Main breaker trip ratings

Status
Not open for further replies.
O

olly

Senior Member
Location
Berthoud, Colorado
Occupation
Master Electrician
The POCO refers to "320A self contained meter socket " is a 400 breaker set to trip at 320A.

My question is, are all main breakers set to trip at 80% ?

Will a 200A trip at 160?

The question is specific to 120/240 residential single phase.

Are mains supposed to be sized at 80% load for residential?
 
Not sure what your poco is referring to?
Breakers trip at their rating, not 80%, bu note the trip point is based on a time current curve, a 200 amp breaker will never trip at 200 amps
 
Sent some breakers rated at continuous use 3 hours or more at full current rating? while others are not, thus 80%?
 
phone changed my wording “Sent” = isn’t some breakers rated at continuous?
 
The trip curve for 80% and 100% continuous rated breakers is the same. What the actual difference is (or was, if this is a vestigial code thing) is a mystery.
 
Gene B said:
The trip curve for 80% and 100% continuous rated breakers is the same. What the actual difference is (or was, if this is a vestigial code thing) is a mystery.
Click to expand...
My understanding is that every breaker passes a test at 100% current in free air (unenclosed). Whereas a 100% rated breaker passes a 100% current test in a specified enclosure, which you need to use in your installation to achieve the 100% rating.

Cheers, Wayne
 
Gene B said:
The trip curve for 80% and 100% continuous rated breakers is the same. What the actual difference is (or was, if this is a vestigial code thing) is a mystery.
Click to expand...
It is all about heat disappation.
Every breaker is UL tested at 100% in open air.
Breakers are applied at 80% when they are mounted into enclosures.
Breakers in properly ventilated enclosures maybe applied at 100%.
 
tom baker said:
Not sure what your poco is referring to?
Breakers trip at their rating, not 80%, bu note the trip point is based on a time current curve, a 200 amp breaker will never trip at 200 amps
Click to expand...
When you say it will never trip at 200A what do you mean? Could you explain plain a little more in depth. Thank you in advance
 
jim dungar said:
It is all about heat disappation.
Every breaker is UL tested at 100% in open air.
Breakers are applied at 80% when they are mounted into enclosures.
Breakers in properly ventilated enclosures maybe applied at 100%.
Click to expand...
So is a fair assumption that your average breaker in an enclosure will trip at 80% load if it is under 80% load for 3hrs or more
 
Gene B said:
The trip curve for 80% and 100% continuous rated breakers is the same. What the actual difference is (or was, if this is a vestigial code thing) is a mystery.
Click to expand...
Would you mind explaining more about the trip curve as it applies to 80% and 100%. I don't understand
 
olly said:
So is a fair assumption that your average breaker in an enclosure will trip at 80% load if it is under 80% load for 3hrs or more
Click to expand...
No.
You need to consider the breaker trip curve as well as it's ambient temperature. For example a UL489 listed breaker can carry up to 130% of its current rating for periods in excess of 1 hour at 40C.

But remember breaker and fuse performance and conductor ampacity are all compatible when applied per the NEC.
 
olly said:
Do breakers have different trip curves based on brand or type? Where can I find more info on trip curves? Would you happen to know?
Click to expand...
All major overcurrent protective device manufacturers have publications on these curves.
 
jim dungar said:
All major overcurrent protective device manufacturers have publications on these curves.
Click to expand...
Last question Jim, I really appreciate your responses. I am reading up on it and I don't understand why a couple people have said: The trip curve for 80% and 100% continuous rated breakers is the same.
 
olly said:
Last question Jim, I really appreciate your responses. I am reading up on it and I don't understand why a couple people have said: The trip curve for 80% and 100% continuous rated breakers is the same.
Click to expand...
Because it is.
The 80% factor is simply a SWAG used to adjust for an open air device rating being mounted internal to an enclosure.

All UL conductors and overcurrent protective devices are evaluated at 100%. The 80% debating basically provides a margin of error for the actual installation condition being different than those in the testing labs.
 
Status
Not open for further replies.
Top