...but not for me since I have no training in electrical. It stems from an experience fixing my car two weekends ago, but the concept could also apply to the work I do.

Long story short, I wound up testing fuses with my DMM and found continuity between different fuses. I did some youtube research and come to find out it isn't uncommon for there to be multiple fuses on the same circuit (huge sigh of relief when I learned that - if you have car problems and are a DIY type, ericthecarguy on youtube is good). That made me wonder why, though, and herein comes the amps and voltage question.

In my example, my guess is that these fuses could all be fed from the same power source in the car with different amperage ratings based on what the accessory would draw...say, the radio draws 20 amps. It's fine connected to the same source as something that draws 30 amps because it is DRAWING that amperage, not having it sent automatically, right? With that in mind, voltage: would one source with a set voltage rating send that to everything connected to it? Like, radio, windows, guages, a/c, etc? Ie, they're all wired in parallel, but with different amperage draws. (If you're a car person and are wondering what happened, yes, my main/alternator fuse blew after my dad hooked his end of the jumper cables up backwards. So did my battery.)

How it might apply to what I do one day: Maybe, in a pinch, if I have to find a way to power, say, two security keypads with one transformer, I could use one that supplies the proper voltage with enough amperage to power them both?

Am I gettin this right?

Thanks for any input!

Yes you are gettin this right. :)

If you have two keypads that require the same voltage... say 24volts (I'm not an CI, so Idk the voltages...) And they each come with a transformer rated for 5amps and each, according to the name plate on the keypad they draw 2amps. Then you can use just one transformer. Because in theory, those two keypads will draw 4amps total out of that transformer.

If however the keypads draw 3amps according to the name plate, then you need to use both transformers. So each will have their own. Even at 2.5amps draw, I will be reluctant to use one transformer.

Keep in mind though, a name plate usually denotes the "highest" amount of average or wattage, that that device can use. So just because it says 2amps, does not mean it will be using 2amps alllll the time. Heck, it may never use 2amps. But still, wouldn't want to play Russian roulette like that either.

KOT

Funny to see this topic tonight. At Cub Scouts tonight I was talking with one of the other parents that's a mechanic. We were talking about pretty much this same issue, and how one person I know crossed the jumper cables and totally screwed up his car. He also brought up how you used to could start a car then pull out the battery and everything would keep running, but that doesn't work now because of all the electrical components.

The purpose of a fuse is fire prevention.

In your car the branch fuses (radio, fan, wipers, lights) are sized according to the current carrying capacity of each wire. This keeps the wire from burning if there is some sort of fault along the way. The source wire to the block of fuses might have its own fuse. The size of this fuse is determined by its own wire size (again for fire protection of that wire) and this fuse does not need to be the sum of the branch fuses -- unless there is a high probability that every branch will be drawing its maximum allowed current simultaneously. For example, a radio connected to a 20 Amp branch circuit might only draw a few amps while operating. Further, the dome lights are usually OFF while driving.

The primary wire connected to the battery terminal is usually not fused because it is sized to carry enough current to run the starter. As such, even if there is a large fault on that wire, the battery voltage will quickly collapse and limit the current.

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You can help yourself to understand what is happening in a battery circuit if you concoct a "practical battery" by drawing a small resistor in series with a perfect battery. There will be a voltage drop across that series resistor (I*R). The imaginary perfect battery will maintain its voltage, even if it needs to supply infinite current. The series resistor represents things that happen inside a real battery that will reduce the practical battery's voltage and limit its current output under various circumstances (temperature, internal connections between cells, overall condition, age, number of charge/discharge cycles, remaining charge, recent current draw history, etc.) When you attach your voltmeter to the battery's terminal, you are measuring the practical battery. Even with no apparent load, you don't get to measure the perfect battery because there is leakage inside the battery case and your voltmeter draws a small current.

[edit]: grammar and minor improvements

Most auto mechanics are scared to death over electrical. During a camping trip I bumped into a mechanic at an opportune time for both of us. I gave him insight to an electrical issue that he was wrestling with, and he gave me some transmission insight.

Yes.
Similar to your house circuit.

You might have 400Amps coming from the meter.

You'll usually find 15Amp breaker for the lighting circuits, but you'll find multiples of those. If you have 15 lamps (recessed cans, whatever), each drawing 1 Amp, that's the maximum for that circuit.

