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Doing math, for many people, is one of the things that holds them back from advancing in their career as an electrician.  “I’m just not good at math,” I often hear.  Knowing how to apply Ohm’s Law calculations in the field is crucial for your ability to think critically, on the fly, about what may be happening in a circuit – and electrician math is really not that scary.  Let’s dive in.

 

OHM’S LAW

First off, what is ohm’s law?  The short answer is a relationship.  It’s a relationship that exists between electrical units of measure, chiefly volts, amps, ohms, and watts.  Each of these values have a nearly perfect mathematical interactions, and when you start to raise or lower one, the others change to keep the relationship balanced.

This becomes extremely useful when you’re troubleshooting and need to find a missing value.  You can take readings in your environment to find some knowns, which you can then plug into an equation and do some simple math to solve for any unknowns.  Ok enough talking about it, let’s look at a few examples:

 

LIGHTS ON A CIRCUIT

One thing we do often as electricians is figure out how many light-fixtures we can install on a circuit.  Too many and we’ll keep tripping breakers.  This one’s easy so pull out a pencil and paper and do this as I go through it.

We have a 120-volt, 20-amp circuit with that was run out to a parking lot for pole-lighting.  Each pole has a 400-watt MH lamp installed in it. It appears 9 light fixtures were installed on this circuit, and the breaker keeps tripping.  Time to do math.

Start with the knowns.  We know we have volts, amps, and watts right?  We also know each fixture consumes a certain amount of wattage, and when we multiply volts and amps together we get wattage.  So 120 x 20 = 2,400 total watts available on this circuit.  And we know each lamp is 400w.  

Now the unknowns. So we could divide 2,400 total circuit watts by 400 watts per lamp and find that there should only be 6 lamps on this circuit.  3 less than how it was wired.  We can also figure out how many watts the 9 existing 400w MH lamps are consuming.  Simply take 9 x 400 = 3,600 watts.  3,600 watts can also be expressed in amperage if we divide 3,600 by 120-volts – which comes out to 30-amps.  This means the existing circuit with 9 lamps on it, is drawing 30 amps as it’s consuming 3,600 watts of energy

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