How quickly will it trip

G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Against my advice, a friend of mine is going to connect his two 1500W (12.5A) greenhouse heaters to a single 120V 15A circuit. How quickly will the breaker be likely to trip when they are both on at the same time?
 
Its less than 200%, so many seconds for sure maybe up to minutes. It depends on the Time Current Curve of that specific breaker and the ambient temperature around the breaker.
 
Since he is not keen on advice, he will likely change it to a 30 amp breaker and tell you he solved the problem :) thus proving he is the wiser one
 
junkhound said:
QO less than 1 second
Click to expand...
Zero chance of that.

Depending on the ambient temperature at the breaker, may never trip. If at 40C ambient, between 15 and 90 seconds if I'm reading the QO 115 trip curve correctly, for 25A current.

But with voltage drop, the current likely won't be 25A. E.g. #14 Cu has a DC resistance of around 3 ohms / kft. So if there's say 300 ft round trip of wiring, that's 0.9 ohms. If each heater is a 9.6 ohm resistor (120V / 12.5A), in parallel they are a 4.8 ohm resistor, for a total resistance of 5.7 ohms, or a current of 21A.

Cheers, Wayne
 
wwhitney said:
Zero chance of that.

Depending on the ambient temperature at the breaker, may never trip. If at 40C ambient, between 15 and 90 seconds if I'm reading the QO 115 trip curve correctly, for 25A current.

But with voltage drop, the current likely won't be 25A. E.g. #14 Cu has a DC resistance of around 3 ohms / kft. So if there's say 300 ft round trip of wiring, that's 0.9 ohms. If each heater is a 9.6 ohm resistor (120V / 12.5A), in parallel they are a 4.8 ohm resistor, for a total resistance of 5.7 ohms, or a current of 21A.

Cheers, Wayne
Click to expand...
His greenhouse is right next to his house. Excluding building wiring I think it's only about a 30 foot round trip.
 
augie47 said:
Since he is not keen on advice, he will likely change it to a 30 amp breaker and tell you he solved the problem :) thus proving he is the wiser one
Click to expand...
FWIW, the electrician who installed his outdoor wiring told him that the conductors from his breaker panel could support a 30A breaker.
 
ggunn said:
FWIW, the electrician who installed his outdoor wiring told him that the conductors from his breaker panel could support a 30A breaker.
Click to expand...
Was he a licensed electrician, or an “electrical guy”?

If he ran #10 UF cable to it, or conduit and THWN, then he could just change the breaker.

If he doesn’t want to change the breaker, I agree with Wayne, 15-90 seconds with 25A on a 15A trip breaker.
 
I had a service call years ago for a tripping breaker. I found 21A on a 15A breaker, part of the load was a 1500W heater. After I found the cause, I reset the breaker out of curiosity to see how long it took to trip. It was about 10 mins before it tripped. It was an Eaton/Cutler Hammer breaker.
 
For an Eaton BR 15A 1P breaker at 167% of rating the trip time is supposed to be between approximately 500 and 850 seconds according to the published trip curve. However I've personally witnessed that same breaker trip in 10-20 seconds at more like 120% of rating, so I take those published curves with a grain of salt.
 
jaggedben said:
so I take those published curves with a grain of salt.
Click to expand...
You need to look at the temperature around, and internal to, the breaker and make adjustments to the curve when it is different. In general the curves are good at showing protective device performance. However, I believe protective devices are supposed to carry close to 133% before the curve becomes 'followable'
 
jim dungar said:
You need to look at the temperature around, and internal to, the breaker and make adjustments to the curve when it is different. In general the curves are good at showing protective device performance. However, I believe protective devices are supposed to carry close to 133% before the curve becomes 'followable'
Click to expand...
There was nothing special about the temperature or situation.
 
You must log in or register to reply here.
Top