I'm working on a self-designed top-secret security system for my '70 Malibu. I will be dealing with some high current switching devices, MY QUESTION IS:

Approximately how many amps are drawn with EVERYTHING running. NOT starting the car, when driving, how much current is being drawn with "everything" in use;

Headlights/tail lights ON
A/C ON
A/C blower on HIGH
Radio (stock) ON
Horn blowing
turnsignals/flashers ON
Cigarette lighter ON
Dome light ON

You get the idea...

So what are we talking about, 40? 50? 60 Amps?

Please help.

Joe

http://www.chevelles.com/forum/wink.gif

Joe, It would vary from car to car. Using a shunt between the postive terminal and the positive cable you can measure the amp draw with an amp meter. I have a meter that I can do it with. Its an old analogue automotive test meter that I still use on occaisions where the high tech digitals won't work. Good luck.

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Philip Valentine
Gold Member #42
philip@chevelles.com

Add up all the watts and convert to amps.

I agree with what 64elcamino is recommending. I'd bet that you would see quite a large difference from car to car. An older car would be even worse.

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Wes. Vann
Technical Reference section
Gold Member #5

Headlamps are about 55 watts per bulb, AC fan is about 20 amps, the AC clutch is (I think) about 6 amps, ....Hey, maybe that ammeter on the cable is easier after all.

But, use all caution!

I had installed an ampmeter in a cluster of gauges, tapping between the positive post and power wire from the battery to the inside of the passenger compartment, when due to road vibrations or my poor installation, one of the leads made contact with the gauge bezel and poof!,..........a whaft of acrid smoke came a pouring out from under the dash! Luckly I was parked, but the damage had been done. Power wire fried, and it took along with it some close buddies that it was bundled with. Recovered from that pretty easily, but man, what a scare! I prefer the anpmeter to the voltagemeter, as it show's me immediately a current drain on the system.

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Any car past 1972 is just transportation.

Most newer mulit-meters won't let you check amp draw mor than like 10, thay can'thandle the current draw, some of the mor expensive dagnostic tools have a clamp that looks like a lobster claw that you can put on the outside of the main wire off the battery that will tel you amp draw, i havw one at the shop but i can't remember the brand, they are fairly accurate though, are youlooking for peac amps drawn, or what it regularly draws, it's probably around 40 or 50 amps, but don't be surprised if it spikes up to around 90 or 100, but that won't always blow a 50 or 60 amp fuse, i am not sure, but i believe the snap on battery checker has a amp draw mwter in it too, good luck, it is also my expirence that most in the car mounted amp meters arent acurate, good luck!

Thanks guys for all the replies. I guess it is obvious now that I will have to determine MY car's particular draw.

Luckily http://www.chevelles.com/forum/smile.gif I own a digital engine analyzer that has an amp measurement with a shunt connector for the battery able to handle high current even in starting the car.

Guess I better go dig it out of the basement.


Joe

The best way I can think of would be to use an ammeter and check each item individually and then add up the totals. If you can find someone who has a Sun VAT 40 and will let you use it, or if you can find a suitable clamp on DC ammeter. here's a way to get an accurate reading with the clamp on amp probe. Disconnect the battery positive cable. Take a length of 12 gauge wire and wrap it around the jaws of the amp probe 10 ten times. Connect one end to the positive terminal of the battery and the other to your battery cable. Now check each of the items you mentioned. Take each reading and divide that reading by 10 and that is your amp draw for that item. For example, if the dome light shows a 20 amp draw, divide 20 by 10 for a result of 2 amps. What you're doing is making an amp multiplier which allows you to get accurate readings of small ampo draws. Just be sure to wrap the wire evenly like a coil, don't overlap the wire. Check each item with the engine except for those taht require the key to be on. Of course, you will have to check the ignition circuit draw and then subtract that reading from the ones for the blower motor and A/C clutch, unless you disconnect the coil. OR, take it to a shop, tell them what you're doing, let them clamp the VAT 40 on and light everything up at once and find the total

How about if you just add up all of the recomended fuse sizes on your fuse block ( and any other fuses there might be, like the high speed A/C fan fuse), then just figure out the headlight current. Or look at the current capability of your stock alternator..63 amps or so?

I'm not sure about the formula, but with all loads in parallel, the "total" current draw on the alternator shouldn't be the sum of the loads. I'm not 100% sure on this, however.

On my household breaker panel, I have a 100 amp main breaker which feeds everything else. If total current equaled the sum of the loads I would never be able to run my mig welder, clothes dryer, air conditioner, oven, and stove all at once.

There are formulae for calculating parallel and series loads, but I'm not sure which one to use for an automotive (DC) layout. I think it can be figured using the formula for parallel resistance and converting to current using ohm's law.

Try running a search on "electrical formulae" through alta vista or another search engine. There are sites with it all available. I found it earlier but can't get my search engine to function right now.

Chad

Parallel loads do add up, particulary in DC circuits. They do in AC circuits too, but you also have to look at whether an AC load is capacitive, inductive (motors, compressors), or purely resistive(toasters, coffee pots,etc). On a hundred amp service for a house, it's usually 100A 220V with a neutral line. From that, if you have a 30A 220V circuit feeding an AC unit or the like, you still have the resources for 140A 110 service (70A * 2).