100% rated OCPD device

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dabatman

Member
I have a situation that I'm trying to verify.

Say I have 100A of non-continuous load and 12A of continuous load, then I end end up trying to size my feeder. This gives me 100A*100%+12*125%=115A for my feeder.

Two things:

1) Do I need a 100% rated breaker if I size my feeder at 125A? I believe I do, just trying to verify.

2) Are fuses considered 100% rated OCPD?
 

kwired

Electron manager
Location
NE Nebraska
No. Read 215.2(A)(1) and exception 1.

If you were to use a 100% rated breaker you would not need to multiply continuous load by 1.25.

I believe most 100% rated OC devices you will find are going to be circuit breakers with electronic instead of thermal current monitoring devices. Heat produced by the thermal sensing element is the main reason for 125% multiplier being needed for continuous loads.
 

dabatman

Member
I guess what I meant was that you can't use an 80% rated breaker because you have to rate the continuous load at 100%. You are correct though, I would be able to rate the system at 112A using the 100% rated breaker.
 

kwired

Electron manager
Location
NE Nebraska
I guess what I meant was that you can't use an 80% rated breaker because you have to rate the continuous load at 100%. You are correct though, I would be able to rate the system at 112A using the 100% rated breaker.

Correct.

Either way your situation would require 125 amp overcurrent device.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I guess what I meant was that you can't use an 80% rated breaker because you have to rate the continuous load at 100%.

This is not correct.


After reading up more on this, I think I can use an 80% rated breaker. Am I wrong on this?

Yes, this is correct per 215.3. Technically, there is no such thing as an 80% rated breaker. All breakers are rated to carry 100% of their rated current for less than 3 hrs (non-continuous load.)
 
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kwired

Electron manager
Location
NE Nebraska
After reading up more on this, I think I can use an 80% rated breaker. Am I wrong on this?

You are correct. As I stated you will still come up with same size on this particular installation. If the 100 amp was continuous and the 12 amps non continuous then it would make a difference. That would be 100*1.25 +12 = 137 amps - you would then need a 150 amp breaker Vs. 100 + 12 = 112 amps needing a 125 amp breaker for a 100% rated breaker.
 

kwired

Electron manager
Location
NE Nebraska
This is not correct.




Yes, this is correct per 215.3. Technically, there is no such thing as an 80% rated breaker. All breakers are rated to carry 100% of their rated current for less than 3 hrs (non-continuous load.)

There are two types of breakers as far as topic of discussion. The term 80% is not typically used, but the rules essentially do not make calling it that an inaccuracy.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
There are two types of breakers as far as topic of discussion. The term 80% is not typically used, but the rules essentially do not make calling it that an inaccuracy.

I don't understand your point. I would think calling a 125A c/b that properly carries a load of 115A (per the OP) an "80% rated breaker" would be an inaccuracy.
 

kwired

Electron manager
Location
NE Nebraska
I don't understand your point. I would think calling a 125A c/b that properly carries a load of 115A (per the OP) an "80% rated breaker" would be an inaccuracy.

To have accuracy the word "continuous" needs to be in there.

Mix continuous and non continuous loads and the 115 that the op has is not one or the other but a combination of both. As I have already said what portion of the load is continuous does make a difference. If the entire 115 (112 in OP's case) were continuous 125 amp breaker is not large enough unless it is rated for 100% continuous loading. Obviously by that statement there must be more than one type of breaker. What one chooses to call them is where this techincal disagreement comes from.

We also do not really size general overcurrent devices according to the load but size them to protect the conductors from heating effects of overcurrent conditions. The conductors need to be sized as a minimum according to the load. The load itself may have overcurrent protection requirements but that can usually be supplemental.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Obviously by that statement there must be more than one type of breaker. What one chooses to call them is where this techincal disagreement comes from.

Yes, and obviously the term "80% rated breaker" has confused to OP into believing that a 125A c/b can't supply a circuit with any more than 100A. That is why it is a poor term to use. "Standard rated c/b" would be a better term, IMO. A standard rated c/b can carry 80% of its rating continuously, 100% of its rating non-continuously, are some combination of cont. & non-cont. as in the OP.
 

Npstewart

Senior Member
I usually look at it this way.

the 80% and the 1.25 (125%) is actually the same thing because:

80% = 0.8
1/0.8 = 1.25 or 125%

in short, 1.25 is the inverse of 80%.
---
SO basically if you have 124.9 amps non-continuous on a 125A breaker it is fine.

If that 124.9 amp load runs for more than (3) hours (ie continuously) than you are required to multiply by 1.25. So:

124.9 *1.25 = 157 A, so now that the continuous multiplier is already factored in, you can load up the breaker to 100%, ie. use 157A breaker or a more standard size would be a 175A breaker.
-----

Once you apply the respective factors to a load, you size the breaker at the next standard size. If you don't have a factor to apply to it then you just size it at the load itself. If you have a load with a factor, and one without a factor, then you apply the factor to the one load, add it to the other, and then size your breaker.

--

One other note,

At sometime or another, the NEC revised there language from 80% to continuous (125%), it gets confusing between the different agencies such as UL and NFPA.
 
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templdl

Senior Member
Location
Wisconsin
100% rated breakers start with a 400A frame

100% rated breakers start with a 400A frame

“Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.”

The minimum circuit conductor size without the application of any ampacity adjustment or correction factors shall have an allowable ampacity equal to or greater than the noncontinuous load plus 125% of the continuous load.

“Exception: Where the assembly including the overcurrent devices protecting the feeder(s) are listed for operation at 100 percent of their rating, neither the ampere rating of the overcurrent device nor the ampacity of the feeder conductors shall be less than the sum of the continuous load plus the noncontinuous load.”

Section 220-10(b) covers standard breakers, and the exception 100% rated breakers. NEC Section 220-10(b) and the Section 220-10(b) exception can be expressed by these formulas:
Standard 80% Rated Design
Noncontinuous Load + 125% of the
Continuous Load
= Total Minimum Load

Special 100% Rated Design
Noncontinuous Load + Continuous Load
= Total Minimum Load
 

kwired

Electron manager
Location
NE Nebraska
Yes, and obviously the term "80% rated breaker" has confused to OP into believing that a 125A c/b can't supply a circuit with any more than 100A. That is why it is a poor term to use. "Standard rated c/b" would be a better term, IMO. A standard rated c/b can carry 80% of its rating continuously, 100% of its rating non-continuously, are some combination of cont. & non-cont. as in the OP.

What is confusing to many people is what is a breaker designed for continuous operation at 100%. They are kind of rare to most people. Everything else is "standard rated c/b".

People read the sections in the code that mention the exceptions for 100% devices and then get to thinking - If only I could use that rule for this installation... - but they don't realize that most every breaker they have ever seen is likely just the standard rated type.
 

templdl

Senior Member
Location
Wisconsin
Yes, and it is common that it is assumned that 100% rated breakers are better than the standard breaker when they are not as it is only in how the 100% breaker is applied.
Actually there probably is no difference in the breakers either electrically of mechanocally as they are most likely identical except for the label and that the 100% will have electronic trip units (not the mechanical thermal magnetic) which would be identical to the standard breakers with electronic trip units.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Yes, and it is common that it is assumned that 100% rated breakers are better than the standard breaker when they are not as it is only in how the 100% breaker is applied.
Actually there probably is no difference in the breakers either electrically of mechanocally as they are most likely identical except for the label and that the 100% will have electronic trip units (not the mechanical thermal magnetic) which would be identical to the standard breakers with electronic trip units.
The 100% rated breakers will also have instructions detailing their ventilation requirements (i.e. oversized enclosures with air slots).
 
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