Power Inverter Question

[JJ Campbell]

I have a Reiff Preheating System installed on my Rotax 912. Since the standard Reiff system for a 912 is 150 Watts, I'm assuming that is what I have. I also have a 300 Watt Power Inverter for use on my automobiles cigarette lighter. My car has a 15 amp fuse on the cigarette lighter circuit.

I googled the formula for calculating amps and what I found was:
AMPS = WATTS/VOLTS

Am I correct in assuming that the Reiff Preheater will pull 12.5 Amps (12.5 = 150/12) and I shouldn't blow the fuse on my car?

[JimParker256]

Your calculation is correct: 12.5 Amps

[drseti]

Actually, it will draw a bit less than you calculated - and a bit more! A "12 volt" car battery, when fully charged, will really put out about 13.5 volts, which would seem to reduce your current to just over 11 amps. But, inverters are not 100% efficient (there are internal heat losses), so they draw more power from the battery than they deliver to the load.

If (just picking a typical number) your inverter runs at 80% efficiency, the actual current draw from the battery will go up 20%, to just over 13 amps. So this is not an exact science.

A typical rule of thumb is to use a source capable of delivering about twice the current you are planning to consume (and fuse accordingly). So you'd be running close to the edge, and might blow the occasional fuse if there are any surges or fluctuations.

My advice: carry a box of spare fuses.

[3Dreaming]

The other thing to consider is that the cigarette lighter circuit might be set up as an intermittent load circuit, and not really had heavy enough wiring to a continuous load of 13 amps for extended periods without the wiring getting hot. When it gets hot the resistance will go up and the circuit will require more amps.

[JJ Campbell]

Actually, it will draw a bit less than you calculated - and a bit more! A "12 volt" car battery, when fully charged, will really put out about 13.5 volts, which would seem to reduce your current to just over 11 amps. But, inverters are not 100% efficient (there are internal heat losses), so they draw more power from the battery than they deliver to the load.

If (just picking a typical number) your inverter runs at 80% efficiency, the actual current draw from the battery will go up 20%, to just over 13 amps. So this is not an exact science.

A typical rule of thumb is to use a source capable of delivering about twice the current you are planning to consume (and fuse accordingly). So you'd be running close to the edge, and might blow the occasional fuse if there are any surges or fluctuations.

My advice: carry a box of spare fuses.

Thank you Dr Seti. I have two cigarette lighters in my car. One is on a circuit for the lighter & ignition switch fused at 20 Amps. The other is on a circuit by itself fused at 15 Amps. Obviously, stay away from the one tied to the ignitions switch and use the dedicated circuit. However, it does make me wonder if there is any risk in replacing the 15 Amp fuse with a 20 Amp? This wouldn't give double the load but it would give me a larger safety margin.

[3Dreaming]

Actually, it will draw a bit less than you calculated - and a bit more! A "12 volt" car battery, when fully charged, will really put out about 13.5 volts, which would seem to reduce your current to just over 11 amps. But, inverters are not 100% efficient (there are internal heat losses), so they draw more power from the battery than they deliver to the load.

If (just picking a typical number) your inverter runs at 80% efficiency, the actual current draw from the battery will go up 20%, to just over 13 amps. So this is not an exact science.

A typical rule of thumb is to use a source capable of delivering about twice the current you are planning to consume (and fuse accordingly). So you'd be running close to the edge, and might blow the occasional fuse if there are any surges or fluctuations.

My advice: carry a box of spare fuses.

Thank you Dr Seti. I have two cigarette lighters in my car. One is on a circuit for the lighter & ignition switch fused at 20 Amps. The other is on a circuit by itself fused at 15 Amps. Obviously, stay away from the one tied to the ignitions switch and use the dedicated circuit. However, it does make me wonder if there is any risk in replacing the 15 Amp fuse with a 20 Amp? This wouldn't give double the load but it would give me a larger safety margin.

Putting in a higher amperage fuse is not a good idea. The circuits are designed for a specific load, and the wire size is chosen accordingly. The fuse is a sacrificial link in the circuit, and designed to protect the wire. If you put in a higher amperage fuse and draw to much current the wire might get to hot possibly causing a fire before the fuse blows.

[JJ Campbell]

Putting in a higher amperage fuse is not a good idea. The circuits are designed for a specific load, and the wire size is chosen accordingly. The fuse is a sacrificial link in the circuit, and designed to protect the wire. If you put in a higher amperage fuse and draw to much current the wire might get to hot possibly causing a fire before the fuse blows.

Ouch! You know that old Chinese Proverb "The Doctor say don't do that!" Thanks for the warning. I ordered a box of assorted auto fuses as Dr. Seti suggested.

[JJ Campbell]

I thought I'd post the resolution:

The 20amp fuse that I that I thought had the cigarette lighter and ignition switch on it because the manual listed it as Cigar Lighter/Ignition was actually only the lighter socket the ignition part was a weak attempt to let you know that it was only powered when the ignition key was in the on position. This socket never worked. A fact that I didn't discover until after I had owned the car for a couple of years. This was something that I wasn't particularly motivated to address since I had the other always live socket on the 15amp circuit. I'm convinced the car left the factory with a defective fuse.

When I visually inspected the fuse, it looked fine but it failed a continuity test. I replaced the fuse with one of the 250 in my recently purchased fuse collection (thank you Dr. Seti) and the socket was alive and well.

The extra 5amps this circuit will give me will really increase my likelihood of not blowing fuses when I use the inverter to feed the preheater.