Gauge wiring help Options

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The 30A range should be ok. Keep in mind that the entire alternator current is not measured by the ammeter. The ammeter should measure just the current going into and out of the battery during normal operation.

See diagrams below. For classic low impedance ammeter applications, the ammeter is directly in series with the alternator and battery. Of the 2 or more wires going to the battery, the smaller ones are for the alternator/vehicle circuits. It's these wires that the ammeter measures. For low impedance ammeters, the wire to the ammeter should be very large like 10gauge.

As the vehicle runs, the accessories/lights/ECU are powered basically by the alternator and the current to these items is not measured by the ammeter.

For high impedance ammeters, a small series resistor is placed between the alternator and the battery. Small gauge wires are run to the ammeter like 20 or 24 gauge.

You can tell if you have a high or low impedance ammeter by measuring the resistance across the ammeter. If low impedance, you should measure resistance in the tenths of ohms. This requires heavy wire routed to the ammeter. If high impedance, this one should measure more than 1000 ohms, probably much higher.