Relay wiring question

[Jarpin]

What size AWG does a ground wire for a 30 amp BOSCH style relay need to match with? Does it need to be the same gauge as the switched (18 AWG) wire? The load and battery wire pins are 16 AWG, so that's a little too large. I can't imagine that there's a great deal of juice passing through it.

Thanks.

 

[clinebarger]

You can activate the control coil side of most automotive relay's with a incandescent Test Light, 20-22 AWG is pretty common for relay coil control.

 

[Jimmy9190]

It depends on what the relay is doing. The rule of thumb (Kirchoff's Law) of a basic circuit says the ground wire needs to be the same gauge as the power wire, but with a relay if the switched power wire is on terminal 85 or 86, then the ground wire on the other terminal could be a smaller gauge without really affecting anything because all that 85 and 86 do is energize the coil. Depending on how the relay is wired the input on terminal 87 and output on terminal 30, or the common 87a would both need to be the same size though to avoid any voltage drop in the circuit/relay application.

 

[Donald]

The whole point of a relay is to use a low draw coil to switch a big draw load. Most auto relays are low draw coils. The starter solenoid is a medium draw coil switching a huge draw load.

 

[qjohn]

It is a good practice to select the gauge of the wire based on the upstream fuse. The wire needs to handle the current of the load (in this case the coil current of the relay). But also; if there is a short circuit then the fuse should blow before the wire melts.

 

[Jarpin]

It depends on what the relay is doing. The rule of thumb (Kirchoff's Law) of a basic circuit says the ground wire needs to be the same gauge as the power wire, but with a relay if the switched power wire is on terminal 85 or 86, then the ground wire on the other terminal could be a smaller gauge without really affecting anything because all that 85 and 86 do is energize the coil. Depending on how the relay is wired the input on terminal 87 and output on terminal 30, or the common 87a would both need to be the same size though to avoid any voltage drop in the circuit/relay application.

It's powering a small air horn. Since the horn circuit is momentary on-off, I thought it would not make too much difference if the trigger wire is 18 AWG and ground is 20-22 AWG. Thanks for the detailed info.

 

[Jimmy9190]

It's powering a small air horn. Since the horn circuit is momentary on-off, I thought it would not make too much difference if the trigger wire is 18 AWG and ground is 20-22 AWG. Thanks for the detailed info.

YW