Ohm's Law is the basic physical relationship that describes how electricity behaves. Typically this is presented as Volts = Amps x Ohms, or E = I x R. By rewriting Ohm's Law as Amps = Volts/Ohms, the law can be used to determine the amount of current flowing in a circuit given a specific resistance and voltage.
Let's assume a customer has built a new facility and is requesting electrical service. The voltage required at the facility will be determined by the type of service requested. The amps can be estimated by adding up the number and electrical size of the lights, outlets, and other power-consuming equipment to determine the maximum quantity of amps required to run the equipment in the building. From experience and history, we know that no more than 70-80% of the total connected load will operate at one time. Now that we know the volts and have estimated the amps, we can calculate the ohms using Ohm's Law. Once we know the ohms value, the type of wire being used, and the distance of the facility to the transformer or distribution line, we can calculate the size the wire must be to adequately carry the facility's load.
Here's an example. Let's say a customer has built a new house and needs to have service installed. The house has been designed for a service voltage of 240 volts. By adding up the various equipment and outlets planned for the house, we know that the number of amps required will be 200. Since we anticipate that at most 80% of the equipment will run at the same time, we can plan for 160 amps, which is 80% of 200. To calculate the ohms required for this service, we divide 240 volts by 160 amps and get 1.5 ohms.