This article will explain the wiring sizes used in the MGA. In automotive use some wires may appear to be slightly undersize for the current load they must carry. This is done for cost and weight reduction. It is feasible because the wire runs are relatively short, so voltage drop will not be excessive. Most of the wiring harness is also run in open air, which will aid in cooling the wires if they do tend to get a little warm.

Original electrical wires in the MGA are composed of multiple strands of thin wire to make up the overall larger conductor. Multiple thin strands make the wire more flexible. Most wires are composed of the same size small strands with larger wires using more strands to make up the overall larger wire size.

A common unit for measurement of cross section of a wire is the circular mil, which is the area of a circle 0.001-inch in diameter (slightly less than one square mil). This is a convenient unit for expressing wire size because it varies as a simple square of the diameter, and multiple small strands can be added together to determine the cross sectional area of the larger stranded wire. The individual strands are 0.011-inch diameter, or 121 circular mil. Wires used in the MGA wring harness are three different sizes, 14, 28, and 44 strands. As such, it is possible to determine the wire gauge by counting the number of small strands. The equivalent wire gauges are like this:

14x121 = 1694 cir-mil, equal to .041 diameter wire or 18 AWG
28x121 = 3388 cir-mil, equal to .058 diameter wire or 14 AWG
44x121 = 5324 cir-mil, equal to .073 diameter wire or 12 AWG

Most of the individual load circuits use 18 AWG. Headlight circuits use 14 AWG (up to the last splitter). Power from starter switch (or starter solenoid) to control box is 12 AWG. Wire from dynamo to control box should be (functionally) 12 AWG, but could be 14 AWG. Wire from control box to lighting switch (terminated on ignition switch) should be 12 AWG (functionally), but could also be 14 AWG.

Wire color is NOT any indication of the wire size. Color is used to designate the circuit connection. When it comes to buying wire, you can find mostly solid colors at a local auto parts store. To get stripes you need a real wire supply house. British Wiring, Inc has all of the wire colors used in British cars (and almost anything else in the world).