sizing 80% of breaker or MCC
sizing 80% of breaker or MCC
(OP)
I dont know if this is a NEC rule, but in this company most of the MCC or Bus or Breaker are only used up to 80% of their max or ratings.
For example, a 600A MCC is only used up to 480A.
My question is this:
1-why do most people use it up tp 80% only?
2-What will happen if I used it up to 90% or even 100% of its rating?
For example, a 600A MCC is only used up to 480A.
My question is this:
1-why do most people use it up tp 80% only?
2-What will happen if I used it up to 90% or even 100% of its rating?






RE: sizing 80% of breaker or MCC
1 - 'cause they have to.
2 - bad things.
RE: sizing 80% of breaker or MCC
Perhaps someone else will chime in with specific references.
You're not permitted to install new loads that will exceed the rating. It probably won't start a fire, but the code prohibits you from trying it. The headroom is to cover unforeseeable thermal effects and to prevent excessive voltage drop.
Good on ya,
Goober Dave
RE: sizing 80% of breaker or MCC
RE: sizing 80% of breaker or MCC
RE: sizing 80% of breaker or MCC
Basically, I just wanted to know what happens if you use more that 80% for standard rated. it is just out of curiosity since I haven't seen it
RE: sizing 80% of breaker or MCC
davidbeach lol at "Time for you to do some code research". I've seen the required 80% but I just wanted to know exactly what happened if you go slightly above it
RE: sizing 80% of breaker or MCC
On the other hand, for example, if you have only one of the breakers in a group mounted assembly exceed 80%, but rest of the devices are below 80%, nothing may happen. But no guarantees.
Rafiq Bulsara
http://www.srengineersct.com
RE: sizing 80% of breaker or MCC
The issue is usually seen with thermal-mag breakers. It depends wether they are designed to operate in a open air or closed section. A good explaination to this is:
http
It might be a bit clearer in how it states everything. But to sum it up, in open air, you have more airflow. Therefore, more cooling. In an enclosed structure, less to no airflow, plus surrounding metal. There will be a higher build-up of heat. This can cause the breaker to trip on its thermal protection earlier than desired.
In breakers that have CT's and electronic trip units, this is not a concern due to the current, not the heat being measured.
RE: sizing 80% of breaker or MCC
Bus bars don't have that restriction, but they are protected by breakers and fed by conductors that do.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: sizing 80% of breaker or MCC
Bus bars don't have that restriction, but they are protected by breakers and fed by conductors, that do.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies