Are You an Electrician? These are 5 Formulas You Should Know!

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Being a great electrician requires a strong knowledge of math. We use it daily from bending conduit, to figuring out what wire to pull, even simply counting light fixtures or circuits. But which formulas should we be more familiar with? In todays episode of Electrician U, Dustin explains the top 5 formulas every electrician should know.

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First on the list is Ohms law. This formula is the relationship between Voltage, Amperage, and Resistance. In many cases, we are not given ALL of the information for a piece of equipment, but still need to determine either the voltage or amperage of it. Ohms law is simply E (voltage) over I (amperage) times R (resistance). So, draw a circle and put a large T in the center of it. Above the horizontal line of the T draw an E. On the left of the vertical line draw an I and on the right of the line draw an R. To assist you, cover up the letter you are attempting to solve. For instance, if I covered up E (voltage) it would leave me with I (amperage) multiplied by R (resistance). If I had a 20a piece of equipment with a 6 ohm resistance (20 x 6) it would be running at 120v!
The next formula is Joules law. This one is slightly different than Ohms law and is the relationship between Wattage, Amperage, and Voltage. The circle is the same as above but with a P on top, an I on the left, and an E on the right. The math is the same also. So, if I was attempting to see how many watts were on a given circuit, cover up the P and I am left with I (amperage) times E (voltage). For a 20a circuit operating at 120v, I would have 2400w. Both of these formulas are very useful because we don’t always get all of the information we need on the equipment nameplate.
Voltage Drop is something that every electrician should know how to figure out. For a single phase circuit the formula is 2 x K(conductor) x I (circuit amperage) x L (length) divided by the circular mils of the conductor you are attempting to use. For 3 phase replace the 2 with a 1.732 (the square root of 3). If you have a copper conductor use 12.9 and use 21.2 if you are using aluminum conductors. The circular mils for electrical conductors can be found in the NEC codebook in Chapter 9 Table 8. The resulting number after crunching the equation is the amount of volts that are lost. You may find that you may need to upsize your wire (and perhaps the conduit) to get your voltage drop down to a reasonable level.
Resistance formulas are needed for every electrical theory class! For a series circuit the total resistance is the sum of all the resistances. For a parallel circuit, it’s the reciprocal of the sum of all the reciprocals. So, 1 divided by 1/R1 + 1/R2 + 1/R3 + etc. An easier way for that last one would be product over sum formula. So, if you had a parallel circuit with resistances of 2, 3, & 4 the formula would be 2 x 3 x 4 divided by 2 + 3 + 4. Much Simpler!
Lastly is Horsepower. Something to just keep in mind is that 1 HP is equivalent to 746 watts. For single phase motors the formula is HP= E (voltage) x I (amperage) x EFF (efficiency) x PF (power factor) divided by 746. For a 3 phase motor, simply insert 1.732 (the square root of 3) in front of the E. Efficiency you can find on the nameplate of the motor. If you have a completely balanced load that isn’t running a ton of motors you may have a power factor of close to 1. The more motors you have on any given system, that number goes down (say to .7 or even more).
We hope this has been helpful in understanding the 5 most popular formulas an electrician uses frequently. Is there a topic you would like to see discussed on Electrician U? Leave us a comment in the comments section and let us know. Please continue to follow Dustin and Electrician U as we are constantly updating our content to assist our followers in becoming the best electricians that they can be.

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[randorguerrero]

Hello, from Dominican Republic
I'm an electrician. But I am watching this video to improve my english.

[danielsilva]

It would be really cool if you could show us some situations where these formulas could be useful in your day to day work as an electrician.

[wmcomprev]

In your example for parallel resistance you came up with 0.9 ohms. You then mentioned product over sum and the equation you wrote was (2x3x4)/(2+3+4). That gives you 24/9 which is not 0.9.

Anyway, I went ahead and looked it up. Product over sum only works for 2 resistors. If you have more than 2, you have to calculate 2 of them then take that result and use it as 1 of the 2 factors in the calculation with the next resistor.

The original formula you showed for parallel resistors can also be written as 1/Rt=1/R1+1/R2+1/R3. It's actually the same formula, but it may be easier for some to visualize than the double fraction.

[stargazerup]

I took electrical theory in high school. 30 years ago. Yes, it's time to brush up. Thanks. Good job.

[billstarr]

Keeping a current copy of an Ugly's book in your tool bag at all times will solve this and many more needs.

[I_love_electricity]

Thank you for this lecture. I am a electronics hobbyist trying to get off the grid at the same time trying to get a job as an electrician apprentice. This will help me get ahead. Can't wait to check out other content!

[Mknpc]

The ending was great 😂 "we're all dummies" my time has come and really helps to remember all of this formula memories back in my tech trading school so wish me luck in my electrician career soon i hope 👍🤞

[Davewest]

Hi Dustin as an engineering student I appreciate what you are doing Keep it up its just another tool we need in our field.

[chriselectric]

Man I’m grateful to run across a knowledgeable craftsman like yourself I went to electrical school before, and just watching and learning is such a refresher it’s like being in school all over again this is like something I will never forget no matter how old I get I can do some electrical work and throw some pipe on the rack u feel me

[nolkavinn]

Excellent formula review boss! I love the way you teach. Much appreciation

[jessesmith]

Dang, just had my fourth year (final year) mid term tonight! Wish this was out a WEEK ago! You da man, Dustin, imma follow up on this come finals weeks and Journeyman exam

[TheWmiller]

Thank you for taking back to school. Learned these formulas and more in my electrical engineering degree

[nateh]

Dustin, as a senior electrical engineering student and commercial electrician, I must say…your teaching and knowledge is phenomenal. Although, as electricians we technically don’t need to know the intense calculus, differential equations, and linear algebra behind RLC circuits, I’m glad you mentioned it. Thank you for all of your informative videos!

[TheMastercheif]

Starting my apprenticeship over after leavening during my 1-2 year. These videos are amazingly helpful for review.

[VesselForHonor]

I'm trying to learn some electrical stuff to get a head start before I start taking classes or doing an apprenticeship, this is very helpful and informative. I appreciate the time and effort you took to make these videos! I subbed👍

[bgable]

I've watched your video's in the past, simple and to the point. Looking forward going down the YouTube rabbit hole viewing the others!!! Thanks!

[KevinCoop]

Another great video Dustin. I did many many calculations to limit my voltage drop on branch circuits to 3% as suggested in NEC. Since I’m really looking conductor size needed to achieve the goal, I exchanged Vd and cm but did 3% of the volts of the system. One calculation and I knew how many cm was needed to be at or below 3%. Go to the table and pick the conductor size. As. Cm=2xKIL/3% of V.
I also did 3 phase slightly different. For 3ph 3w balanced load same as single phase and multiply by .66. For 3ph 4W balanced load multiply by .5. In the thousands of designed circuits over 35 years, I never had a low voltage or breaker tripping issue.

[Awegner]

Hey Dustin! Might be worth mentioning the reasons behind the 1.73 with respect to calculating 3 phase power as well with the P=I*E bit. Also, for those asking about E vs V for voltage, it was called Electromotive force before Alessandro Volta put his last name on it!

[raymond]

Lots of theory to learn in the field of Electrical installation, as they say we live and learn every day.

[abdullahgervin]

The Pythagorean theorem is good to know for figuring out how to roll offsets and helps you calculate degrees of bend to recalibrate old benders. Makes conduit bending easier when smart benders are available.