Why Use 12 AWG Wire For My Ham Radio?

[SSTJ]

Yaesu dual-bander owner's manual states that the wiring should be capable of handling 10A continuously. Max current draw, on UHF, is 9A. It recommends a fast-blow 15A fuse. There is one pair of fuses by the battery, and another pair by the unit.

Based on that, I figure 14 AWG wire would suffice. But they recommend 12 AWG. Are they just covering their backsides? And yes, I'm asking because I just finished wiring it with 14 AWG and would like to know now, rather than later, whether I've done something stupid.

I thought it could be something about avoiding voltage drop, but it's only about 7' of wire. I just tested the voltage at the battery and at the ends of the wires. There is no difference, down to the second decimal.

Thanks!

 

[Zorba]

Did you test your voltage when you keyed the mic?

 

[SSTJ]

Nope! Tell me more!

Thanks.

 

[Zorba]

That's when the radio pulls the max power - and will create the most voltage drop.

 

[SSTJ]

Ah, I see ... Well, I will finish the installation, key up and see what kind of voltage drop I get when I do.

Many thanks.

 

[Mr. Bills]

Don't overthink this.

14 gauge will work just fine for most mobile installs. It is definitely sufficient if you ground to the body/frame near the radio rather than run the negative wire back to the battery. [Wire runs in nearly every wire gauge chart are measured by the total length of the positive wire + negative wire, not just the distance between power source and appliance.]

I use 12 gauge wire, primarily because I buy it in large rolls and always have some on hand, and secondarily because I follow the old school practice of running positive and negative wires from battery to radio thus I have a relatively long wire run. Even so, 12 gauge isn't mission critical for the ham, CB or GMRS radios typically used in mobile installs, and any voltage drop resulting from using 14 gauge wire instead will probably be undetectable in real world use.

I wouldn't hesitate to use 14 gauge if is what I had on hand.

73.

KG6LMV

 

[SSTJ]

Don't overthink this.

14 gauge will work just fine for most mobile installs. It is definitely sufficient if you ground to the body/frame near the radio rather than run the negative wire back to the battery. [Wire runs in nearly every wire gauge chart are measured by the total length of the positive wire + negative wire, not just the distance between power source and appliance.]

I use 12 gauge wire, primarily because I buy it in large rolls and always have some on hand, and secondarily because I follow the old school practice of running positive and negative wires from battery to radio thus I have a relatively long wire run. Even so, 12 gauge isn't mission critical for the ham, CB or GMRS radios typically used in mobile installs, and any voltage drop resulting from using 14 gauge wire instead will probably be undetectable in real world use.

I wouldn't hesitate to use 14 gauge if is what I had on hand.

73.

KG6LMV

Very helpful, thank you. If you don't mind continuing my education, can you tell me more about the old-school practice of running both wires to the battery? I was advised to do the same on my CB (and I did), and then saw that my Yaesu manual recommended it too (and so I did).

 

[Mr. Bills]

There are usually two reasons advanced by those who advise running the power wire directly to the battery:

1. Reduction of interference picked up by the wiring, and inductance (which is undesirable in power supply connections).

2. Minimizing resistance. A radio can be a fairly heavy load as car accessories go, so you want to avoid any voltage drop you can, and the chassis may not give you the best conduction to the battery.

According to one post I read somethere on the Internet, neither of these is a reason to wire specifically to the battery per se, only to avoid using a completely different conductor (the chassis). According to K0BG.com, a site dedicated to mobile amateur radio, some modern cars have electrical systems in which you shouldn't wire direct to the battery terminals, for example:

"The use of these sophisticated subsystems have necessitated the relocation of the ELD to the negative lead of the SLI battery as shown at right (surrounding the battery ground lead), and below right (incorporated in the battery negative connector). The photos are of a 2014 Nissan Titan, and 2013 Honda Accord respectively, but other makes are similar such as Ford's F150 shown below at left.

"Ford's Battery Monitoring SystemIt should be obvious that transceiver ground connections cannot be made directly to the battery as recommended in the past, as doing so would bypass the BMS. Thus in the examples shown, the negative lead would be attached to the battery's chassis connection point (Titan), or on the ground side of the ELD (Honda).

"(Note that the reference to connecting to the chassis is to the point at which the battery is connected to the chassis, not elsewhere on the chassis.)"

The article goes on to say, after discussing other possible complications:

"The bottom line here is, if in doubt, read your Service Manual, or contact your dealer's service department before undertaking your installation."

I have a third reason - simplicity. Should there be a problem I know that my communication device wiring is completely independent of the vehicle wiring and can be disconnected/isolated in seconds . That doesn't matter much in an old jeep, but in a newer vehicle it might, especially if still under warranty.

 

[SSTJ]

Many thanks. Very helpful.

 

[Zorba]

Myself, I run the positive direct to the battery, but ground the negative at the nearest convenient GOOD grounding point. Lower impedance on the negative side, less chance of RFI coupling on the negative side. YMMV and all that.