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I am using 3 of these Mighty Max 12V 12Ah batteries on my Razor MX650 electric bike and they are connected with 12 gauge wire. I would like to tap into the power on one of the batteries with a 20 gauge wire. Is it ok for me to tap into the 12 gauge wire with a 20 gauge wire to pull power? I am actually just going to use it to power a voltmeter on the bike.

UPDATE: @Cubic273.15 Here is an actual diagram of the wiring for the bike, I added what I am trying to do with the voltmeter. I just want to make sure you are saying it's ok to tap into the 12ga wire with the 20ga wire.enter image description here

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  • \$\begingroup\$ The devil is in the detail - what you propose is fine but, how you choose to do it may be bad. \$\endgroup\$ – Andy aka Jan 1 at 11:27
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In the amp-gauge table and many websites, the 12-gauge wire capable of delivering 20A. Check here

Usually, a Voltmeter is not drawing over 0.5A. and using 20ga wire is okay, tapping into 12ga wire is okay too.

But for safety reasons, you can fuse 0.5A or 1A fuse in between your 20ga wire to the voltmeter.

Example how to fuse

Image source: 12 Volt Planet - Fusing Guide

EDITED: adding picture of how to How to add the fuse

0.5A. but I do prefer to tap directly to the battery pin because it is quicker for me. :)

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  • \$\begingroup\$ please see my updates in the post.thanks \$\endgroup\$ – iambdot Jan 1 at 19:21
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    \$\begingroup\$ @iambdot that is okay :) you can tap, but before the wiring + to switch for voltmeter please add fuse 0.5A. \$\endgroup\$ – Cubic273.15 Jan 2 at 3:50
  • \$\begingroup\$ ok thanks. where should the fuse go? between the tap and switch or between the switch and voltmeter? \$\endgroup\$ – iambdot Jan 2 at 3:58
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    \$\begingroup\$ @iambdot added the picture of how to add the fuse. and personally, I do prefer to tap on the pin directly not on the cable :) \$\endgroup\$ – Cubic273.15 Jan 2 at 4:23
  • \$\begingroup\$ i looked up the power consumption for the volt meter i ordered.it says Power consumption 15-20mA, so the 0.5A fuse should still be good? \$\endgroup\$ – iambdot Jan 2 at 5:14
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When wiring low voltage circuit not only amperage should be taken in consideration. Drop voltage is important too because voltage drop should not be over some percentage of source. In case like yours voltmeter consume milliamperes or even micro so it will be OK. Fuse is protecting wires from overheating and catching fire, so value should be chosen according to wire size.

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Tap into the fused power switch seems like the best access point. Wire gauge is irrelevant for the expected uA of current to a DMM.

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A voltmeter, if you need to power exactly this kind of device, draws very little current. A thin gauge is more appropriate. Even wire as thin as 28 AWG can handle 226 mA for power here) and differently from the monstrous 12 AWG is more likely to burn out without other damage if you short circuit anything by chance. 12 AWG may hold over 40 amps when you do not want, and the bike battery may actually deliver that much to no good. A circuit breaker is also a good idea but there is always a wiring also before the circuit breaker.

Especially if it is some LCD voltmeter drawing just a few mA or less, use thin wires for it. If the power input is separate from the measurement input, use separate pair of wires to connect input where it should measure (exactly the battery, not the voltage at the power terminals of the voltmeter).

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  • \$\begingroup\$ Sometimes in a bike or boat or car, there is a minimum wire gauge used. Not only for current but because if the wire is too thin it may be broken accidentally. If it is possible to use 20AWG, that is better than 28 AWG because of other concerns (not current). \$\endgroup\$ – mkeith Jan 1 at 19:56
  • \$\begingroup\$ This may depend on how well is the wiring protected. It is also possible to use a thin ribbon cable with all wires bundled together for more robustness. \$\endgroup\$ – h22 Jan 1 at 20:31
  • \$\begingroup\$ True. Also, maybe if the wire is inside of some kind of tubing or conduit it could be OK also. \$\endgroup\$ – mkeith Jan 1 at 23:37

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