Watts to amps conversion query

[Trevor]

Accorduing to the manual for my Treg 3.o TDI it can handle any electrical appliance as long as its current draw does not exceed 120 watts. Now how much does that boil down to in amps drawn?

[Koebelwagen]

I = P/V

Thus 120/12

10A

[HenryT]

V does not always = 12V it can go as low as 11.5 and as high as 14.3.
Therefore 120W at 11.5V = 10.43A and at 14.4V = 8.39A.

Another thing to remember is the thickness and length of wire you use to the appliance/equipment. This induces a volt drop (the wire consumes power) and can increase the amperage.

Another thing to remember is the thickness and length of wire you use to the appliance/equipment. This induces a volt drop (the wire consumes power) and can increase the amperage.

[Koebelwagen]

V does not always = 12V it can go as low as 11.5 and as high as 14.3.
Therefore 120W at 11.5V = 10.43A and at 14.4V = 8.39A.

Another thing to remember is the thickness and length of wire you use to the appliance/equipment. This induces a volt drop (the wire consumes power) and can increase the amperage.

Another thing to remember is the thickness and length of wire you use to the appliance/equipment. This induces a volt drop (the wire consumes power) and can increase the amperage.

Yes what you say is true, but I am willing to bet that the aux power circuit he is refering to is protected by a 10A fuse.

[ADUP]

Word of caution.......

Electrical appliance..........means 240 volts !! If it the same as the Tiguan, it comes with a 12 volt to 240 Volt invertor and in the case of the Tiguan it means that on 240 volt only 1/2 Amp on the output is already on the limit of the invertor.

Now....if 120 watts can be delivered on 240 volts from the invertor, it means 1/2 amp on the 240 volts. If the invertor has an efficiency of 100%.....means 12 Volt at 10 amp from the battery. Unfortunately there is no invertor available on the market with 100% efficiency. Maybe 80%....means 150 watt from the battery.....this means 12.5 amp!!!!!

Maybe you can run a laptop, or a couple of energy savers or a cellphone charger but not a hairdryer or a heater or kettle..........

[Trevor]

I was reading the handbook of my Treg and saw the reference to the wiring on the assessories outlets in the boot and elsewhere being able to take any appliance so long as it does not consume more than 120 watts. I want to run a Campcraft freezer on the rear outlet and was scared it would not be able to take the power drain but as this unit only uses in the region of 2.5 amps I should be safe.

[Fluffy]

OK,

First things first the Egg supplies nominal 12v. Unless your one is different to my one.

Secondly Power (watts) is power from any angle. If an appliance states x watts then it is x watts no matter what voltage or technology is inside. If it states current then you have to know at what voltage and do the sums youself as above.

One slight error above ( sorry HenryT ), on an electrical circuit with a resistive load,ie Peltier effect or DC motor compressor fridges, lights etc , then if you use a THIN wire the system will draw LESS AMPS amps. By your own definition the wire "consumes" some of the power, effectively stealing it from the appliance. Concider an extreme example, attach a 12 volt car headlight to 1m of cable, the light will be bright. Attach the same bulb to a 1000m of cable the light will not even glow, because all the voltage will be dropped across the cable.

I am not trying to appear "clever", and if I can share my knowledge I will gladly write a detailed explanation with all the calculations and theory behind this issue so that anybody can use the info. (Ohms LAW 101)

The above is not conjecture, I own a very high tech electronics company, and I have both used the above thoery, and tutored clients and staff on the above issue for over 30 years.

I hope this helps.

[Trevor]

Thanks Fluffy. Am I correct in assuming that I can safely connect a freezer to the 12v outlet if it draws between 2 - 2.5 amps per hour. I don't have a clue how many watts it consumes.
??

[Fluffy]

Very safe, it would be 20-30 watts which is very roughly equivalent to leaving your parking lights on, depending on the vehicle.

I checked and your spec is par for the course for something like an Engel 40L fridge/freezer.

No effect on your vehicle what so ever with the motor running

[Trevor]

Thanks Fluffy.

