MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - electricsl wire gauge

Last night I began restoring one of a pair of Lucas "Ranger" driving lamps which I originally bought back in 1966!! For the many, many 1000's of miles they have traveled, they are in pretty good nick. They look like a Lucas "Flamethrower" but the shell isn't nearly as deep. They use a 55wt QH bulb and I believe I recall them being rated at 20,000 candlepower apiece.
Any way I intend to mount one of them on my MGA 1600. They were fitted with what appears to be 14 or perhaps 16 gauge line. I intend to take the feed directly off the hot line at the regulator or off the starter solenoid to minimize amp drop.
My question is; what gauge wire is appropriate to drive one of these lamps? Is a 14 or 16 wire OK or should I go to a heavier 13 or 12 gauge?
My thanks to all.
John
John

John - At 55 Watts, the lamp would be drawing a little less than 5 amps (Current = Power/voltage = 55/12 = 4.58)
For 5 amps, 16 gauge wire is sufficient or you can go with 14 gauge and have a large safety factor.
Good luck - Dave
David DuBois

Even 18 gauge wire is enough. Reference : http://www.allaboutcircuits.com/vol_5/chpt_3/2.html

There are 2 basic concerns with the determination of the needed wire size, 1) temperature rise and 2) voltage drop. From a temp rise standpoint, 18 gauge wire in open air is no problem. 18 will easily handle 13+ amps in open air depending on the ambient and the insulation grade. Put it in a closed in harness with other wires carrying current and that's a different story. There are many ways to estimate the rise but best to actually make a measurement if it is in a harness.

The second, voltage drop, is much easier to calculate there are many references which list the resistance of wire. 18 ga is 6.45 milliohms per foot. So a run of 10 ft at 5 amps will result in 0.064 volts drop (10*6.45milliohms) increasing it to 16 ga results in 0.040 volts lost and 14 gauge get us to 0.025 volts. Regardless of which wire size you use. Any of these will no have a discernable effect on lamp brightness. I think the largest voltage drop is going to be the switch contact resistance, the lamp socket terminals and any wiring connections.

Myself, I would not think twice about the use of 18 gauge, but would probably use 16 ga if I had it, just on general principles. Anything heavier than 16 gauge is definitely overkill.

Flames on!
Chuck
Chuck Schaefer

Chuck - Great web site! I agree that 18 gauge is probably sufficient for the 5 amps required by the light that John is using, but I usually like to error on the side of greater capacity, especially since a higher wattage bulb may be used int he future. And yes, 14 gauge would be overkill. So there is your answer John, eithe 18 gauge or 16 gauge if you want a bit more capacity for future upgrades. Cheers - Dave
David DuBois

My thanks to all who responded to my question. Thank you, Chuck, that is a great web site.

A friend locally who also has an MGA suggested using a relay for this application.
So I'm planning on using 14 gage to supply the hot side of the relay ( 30 amp cap)and use 16 gage on the lamp side. This way if I want to I can add on the second lamp; also fed from the relay
This is a great forum - there's so much knowledge out there.
John
John

This thread was discussed between 09/11/2004 and 11/11/2004

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.