Amps (Amperes)
The unit of electric current that sets breaker and feeder ratings.
Quick Answer
An amp, short for ampere, is the unit of electric current: the rate at which charge flows. One ampere is one coulomb per second. On electrical plans, amps appear as ratings on breakers, panels, switches and feeders, and those ratings decide which size of equipment and conductor the estimator prices.
The Full Picture
Current is the flow of charge through a conductor, and the ampere is its unit. The SI definition is now based on a fixed value of the elementary charge, and one ampere equals one coulomb passing a point each second. Voltage is the electrical push and watts are the power delivered, so the three are tied together: for a single-phase load, watts equal volts times amps, which is why a 1,800 W load on a 120 V circuit draws 15 amps.
Amps matter in preconstruction because they are the main rating printed on the equipment that gets bought and installed. A breaker is sized by the current it will carry before it trips, a panel by its bus rating, a switch or disconnect by its ampere rating, and a feeder conductor by the current it must carry. The word the codes use for any current above a device or conductor's rating is overcurrent, and overcurrent devices are rated in amps. An estimator reads those ratings off the drawings and prices to them.
Amps show up in several places in the document set. The one-line diagram labels the service, switchboard and panel ampere ratings and the feeder breaker sizes. Panel schedules list each breaker's pole count and ampere rating, with the connected load per circuit. The riser diagram repeats feeder sizes floor by floor, and equipment schedules list the amperes or minimum circuit ampacity of motors and mechanical units. Specs and general notes may add requirements such as interrupting ratings.
Amps do not have a takeoff unit of their own. Nobody prices an amp. The ampere rating is an attribute that sorts each counted item into the right price line: a 20 A one-pole breaker, a 400 A fused switch and a 200 A panel are three different items, each counted EA. The common pitfalls are mixing up ampere ratings with ampacity of a conductor, missing that a feeder breaker on the one-line has been upsized by a revision, and pricing a 2-pole or 3-pole breaker as if it were a single pole.
Real Examples
Common Misconceptions
People assume: Amps and watts are two names for the same thing.
Actually: Amps measure current and watts measure power. They are linked by voltage, so the same 1,800 W load draws 15 A at 120 V but 7.5 A at 240 V.
People assume: A change in ampere rating only affects the breaker line.
Actually: The rating cascades. A change to a feeder breaker can change the conductor, conduit, switch and panel it serves, so a rating change on the one-line has to be traced through every line that depends on it.
Frequently Asked Questions
What does amp stand for?
Amp is short for ampere, the SI unit of electric current. It is named after the French physicist Andre-Marie Ampere. One ampere is one coulomb of charge passing a point each second.
How do you convert watts to amps?
For a single-phase load, divide watts by volts. A 2,400 W load on a 120 V circuit draws 2,400 / 120 = 20 A. Three-phase loads and power factor add more terms, so estimators usually take the ampere value from the schedule instead.
Where are amps shown on electrical plans?
Look at the one-line diagram for service, switchboard, panel and feeder breaker ratings, at panel schedules for each breaker, at the riser diagram for feeder sizes, and at equipment schedules for motor and unit amperes.
Is there a takeoff unit for amps?
No. Amps are a rating, not a quantity. Estimators count the equipment that carries the rating, such as breakers, switches and panels, in EA and group it by ampere rating and poles.