Common & Useful Electrical Formulas

Ohm’s Law

Ohm's Law describes the mathematical connection between power, voltage, current, and electrical resistance. 

It explains that in an electrical circuit, the current increases with voltage and decreases as electrical resistance rises.

We recommend using the Ohm’s Law Pie Chart to make quick electrical calculations regarding this principle.

The basic electrical formulas provided in the Ohm’s Law Pie Chart allow you to calculate:

• P = Electric Power/Wattage (watts)

• I = Electric Current (amps)

• R = Electrical Resistance (ohms)

• E = Voltage (volts)

You only need to know two parameters to calculate the other two parameters.

AC/DC Formulas

AC/DC formulas calculate essential parameters in electrical systems that operate on AC (alternating current) or DC (direct current) power.

Formula keys you need to know:

• E = Voltage

• I = Amps

• W = Wattage

• PF = Power Factor

• Eff = Efficiency

• HP = Horsepower

Here are the electrical parameters you can calculate using AC/DC formulas.

Find amps when horsepower is known

• Direct current: (HP x 746) / (E x Eff)

• AC / 1 phase 115V or 120V: (HP x 746) / (E x Eff x PF)

• AC / 1 phase 208V, 230V, or 240V: (HP x 746) / (E x Eff x PF)

• AC 3 phase all voltages: (HP x 746) / (1.73 x E x Eff x PF)

Find amps when kilowatts are known

• Direct current: (kW x 1000) / E

• AC / 1 phase 115V or 120V: (kW x 1000) / (E x PF)

• AC / 1 phase 208V, 230V, or 240V: (kW x 1000) / (E x PF)

• AC 3 phase all voltages: (kW x 1000) / (1.73 x E x PF)

Find amps when kilovolt-amperes (kVA) are known

• Direct current: Not applicable.

• AC / 1 phase 115V or 120V: (kVA x 1000) / E

• AC / 1 phase 208V, 230V, or 240V: (kVA x 1000) / E

• AC 3 phase all voltages: (kVA x 1000) / (1.73 x E)

Find kilowatts

• Direct current: (I x E) / 1000

• AC / 1 phase 115V or 120V: (I x E x PF) / 1000

• AC / 1 phase 208V, 230V, or 240V: (I x E x PF) / 1000

• AC 3 phase all voltages: (I x E x 1.73 PF) / 1000

Find kilovolt-amps

• Direct current: Not applicable.

• AC / 1 phase 115V or 120V: (I x E) / 1000

• AC / 1 phase 208V, 230V, or 240V: (I x E) / 1000

• AC 3 phase all voltages: (I x E x 1.73) / 1000

Find horsepower (output)

• Direct current: (I x E x Eff) / 746

• AC / 1 phase 115V or 120V: (I x E x Eff x PF) / 746

• AC / 1 phase 208V, 230V, or 240V: (I x E x Eff x PF) / 746

AC 3 phase all voltages: (I x E x Eff x 1.73 x PF) / 746

AC Efficiency and Power Factor Formulas

AC (Alternating Current) efficiency and power factor formulas help assess the effectiveness of electrical power usage in electrical equipment, motors, and systems.

Electricians can use these formulas to optimize electrical energy consumption, reduce power losses, and improve the reliability of electrical installations.

Formula keys you must be aware of:

• E = Voltage

• Eff = Efficiency

• HP = Horsepower

• I = Amps

• PF = Power Factor

• W = Watts

Here are the most common electrician formulas for calculating AC efficiency and the power factor:

Single-phase efficiency

(746 x HP) / (E x I x PF)

Three-phase efficiency

(746 x HP) / (E x I x PF x 1.732)

Single-phase power factor

Input W / (Volts x Amps)

Three-phase power factor

Input W / (E x I x 1.732)

Voltage Drop Formulas

Voltage drop formulas calculate the reduction in voltage that occurs as electrical current flows through an electrical circuit or conductor, like a resistor or wire.

Resistance or impedance leads to voltage loss when current flows through wires or various components, decreasing system efficiency.

Therefore, these basic electrical formulas can help you ensure an electrical system provides enough voltage to the connected equipment.

This is particularly important when dealing with high current levels or long-distance installations.

Formula keys you need to master:

• VD = Voltage drop

• CM = Circular mil area of the conductor

• I = Current in conductor (amperes)

• K = ohms per mil-foot (copper = 12.9 at 75°, aluminum = 21.2 at 75°)

• L = Length of conductor in feet

Keep in mind that the value of K changes with temperature.

You can effectively calculate the voltage drop and circular mil area of the conductor using these formulas.

