Simple wiring question, I'm 99 44/100ths % sure I'm right, but I have to ask

[kell]

What will probably happen is one light will be lower resistance than
the rest and take a lot of current and blow itself up as it lights
first and the others remain almost a short circuit.

I know they do this with xmas tree lights but they are usually run
well below 12 volts each.

They are in series, so current-hogging shouldn't be a problem.

 

[Fred Bloggs]

OK, I have a situation at work where I have to light a rather large
area, but the lights have to be very bright and extremely impact and
shock proof. So that pretty much rules out conventional incandescent
light bulbs and fluorescent tubes. Plus, I have to do this on kind of
the cheap.


So this is what I came up with: Wire 8 automotive clear fog lamps in
series. The lenses are rated for rock strikes and road debris at
70MPH, perfect. They are absolutely vibration proof, being off-road
lights, and because of the lenses, they are bright at hell. As a
bonus, I can get them for $12 each.

That said, they are rated 55 watts at 12VDC, but of course they easily
go to 14.4V: the output of a car alternator. And being light bulbs,
they couldn't care less if they are seeing AC or DC current. So,
according to Ohm's Law, 8 wired in series, assuming 14.4V, they should
now want for 115.2V. Perfect for plugging into a standard wall
outlet.

Am I right, or am I going to electrocute myself?

The line is rarely 115VAC, more like 125VAC, which would mean 125/8=15.6
per bulb, and if truly incandescent rated at 14V nominal would make for
a (14/15.6)^12= 73% reduction in operating life. Anyone have a problem
with this? The method I have seen used, and probably mandated by code,
is to install a stepdown transformer at each lamp, lamps wired in
parallel off main branch, you only need a 60VA or so which is not too
big, but there goes the budget. There may also be a fire safety issue
with applying the full line across a failed lamp which is not designed
to hold it off predictably.

 

[Jasen]

OK, I have a situation at work where I have to light a rather large
area, but the lights have to be very bright and extremely impact and
shock proof. So that pretty much rules out conventional incandescent
light bulbs and fluorescent tubes. Plus, I have to do this on kind of
the cheap.


So this is what I came up with: Wire 8 automotive clear fog lamps in
series. The lenses are rated for rock strikes and road debris at
70MPH, perfect. They are absolutely vibration proof, being off-road
lights, and because of the lenses, they are bright at hell. As a
bonus, I can get them for $12 each.

That said, they are rated 55 watts at 12VDC, but of course they easily
go to 14.4V: the output of a car alternator. And being light bulbs,
they couldn't care less if they are seeing AC or DC current. So,
according to Ohm's Law, 8 wired in series, assuming 14.4V, they should
now want for 115.2V. Perfect for plugging into a standard wall
outlet.

Am I right, or am I going to electrocute myself?

mount them out of reach, and on an insulating surface (because they
won't have insulation rated for mains) affix "live wires" warning
labels. after all that there's still an electrocution risk when one
of the bulbs blows and someone who doesn't understand what you've done
tries to replace it.

The safe way to power them would be from a 12V or 24V transformer.

 

[Homer J Simpson]

The method I have seen used, and probably mandated by code, is to install
a stepdown transformer at each lamp, lamps wired in parallel off main
branch, you only need a 60VA or so which is not too big, but there goes
the budget.

Code MAY require a certain maximum VA transformer - watch that one.

 

[redbelly]

You know what... regarding that 56/100% that's not so sure... does
anyone know how the resistance of a light bulb filament varies from
room temperature to operating temperature? (All within one second, of
course...)

It's a factor of 15 or 16 higher at operating temperature.

Mark