FYI, since I know you like to educate yourself -- alarm keypads are 12VDC.
Because they're DC and not AC, they require a power supply that converts AC into DC.
Transformers only step AC voltages up or down.

Alarm panels take 16 or 24 VAC and convert it to 12VDC to power keypads, motion detectors and send trickle voltage on zone loops etc.

A key concept is that the meter Mario mentions is capable of delivering 400 Amps. There's not 40p Amps "coming from the meter" unless all the loads in the house DRAW a total of 400 Amps.


There are many more picky details we could go on and on about. Keep learning!

Cars have one main wire supplying current to the fuse blocks (some have one or more inside and another under the hood. In addition, some of the supply wires may have a 'fusible link', which is a section of wire that acts as a fuse if the current exceeds its rating (sometimes, the feed to the ignition switch will have this- I know Chrysler used these in the '90s, but they may or may not have changed this).

The wires going to the fuse blocks are sized to handle all of the load for all fuses in a 'worst case' situation, but I wouldn't be surprised if they under-size them to save money. The same power distribution could be accomplished by connecting a fuse holder for each wire/circuit to the same battery post, but that would become a big mess very quickly, so it's condensed by using the fuse blocks.

If you want to test for blown fuses, it's better to set the meter to measure DC Volts, then touch the + probe to both contacts on each fuse. If you see voltage on one but not the other, you have found the blown fuse.

Also, when jump starting a car, DO NOT leave the donor engine running when the car being jump-started is cranking. Alternators are sized to handle one car's electrical system and no more than that. Its primary function is to recharge the battery after cranking and then, supply current for the engine and accessories while it's running.

The proper way to jump start a car is:

-Shut off the donor car's engine
-Connect the battery cable + to the dead battery and the cable - to the dead car's engine (the alternator bracket is a good place, if it's easy to reach) before the donor battery
-Connect the jumper cables to the donor battery + before connecting to the - and do it cleanly, not repeatedly making and breaking contact with the battery post
-Allow a few minutes to pass, so the dead battery can partially recharge, rather than trying to start the engine immediately- a dead battery presents a huge load to the alternator
- Start the donor engine and raise the engine speed to about 1000 RPM for a couple of minutes to partially recharge it
-SHUT OFF THE DONOR ENGINE!!!!!!!!!!!! DO NOT CRANK THE OTHER CAR WITH THE DONOR ENGINE RUNNING!!!!!!!!!!!!!
-Crank the engine of the car being jump started. Once it starts, disconnect the - clamp from the car that was jumped before disconnecting the + clamp and hold the cables in a way that won't allow contact between the clamps (I think one lead should be shorter than the other) and then disconnect the clamps from the donor engine in the same order.

If the car with the dead battery is cranked while the donor engine is running, be prepared to replace the donor car's alternator- the voltage regulator IS NOT sized to handle the load of a cranking starter, which can draw more than 100A (most alternators are rated for about 75A and typical draw when the engine is running is around 50A). Ditto on replacing the starter if the cables are reversed when they're connected- the bridge rectifier (makes DC from AC) doesn't tolerate being connected with reverse polarity.

One of my vehicles is 24VDC which makes it a PITA to jump start.

Any chance you could rent a fork lift or golf cart?

Bummer. You need to flag down two friends and carry one and a half sets of jumper cables. (Take care not to touch bumpers while jumping)

Note also if there is no voltage on either side of a fuse. Then you may have turn the ignition switch on. As some fuses receive the power from there or a relay that is controlled by the ignition switch.

KOT

You ought to watch boat mechanics when they're at a training session that involves electricity- it's funny AND sad at the same time. I worked for two boat dealers and we sold Mastercraft ski boats, which is one of the few companies that require service techs to attend these sessions. The first time I went, about half of the class was guys from the SE part of the US and they had been working on boats since before fuel injection. The problem was that the main part of the week was spent on the fuel injection systems used by Mastercraft (one of the first) and others and therefore, very heavy in electrical concepts. On the day they presented the basic concepts and Ohm's Law, one part of the test was to wire a light to illuminate when a switch was open and a blower to run when the switch was closed, using a Bosch-style relay. Some of these guys had very good reputations for their work, but they had no freaking idea about wiring one of these simple relays, even though the diagram was molded into the plastic case. It was like watching Dan Aykroyd and Steve Martin on SNL when they'd look at something and wonder, "What the hell is that?????????"