[Soutter]

Just take some spare fuses for the circuit along too. Just incase the startup surge current of the freezer is for some reason higher than the fuse rating on the circuit... I doubt it will be though, circuit is most probably 10A, but just to be sure

[Trevor]

Thanks -- a very good idea.

[Dif]

Watts(36) = Volts(12) x Current(2.5)

I'm no electrician though! Hope I'm right, if not, shoot me!!! My wires aren't that long either!

[ArthurB]

12 replies to one question about Ohms law.... remarkable :-)

On a more practical note, I prefer to lay my own 15Amp connection to the boot of any car I buy with a fuse at the connection to the battery - in that way, I know I can power my fridge, an inverter or a tyre pump without stressing about 2.5 Amps fuses and thin little wires the manufacturers use to save their own profits.

[ADUP]

More remarkable is that fact that the conclusions are still not correct.

Watts(36) = Volts(12) x Current(2.5)

??
wow.....my maths produce...

Watts(36) = Volts(12) x Current(3)

Dif, IMHO, it's a good thing you are not an electrician.

[RedLineR]

Just be careful when trying to apply this to induction motors. Then it don't work. But for 12V DC stuff it works fine...
Volts = Amps x Resistance
Amps = Volts/Resistance
Resistance = Volts/Amps
Watts = Volts x Amps
Volts=Watts/Amps
Amps=Watts/Volts

[Ockert123]

<12v outlet if it draws between 2 - 2.5 amps per hour>
<Very safe>

Normally the danfoss compressors draws around 5 amps when running. Because it switches off at some or other point, they will draw roughly in the region of 2-3 amps per our, which is then mistakebly given as 2-3 amps per hour. Not the same and not safe. This will actually be in the region of 60watts (12V x 5 amps). Need to double check on the fuses and wiring of the <12v outlet>. It could cause problems. Rather install your own wiring and 12v outlet, then you know whats going on.

[Ockert123]

<Just be careful when trying to apply this to induction motors>
Redliner, that is true, for all AC power. There are losses etc to consider. Therefor, what ever your answer is, X 0.7 (cos factor) to give a more accurate value.

[ArthurB]

And another point - the Engel swing motor probably does not have a much higher current draw at start-up, while the danfoss and other motor driven compressors certainly draw higher amps during startup. If your fuse is the fast-type, it may blow at startup already, and a slow fuse is not as safe as a fast one.....

That is why I still say - install a proper wire set and fuse from the battery to the back.

2.5A in the boot is a serious waste of time except of you want to power a small light or thermo-electric cooler there.

[Jengel]

Most fuses used on cars are slow blow types. If the rating say 120 watt it means continous rating and a surge is allowed to normally 150%. A major type of Time Delay fuse is the dual-element fuse. This fuse consists of a short circuit strip, soldered joint and spring connection. During overload conditions, the soldered joint gets hot enough to melt and the spring shears the junction loose. Under short circuit conditions, the short circuit element operates to open the circuit. Slow-blow fuse allows temporary and harmless inrush currents to pass without opening, but is so designed to open on sustained overloads and short circuits. Slow-blow fuses are ideal for circuits with a transient surge or power-on inrush. These circuits include: motors, transformers, incandescent lamps and capacitate loads. This inrush may be many times the circuit's full load amperes. Slow-blow fuses allow close rating of the fuse without nuisance opening. Typically, Slow Blow fuses are rated between 125% to 150% of the circuit's full load amperes.
I checked my fridge with a Danfoss unit on the osciloscope. The surge current highest was 6.2A while running at 13.6V around 3.3A (80 liter fridge).
This power might not be enough for a compressor but surely a fridge and some other stuff like camera battery charger, laptop etc can be used from here as well. The rating of most cigarette or aux power plug is 10A @ 12-14V. Maybe a good test is to idle the car fast at say 1500rpm and to measure the voltage. Connect a 100W spotlight and note the voltage drop at the plug. If the voltage drop is less than say 10% your wiring is adequate. More than this, I'll rather replace with heavier duty wiring and directly from the battery if possible.
Johan