# a trip curve for a plain-vanilla resi

Discussion in 'Electrical Forum discussion & Blog' started by Thatguy, Nov 19, 2008.

1. ### ThatguyHomeowner

Joined:
Aug 27, 2008
Occupation:
A bounty hunter like in "Raising Arizona"
Location:
MD
15A breaker to be used with #14 AWG copper; anyone have a link?

Thanks.

2. ### jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina

Just what are you looking for?

A good rule of thumb concerning the trip curve of a breaker is six times the rated current.

Your 15 amp breaker will carry 90 amps for 12 cycles without tripping.

4. ### ThatguyHomeowner

Joined:
Aug 27, 2008
Occupation:
A bounty hunter like in "Raising Arizona"
Location:
MD
Good. That's exactly what I'm looking for.

So the I-squared-T rating for this breaker is 8100 x 12/60 = 1620 amps-squared-seconds.

For one-half cycle at 60 Hz this should trip at 440A.
With a dead short at the end of 1/4 ohm worth of Romex the rms current for this half cycle would be 120/(1/4) = 480A.
Close enough.

If #14 AWG copper fuses at ~170A (in one second?) then its rating is about 30,000 (A^2)-sec. So the breaker interrupts the current before the wire gets anywhere near the melting point, maybe when the wire reaches 90Â°C or so. . .?

Thanks.

Last edited: Nov 19, 2008