The Electrical Code

220.5 Calculations. This article is laying the ground rules for what voltages are used commonly. And it does this because some people are going to read a voltage on there voltage meter and try to calculate with that reading. Now this reading is not absolute(it can change because it is fed by public utility). When the article is giving you these rating of what voltage are it will be similar not exact to what you meter will tell you. For regular US household we are going to use 120 and 240. If you are an electrician in training, welcome to this wonderful industry and I hope you learn as much as you can and show it to the next guy. Hand it down. Even to home owners that you feel are being cheap and don’t want to pay for your knowledge. Those people are people and they will not ever pay for your labor that is necessary to install something correctly. Don’t give that person your personnel phone number just your business cell phone number. They might just be a customer some day. You never know.

“(A) Voltages. Unless other voltages are specified, for purposes

of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.

(B) Fractions of an Ampere. Calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped.”