using a multi meter

[tomcat rio]

not a big deal, but i've found that life is easier with a length of two wires with aligator clips at each end. clip one end to whatever needs to be checked and measure the volts or ohms on the other end. easier than trying to hold the meter probes on the test point directly, i usually have the wrong scale or whatever selected on the meter anyway.
i don't think i have ever had to measure amps.
P=IE

[Denis]

I=E/R

R=E/I

E=I*R

GOOD SUGESTION ABOUT THE CLIP LEADS!

[Elizabeth Ann]

Maybe it would help to name the variables:

I = Amps
E = Volts (I use V)
R = Resistance (which is actually called an Ohm)
P = Power (or Watts)

Darn it, I forgot the difference between series and parallel circuits.

[Mariner]

To remember series vs. parallel, just think of a bunch of little AA batteries. If you've got them lined up end to end, they're in series. If you've got them stacked side by side with wires (or bus bars) connecting them, they're parallel. Series increases voltage, while parallel increases wattage.

At least I THINK that's right.

[Mariner]

Ohm's Law:
W=AV

Where:
W=Watts
A=Amps
V=Volts

Sample calculation:

Your shorepower connection is 30 amps and you want to know if it is safe to run your TV, hair dryer, and three 60 watt light bulbs all at once. You are in the US, so you know that the voltage of the shorepower is about 120vac. So you take 30 amps and multiply it by 120vac and you get 3,600 watts. You can run up to 3,600 watts. The tv uses 400, the hair dryer uses 500, and the three light bulbs use 180 combined. That's 1,080. You're fine! (so long as the individual circuits are also up to their respective tasks)

You have a 5KW generator and when you turn on your air conditioning system, it spikes the Amp meter on your power panel to 45amps. You want to know if you're running a risk of burning up your genset, or throwing a breaker which would require a trip to the bilge to reset it in the summer heat. 5KW is 5,000 watts, so divided by 120vac, that's 41.66amps. If the amp meter is hitting 45, you are, in fact, in danger of the above. You need to turn things off until the amp meter is safely below 40 amps.

[pbirman]

Posted: Wed Sep 27, 2006 3:39 am

To remember series vs. parallel, just think of a bunch of little AA batteries. If you've got them lined up end to end, they're in series. If you've got them stacked side by side with wires (or bus bars) connecting them, they're parallel. Series increases voltage, while parallel increases wattage.

At least I THINK that's right.


No, parallel connection increases the current. An increase in either voltage (series) or current (parallel) increases the power (power = voltage X current)

BTW, putting batteries in parallel without an isolating diode is NOT a good idea as the batteries will try to self discharge into each other because of small differences in their terminal voltage.

Paul (retired Electrical Engineer)

[Mariner]

Thank you Paul for the clarification!

So long as the batteries are all in good condition, the isolator is not strictly necessary, but if one cell goes bad, it's going to draw down the others and you'll be left with no power, so it's still good advice.