Gear Tooth Sensor Wire Gauge

[usbcd36]

<R47> All main and branch circuits shall be wired with appropriately sized wire:
 12 AWG or larger diameter wire must be used for all circuits protected by a 40A circuit
breaker.
 14 AWG or larger diameter wire must be used for all circuits protected by a 30A circuit
breaker.
 18 AWG or larger diameter wire must be used for all circuits protected by a 20A circuit
breaker.
 24 AWG or larger diameter wire must be used for providing power to pneumatic valves.

<R55>
…
 Power feeds to custom circuits and additional
electronics must be protected with a 20A circuit
breaker.
…

The rules specify that custom circuits (including sensors) must be protected by 20A snap-action breakers, but I can't figure out how I would hook 18 AWG wire up to a gear tooth sensor. Furthermore, I know that using a smaller gauge wire would cause an unsafe situation because the wire would overheat before popping the breaker.

<R45> Items specifically PROHIBITED from use on the ROBOT include:
…
 Fuse panels different from those provided in the Kit Of Parts

Ok, so having a sub-panel is out.


What do I do? Do I just hook up the 24 AWG wire I've been using for the GTS to the breaker panel and hope they don't notice at inspection? Since the sensors have been around for awhile, I assume there's an acceptable solution.

[MrForbes]

<R55> says also:

"In addition to the required branch power circuit breakers, smaller value fuses or breakers may be incorporated into custom circuits for additional protection. "

But it looks like they really do want us to run 18 AWG power wires, protected by a 20A breaker, to all of the sensors that need 12V power.

I wonder if you could make a custom circuit that feeds power to the sensors, with smaller wire protected by lower current fuses? ie. 22 AWG and 5A fuses.

Might be a suitable question for Q&A

[usbcd36]

Ah…

Completely missed the bit about fuses for custom circuits. I'm sure that's what they'd want us to do, though.

How would I submit this to Q&A anyway?

[MrForbes]

I would not be so quick to assume that this is what they want us to do, which is why I suggested the Q&A. Your team FIRST contact has to be the one to ask...

[usbcd36]

I sent the question to my team's FIRST contact.


Here's a question for other teams that have used these sensors in the past:

What did you do in this situation?

[DonRotolo]

In the past, the rules were different it seems. We used PWM cables for the GTS in 2006 without any problems.

Don

[TubaMorg]

In the past things like sensors were allowed to use smaller gauge wire. Also several sensors can be powered from the same 20 amp breaker: The sensor breaker. Exceptions are also made for components such as the pneumatic solenoids which come with wire smaller than that stated in the rules. You will notice that the solder headers for the sensors in the kit are appropriately spaced to accept the male end connector of the supplied PWM cables.

In response to Don, I think the wiring rules have been pretty constant, but the GDC allows exceptions every year which will have to be done again this year. I think the sensor manual and Tips and Good Practices also addresses this issue.

[Gamer930]

With just wiring up the Gear Tooth Sensor on Friday you will not be able to get all the strands of any wire larger then the PWMs.

That is what we used without realizing this rule. . . . I really don't want to change it so I hope they add this to the next team update

[Al Skierkiewicz]

The Sensor manual on the FiRST website details the wiring. It states...
Two PWM cables are needed to interconnect the module. One cable is used to supply a
12Vdc operating voltage from the sensor distribution circuit breaker. The second PWM
cable connects to an available digital input port on the robot controller to deliver sensed
pulse counts from the “S” port and to receive +5V from the RC unit. “G” is always the
ground connection. The current draw for these sensors is relatively low at 16mA.

As always, the manuals are your friends.

[MrForbes]

Al, it appears to me that the info in the sensor manual specifies a 24 AWG power cable (in the form of a PWM cable) to connect the 12V power connection on the sensor to the 20A sensor circuit breaker, while <R47> specifies that we must use 18 AWG wire for all circuits protected by 20A circuit breakers.

Do you see the source of our confusion? Does the sensor manual information have priority over the rule? That doesn't make sense.....

[Al Skierkiewicz]

Jim et al,
Under robot rules..
<R70> 24 AWG or larger diameter wire must be used for:
 connecting sensors, switches, potentiometers, accelerometers, and other detection
devices
 connecting a vision system to Robot Controller inputs,
 extending the PWM cables,
 connecting small muffin fans,
 wiring LEDs

I believe that is the rule that is referenced by the sensor manual. That being said, I agree that protection with a 20 amp circuit breaker is contradictory in this case. However, this rule (partial) is a better way to protect #24 wiring to sensors...

<R55> All active circuit breaker / power distribution panel branch circuits shall be protected from
overload with an appropriate value auto resetting Snap Action circuit breaker from the Kit Of Parts.
 Power feeds to custom circuits and additional
electronics must be protected with a 20A circuit breaker.
In addition to the required branch power circuit breakers, smaller value fuses or breakers may be incorporated into custom circuits for additional protection.

When in doubt, add a smaller breaker on your custom circuit board in order to feed sensors with #24 wire.

[mneary]

It would be so nice if the Rules allowed using a COTS 5A (or smaller) automotive fuse in the fuse block.

As a design consideration, I would normally fuse 22AWG wire at 5A, and I wouldn't use 24AWG on a high current source.