Wiring Rules

[sparkyone]

Section 5.3.9 Wiring Rules states the minimum wire size that must be used for the various circuits. These seem way too small, but I'm used to working with A.C. power not D.C. Is 12AWG wire correct for using on a CIM motor with a 40 amp circuit breaker? If so, would it be best to go with 10AWG wire instead?

[Cory]

In the past the rule has been 10 AWG wire for CIM motors. I suspect that many teams will continue to wire them with 10, even though 12 is now the minimum.

[Al Skierkiewicz]

The rules read...
<R82> You must use 12 AWG or larger diameter wire for all circuits protected by a 40A Circuit Breaker.
<R83> You must use 14 AWG or larger diameter wire for all circuits protected by a 30A Circuit Breaker.
<R84> You must use 18 AWG or larger diameter wire for all circuits protected by a 20A Circuit Breaker.

However, I recommend #10 feeding Chalupas with 40 amp breakers. You can cheat this a little by running #10 to the speed controller and placing it close enough to the motor that you can then use the motor leads to attach to the controller directly.

[lukevanoort]

Remember that a bigger guage helps resistance, so a bigger guage is better for other reasons.

[DonRotolo]

In regular house wiring, #10 is the minimum for 40 Amp circuits according to the NEC. However, in FIRST you can cheat that value a little bit, because we're dealing with generally accessible and open wiring, also generally used for 2 minutes or so.

The reason behind specifying wire gauges, such as for house wiring, is to prevent overheating, which can start a fire. Hot wires inside a wall is a disaster, but hot wires in a robot, in which the wiring is visible (if it smokes, people will see it), better cooled (a wire in free air can dissipate more heat than a wire inside a vinyl jacket (Romex) surrounded by fiber glass insulation), and used only intermittently (2 minutes at a time) is not as big of an issue. Not to mention the motors aren't drawing 40 A continuously.

There is nothing wrong with using larger wire - we do it at 1676, the added weight is small but the added power (reduced voltage drop) is compensation.

Don

[I'mwithstupid^]

we use bump all our wire sizes up by one, for basically the same reasons, also we have free supply of the regularly used house sizes because our school just got a new building so all left over wiring came to us :)

[mechanicalbrain]

There is nothing wrong with using larger wire - we do it at 1676, the added weight is small but the added power (reduced voltage drop) is compensation.

Don
I know it's a pain to calculate,due to a bunch of dependent variables, but I've always wondered how the resistance gained from lower gauge compares to the loss of power due to weight. I guess the lesson hear is to plan your robot to use as little wire as possible (including enough slack that nothing gets pulled).

[Al Skierkiewicz]

I know it's a pain to calculate,due to a bunch of dependent variables, but I've always wondered how the resistance gained from lower gauge compares to the loss of power due to weight. I guess the lesson hear is to plan your robot to use as little wire as possible (including enough slack that nothing gets pulled).

A rule of thumb I have been using is the "wire foot". At stall several of the kit motors draw about 100 amps. For #10 wire (resistance =.001 ohm) solving for Ohm's Law, V=RxI every foot of wire is equivalent to a 0.1 volt drop. Remember that you have two wires, so if the wire run to a motor is 4 feet, you have eight feet of wire, 0.8 volts at stall will be dropped in the wire. So for our wire sizes (rough approximations) 1 foot of #6 is 1/2 wire foot and #12 is 1.5 wire feet, and #14 is 2.5 wire feet.
As to weight, there are several factors like strand count, type of insulation and temperature rating of insulation that add to the values. Suffice it to say that #6 wire is 50% larger in diameter than #10 and #12 is only 20% smaller in diameter. I have found that there is negligible weight savings of #12 over #10 but a huge savings in voltage drop. #6 will have half to voltage drop but twice the weight. For that reason, I would tend to keep #6 runs short (battery to fuse panel(s) and main breaker) and keep #10 for all motors except the globe. The overall series resistance of this choice actually goes down due to the splitting of the current loads. Remember all robot current flows through the #6 primary wiring but only single loads flow through the individual branch circuits.

[John Gutmann]

Least year I helped make alot of the wires shorter, and amazingly it dropped 1/2 a pound. So even though some people don't think it matters much just think about the day before ship where your 1/2 pound over weight and can't find anything to lighten up.

[Sscamatt]

A useful tool to calculate voltage drop can be found here: http://www.powerstream.com/Wire_Size.htm Scroll down until you see the "Voltage Drop Calculator". Play around with the different gauges to see what is fitting for your robot.