The full output of Alternator feeds directly through the Amp gauge, rather than having it just monitor the load. The gauge by design is marginal, especially when one has installed a higher output alternator, or has an extreme load on the system when alternator is at full output. There have been several cases of fires resulting and I have 2 failed & fried Amp gauges from different trucks.

The alternator puts out sufficient amperage & voltage to weld with so it's a considerable amount of electricity there. Plain & simple, the gauge can fail and start a fire. If you have a mechanical oil pressure gauge that had a slight leak, or more commonly, one that has vinyl tubing running to it, you've got some serious fuel for a fire & switching off the ignition & gauge won't stop the flames or the oil. It's also a hard place to access, to know that a fire has started or worse, to try & put out the fire.

A Volts gauge on a light-gauge leg [20 ga] running off the main circuit, without the Amp gauge's heavy gauge wiring & high current flow, is much safer by design for monitoring system voltage.

A Volts gauge that's junk by design, such as the Autogage unit by Autometer, can cause the same disaster when full current passes through it, which in fact happened to me. The vinyl tubing to mechanical oil pressure gauges also melted through because of the wire fire, so plenty of fuel for the cab fire that resulted. The engine oil from melted vinyl tubing also squirted all over my pants legs which could have caught fire too, complicating the problem somewhat.

Just because a person has never had a house burn down isn't good reason to not own any fire extinguishers or to say that fire departments are a stupid idea. Come to think of it, I've got 3 10lbs fire extinguishers in my Town Wagon, 2 in my Ram. To each his own?