Re: [MH-list] series or parallel

Simon Whitehead

On 31 Mar 2004, at 23:00, DABurleigh wrote:

can anyone (or Andy) tell me when running a feed to a row of running lights
(top front and rear) if they are wired in series does that reduce the
intensity of each bulb by a factor equal to the number of bulbs i.e. 4
of 12 watt on 12 volt supply would they give out 3 watt luminance each. Or
if wired in parallel would they all give 12 watt luminance but be too much
for the in line fuse (48 watts)

In parallel, a 48 watt fuse would indeed be on or close to the limit, as at
these low currents there will not be much voltage drop in the wiring.

Fuses are rated in Amperes, not Watts.
48 Watts at 12 Volts gives 4 Amps. However, if the lamps are halogen you would need at least a 10 Amp fuse (and I would be prepared to accept that other's experience might show that a higher rating is required) because of the very high starting current of that type of lamp.
It is also the case that 4 Amps will cause quite a surprising amount of voltage drop in the wiring of the average MH (including my new Hymer!). The drop that I measured was about 1.5 volts from the distribution board to the last lamp. This drop doesn't matter to the lamp though as the lamps are actually domestic versions designed for 12 Volt operation and a fully charged battery is running at about 13.8 Volts.

Filament lamps use a resistive element that gets hot when a current is passed through. The filament is tungsten (or similar) and it is surrounded by an inert gas such as argon (for conventional lamps) or something more exotic like Krypton or even a halogen. The gas takes the excess heat away (and can also do some other clever things) without allowing the filament to oxidize. When the filament is cold, its resistance is small and if one was to calculate the power based on that resistance the result is a surprise. I have just measured the resistance of a 12 Volt halogen lamp rated at 20 Watts and when cold it is 0.8 Ohms. This would seem to correspond to a power of 180 Watts. Fortunately, the resistance increases as the filament gets hot so that at its proper operating voltage its resistance will be 7.2 Ohms to give the correct power. The running current for a 20 Watt lamp at 12 Volts is 1.67 Amps, but the starting current for this lamp is 15 Amps which is 9 times the running current. Fortunately, the lamp will heat up very quickly and the fuse will only get a short shock but it is easily seen that a fuse rated at the running current of the lamps will probably blow often, if not every time. The safe fuse rating is one which is less than the wire rating and not one set for the lamps. If the rating of the wire is too low, then it is unsuitable for use with those lamps. If you are stuck with the wire you have, use fewer lamps or lower power lamps. This is not an area to take risks.
Faults in vehicle wiring is one of the main causes of vehicle fires and current overloading is only one of the factors. I would always use a heavy gauge wire in my MH. The investment in wire is trivial when compared to everything else.

Simon Whitehead

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