The prior article was about why additional fuses and relays are usually not needed (at least in the MGA). This one is about when they may be desirable or necessary.

On 9/3/2016, Mark Wellard in Australia wrote:
"I was looking at the twin horn page and thinking about the current. If the twin horn loop is heavier gauge than the rest of the loom, then why was a relay not used? Other cars, such as the Magnettes, had a horn relay".

Most likely they wanted to use smaller wires for the horn switch in the Magnette. This depends on the type of switch and where it is located. When the horn switch in on or near the steering wheel, you don't want to run very heavy wires up the steering column. So they run small gauge wires to the switch and use a relay for remote power switching.

A relay is just a remote controlled switch. It has one of two different purposes. Logic switching, or remote power switching.

Logic switching allows me to make my turn signals double as 4-way flashers, or a lighting switch to control 2-speed wipers (needing 5 wires on the motor). It can also make driving lamps go off when dipping the headlamps, or flash high beams with a momentary touch of a ground wire.

Remote power switching (with a relay) allows use of smaller wires on the switch and lighter duty (lower current) switch. MGA uses high current switches, and nothing in the car draws current high enough to worry about the size of wires behind the dash.

The generator can put out more than 20 amps and needs large wire. The dual horns can draw up to 25 amps momentarily so need large wire. Power feed from control box to ignition and lighting switches draws up to 20 amps and needs large wire. Any wire must be sized to carry the current required by the load device. From the other side, any load device needs wires large enough to carry the current draw of that device.

Minimum wire size for anything in an automobile should be 18-gauge. This is a mechanical requirement to prevent breakage of the wires with physical shock and vibration. Parking lights 4 x 6-watts each = 24-watts = 2-amps at 12 volts. Brake lights 2x21w = 42w = 3.5a. Head lamp high beams 2x60w = 120w = 10a. Horn is high power momentary, maybe 10a each x 2 - 20a. The horn definitely needs large wires. Headlights could work 10a on 18-ga wire, but it involves a small voltage drop due to resistance and length of the wire so they use a larger wire to improve voltage at the lamps for more light output. Given wires large enough to carry the current, and a suitable switch, you don't need relays for any of this.

When engineers began installing "multi-function" switches for turn signal, headlight dipper, and horn all in one switch, they wanted to use lighter duty switch contacts, and they didn't want to run that many heavy wires up the harness to the steering column. So they used relays to allow light duty switch contacts and smaller wires at the switches. This then requires a relay to remotely switch power on the heavier wires going to the load devices.

There is one application in the MGA that may require a power relay, perhaps due to short-sightedness of the designers. The Fog light circuit takes power from the parking light circuit, and runs one 18-ga wire from the "F" switch to front of car. This is sufficient to power one fog lamp, which was a customary practice, back in the day. If you install dual fog lamps or dual driving lamps, the single 18-ga wire is marginal in capacity. It can work for dual fog lamps with moderate power when maximum illumination is not mandatory. A relay is desirable for dual fog lamps. A relay might be required for dual driving lamps with higher power, as a single 18-ga wire may cause too much voltage drop with the higher current (similar to high beam headlamps).

Without a relay you only need a single wire running around the circuit loop. Battery to switch, switch to load, and ground return to battery. To use a relay you need more wires. Battery to relay, relay to load, and ground return to battery, plus battery to switch, switch to relay coil, and relay coil to ground. Or for switching you can go power to relay coil, relay coil to switch, and switch to ground. That last configuration allows a single power wire to supply both the load and the relay coil (one less wire). The additional wires are added expense, as well as cost of the relay. So if you don't need a relay don't use it.

There are some people who advocate use of relays for remote power switching to reduce voltage drop for improved performance of headlamps or heater blower or dual horns. If this is to be of benefit, it implies that the original wire size is too small or the switch may have undue internal resistance to cause some voltage drop. To make the relay work, you need to install an additional heavy gauge power wire (thicker than the original harness wire) from battery supply to the relay. You can then use original switch wires to operate the relay (if the relay is located close to the output end of the switching wire).

In the case of vintage car wiring being too small for electrical loads of modern devices, you will always be installing at least one additional thick power supply wire to the new relay. If this heavier wire was originally built into the harness, then the switching wires could be smaller. To put the new heavy gauge wire into the harness and make switch wires smaller, you would have to build a completely new custom wiring harness (very expensive).

There are some relays with internal ground designed to ground both the coil and the load to the car body. For this you run power to the horns and the horn return to the relay. Also run power to the switch, and the switched wire to the relay. Then touching the button triggers the relay (which is internally grounded) and the relay grounds the horn circuit to the car body. In that case the relay may only have two wires and case ground to the car body. You can do this in the MGA using the original wiring by changing the horm button from ground to power, and putting the relay between the horn return and switch wires. The relay will then shunt horn return directly to ground rather than passing it back through the harness wire to the switch. Shortening the ground return path may improve voltage at the horn without adding any new wires (but you have to use the correct "2-wire-and-case-ground" relay).