Single-phase (two- or three-wire) installation

• VD = (2 x K x I x L) / CM

• CM= (2K x L x I) / VD

Three-phase installation

• VD = (1.73 x K x I x L) / CM

• CM= ( 1.73 x K x L x I) / VD

Conversion Formulas

Conversion formulas allow electricians to convert electrical quantities and units accurately.

Proper conversion ensures efficient system performance and compliance with standards.

Some fundamental conversion formulas are:

• Temperature conversion formulas: 

℃ = (5 / 9) x (℉ - 32) 

℉ = (9 / 5) x ℃ + 32

• Standard-to-metric length conversions:

1 inch = 2.54 cm

1 inch = 25.4 mm

3.28 feet = 1 m

1 yard = 0.91 m

1 mile = 1.609 km

144 square inches = 1 square foot

9 square feet = 1 square yard

640 acres = 1 square mile

1 cubic foot = 7.48 gallons

For instance, standard-to-metric length conversions are practical when calculating voltage drop for different electrical runs.

However, there are also complex conversion formulas electricians should know, like:

• 1 siemens = the reciprocal of one ohm

• 1 ampere = 1 coulomb per second

Empowering Electricians Beyond the Technical Knowledge

Every electrician must possess the technical knowledge and skills to perform the job well. 

However, if you want your team to provide stellar services, you must build on that foundation and push them to improve.

Here are a few practical ideas on how to achieve this.

1. Encourage continuous learning

Continuous learning helps electricians improve their skills, specialize in new areas, and stay current with industry developments.

Therefore, you should regularly enroll your team members in electrical training courses.

A good example is the National Electrical Code (NEC) Online Training Series, which aims to improve electrical safety in residential, commercial, and industrial environments.

At the same time, you can offer your staff useful electrician books that will advance their knowledge or pocket-sized books for quick access to essential information in the field.

For instance, Ugly’s Electrical References, by George V. Hart, is a good on-the-job reference tool that also includes basic electrical formulas, like conductance, inductance, reactance, and electrical potential.

2. Equip them with the necessary technology

Modern electrician tools and technology facilitate electrical work, allow professionals to perform complex tasks with more precision, and improve safety.

Even something as simple as a phone app electrician calculator can help your team perform complex calculations in seconds and with less room for error.

However, technology can do much more than that.

For instance, if you want to take your business to the next level, we recommend investing in ServiceTitan’s Electrical Software.

From improving workflow efficiency and customer service to helping you achieve maximum profit, our tool is perfect for managing and growing your business.

Moreover, ServiceTitan’s Field Service App will enable your electricians to arrive well-prepared at any job, streamlining tasks, improving productivity, and impressing customers.

Specifically, the ServiceTitan electrician app gives your team access to all the information collected during the intake call.

That includes the client’s contact information, a brief description of the electrical issue or the type of job requested, and even detailed property information.

Furthermore, if your team visits a returning customer, they can access the customer's history, including existing equipment, photos and videos, and recurring services.

All this information will help your electricians provide better service and enhance the customer experience.

Lastly, you can forget about missing paperwork since you can upload all the necessary forms and documents to the Field Service App. You can even make these documents appear automatically on your techs’ devices during specific parts of the job so they won’t be forgotten.

3. Teach them sales skills

By teaching your electricians sales skills, you can turn them into field sales reps for your business.

Specifically, this will enable your team to identify a client’s needs beyond the job at hand and recommend additional electrical services or equipment.

Your electricians will also be able to educate clients on the long-term benefits of specific electrical work and upgrades, making customers more likely to opt for those services.

Moreover, they will be able to communicate effectively with clients and confidently discuss pricing, building strong client relationships.

ServiceTitan’s Field Service App facilitates this process by offering your electricians in-app estimates to present to clients.

Moreover, you can choose ServiceTitan’s Pricebook Pro solution, which makes it easy for your team to find, display, and sell electrical equipment and services in the field.

Your Pricebook Pro catalog will include high-quality product images, descriptions, and explainer PDFs that denote professionalism and trustworthiness.

That, in turn, will make clients more likely to purchase.

Over to You

Our crash course on the most common and valuable electrician formulas is just the tip of the iceberg.

Continuous learning, developing sales skills, and making use of technology will empower you and your team to provide excellent services and scale your business.

Book a demo to learn more about how ServiceTitan’s Electrical Software can help you in this endeavor.

ServiceTitan is an all-in-one solution that facilitates business management, increases your team’s productivity, and enhances customer experience. You can even use it for bookkeeping, invoicing, dispatching, and communicating with customers.