What am I missing here?

Old 09-17-03, 12:38 AM
Liquid plumber
Visiting Guest
Posts: n/a
What am I missing here?

According to my Black & Decker Advanced Home Wiring book, a 15 amp circuit can carry 1440 watts of electricity at 120 volts. A 20 amp circuit can carry 1920 watts at 120 volts or 3840 watts at 240 volts. It goes on like this up to 50 amps. It seems to me that this book is either understating the capacity of various circuits or circuit breaker manufacturers are overstating the amperage rating of their products. After all 120x15=1800 and 120x20=2400. If this is true, then a typical 15 amp circuit should be unable to run a 1500 watt electric space heater or hair dryer. What gives?

Old 09-17-03, 05:56 AM
Join Date: Nov 2001
Location: Taylors, SC
Posts: 9,261
Upvotes: 0
Received 0 Upvotes on 0 Posts
80% rule along the lines that a circuit carries a steady load at 80% of it rating.
Old 09-17-03, 06:01 AM
Join Date: Dec 1999
Location: Fort Mohave, AZ
Posts: 99
Upvotes: 0
Received 0 Upvotes on 0 Posts
I think, I know where you are coming from, but the pros can answer it better than I. Since the hair dryer/space heater are plug in , then the 15 amp circuit can handler them. 15X120=1800 watts. Now if the circuit was a hard wire situation, then it is usually downgraded by 80%. 1800 wattsX.8 = 1440 watts. HTH
Old 09-17-03, 06:44 AM
Join Date: Sep 2000
Location: United States
Posts: 17,733
Upvotes: 0
Received 1 Upvote on 1 Post
In most cases, you are permitted to design to use 100% of a circuit's capacity. In a small number of cases, you may only design for 80%. Since most people don't fully understand which is which, it is safe to simply always design for 80%.

It is a widely believed myth that you must always design for 80%.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: