Lucas wiring systems as used on virtually all British cars since the 1930s are a source of frustration and bewilderment to a great many sports car enthusiasts. In fact, Lucas wiring is clearly engineered around a standardized color code and cable size formula. This system is used on all British sports cars, and once understood, is very simple.
The following detailed explanation has been excerpted from a Lucas technical manual which dates from the mid-1950s.
With few exceptions, the electrical system of a motor vehicle can be considered as a series of simple circuits, each consisting of the component, its switch, and three wires: feed, switch wire, and return. On earth return systems, the return circuit is provided by the frame of the vehicle, although in the case of components insulated from the chassis, an earthing lead is also necessary. Some variations are to be found, such as fuses, two-way switching, and so on; but the principle of feed wire, switch wire, and return remains, and it is upon this principle that the Lucas color scheme is based. The insulation on feed wires carry a main color only, switch wires have the main color of feed with a colored tracer running the length of the wire, while return earthing leads are black.
Where components are switched or controlled in the earthed side (that is, with the switch wire in the return side of the unit, instead of on the feed side), this is normally indicated by the use of a black tracer.
Main colors, of which there are seven, are allocated to the circuits as shown below. The practice of feeding certain of the accessories through the ignition switch, and auxiliary lighting circuits through the side and tail lamp switch, is recommended, so that the side and tail lamp switch and ignition switch wires become feeds to other circuits or, in effect, master switch wires.
Brown: Battery circuit. From battery or starter switch to ammeter or control box and (with compensated voltage control feeding lighting and ignition switches and radio, when fitted) from control box terminal. Also, from starter switch to electric clock, inspection sockets, and battery auxiliaries fuse (from which are fed electric horns, cigar lighter, interior lights, etc.).
Yellow: Generator circuit. From generator terminal to corresponding control box terminal and to ignition warning light.
White: Ignition circuit and all requirements essential when ignition is switched on but which do not require fusing, e.g., electric fuel pump, starter solenoid switch, etc.
Green: Auxiliary circuits fed through ignition switch and protected by the ignition auxiliaries fuse, e.g., stop lamp, fuel gauge, direction indicators, windscreen wipers, etc.
Blue: Headlamp circuits. Fed through terminal on lighting switch.
Red: Side and tail lamp circuits. Fed from terminal on lighting switch. Included in these circuits are fog lamps, panel lights and other lamps required only when the side lamps are in use.
Black: Earth circuits. If a component is not internally earthed, a cable must be taken to an earthing point on the chassis.
Hopefully, the above information, combined with a proper wiring diagram for your car, will help turn that multicolored mass of spaghetti into an understandable wiring system. Don’t get discouraged; Lucas really did make an effort to use logic in their wiring.