires. not that there is a problem.

It's really solving a problem that doesn't exist. yes you can pull a fair bit of current on a winch launch because you have a lot of deflection on the surfaces and a lot of speed, but the wire can take high loads for a short period quite easily.
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Old Jun 22, 2012, 03:48 PM
Build A Rocket Boys!
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Portland, Oregon
Joined May 2002
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Good topic and something I have always wondered about...
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Old Jun 22, 2012, 04:10 PM
F3B
satinet's Avatar
Warwickshire, England
Joined Sep 2006
5,488 Posts
some info about servo wire ratings here:
http://www.smservices.net/acatalog/Cable_and_Wire.html

in metric but you can convert it. I assume 0.34mm is the same as 22awg (looks like when I bought some). That takes 6amp continuous and 12+amp for 5m minutes. So even if you go down a grade i very much doubt you will have a problem.

IIRC the typical stall current of a digi wing servo is something like 1-1.5amps.
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Old Jun 22, 2012, 07:26 PM
Acacia II Lunar Lander
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United States, CA, LADERA RANCH
Joined Feb 2011
640 Posts
Really helpful posts, Satinet! Thanks very much!
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Old Jun 23, 2012, 01:26 AM
In F3J size does matter!
roydor's Avatar
Israel
Joined Nov 2006
834 Posts
In the following link you will find a chart with max current and a calculator for the voltage drop.
http://www.powerstream.com/Wire_Size.htm

In general it is wise to de-rate the wires by a factor of 2, this means that for a servo with a current draw of 1 amp you wouldn't use a wire with less than a 2 amps for its max rating.
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Old Jun 23, 2012, 03:04 AM
F3B
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Warwickshire, England
Joined Sep 2006
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Quote:
Originally Posted by roydor View Post
In the following link you will find a chart with max current and a calculator for the voltage drop.
http://www.powerstream.com/Wire_Size.htm

In general it is wise to de-rate the wires by a factor of 2, this means that for a servo with a current draw of 1 amp you wouldn't use a wire with less than a 2 amps for its max rating.
With this information plus the stall current of your servo you should have no problem working out what you need.
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Old Jun 23, 2012, 07:54 AM
Acacia II Lunar Lander
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United States, CA, LADERA RANCH
Joined Feb 2011
640 Posts
Nice resource! Roydor, do you know if that data is for single- or multi-conductor wire? Servo wire is all multi-conductor.
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Old Jun 23, 2012, 08:19 AM
Acacia II Lunar Lander
RCRod's Avatar
United States, CA, LADERA RANCH
Joined Feb 2011
640 Posts
Here's another good resource...

Hansen sells wire and connectors. This PDF is his a how-to for creating servo leads and extensions.

http://www.hansenhobbies.com/product...Connectors.pdf

He includes a table of gauges and resistance. The only data missing is max amperage. Is there a formula we can use to determine that?
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Old Jun 23, 2012, 11:04 AM
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United States, AZ, Buckeye
Joined Mar 2010
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There is a formula it is current=voltage/resistance of wire per unit length. So if you are using a 3 foot piece of wire and it measures 5ohms

Current=6.0volts/5 ohms
Current = 1.2 amps
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Old Jun 26, 2012, 07:32 AM
Hot Dawg Glider Pilot
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United States, TX, Weatherford
Joined Nov 2002
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I used the heavier wire not for current carrying capacity, but for the drop in voltage over the long leads.
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