100% rated breakers vs their 80% standard rated counterparts

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Carultch

Senior Member
Location
Massachusetts
As the title says, what is the difference between a full 100% rated breaker and its standard rated counterpart?

Example: how does an 80A/100% breaker compare to an otherwise equivalent 100A standard 80% rated breaker? I take it that this isn't just a re-labeling of the same device, otherwise it wouldn't make sense for the 100% rating standard to exist. Although at first glance, it might appear this way.

I understand the sizing algorithm for selecting the breaker, but how does this affect the rest of the design, such as circuit ampacity? Using a 100A standard rated breaker, one would need greater than 90.1A worth of wiring to be compliant (presuiming all other rules also comply). But do you still need 90.1A worth of wiring to be compliant with an 80A/100% rated breaker? Or is it 70.1A worth of wiring?

If you are wondering where I'm getting 90.1A and 70.1A, this is from 240.4(B), the "next size up" rule. And like I said, assume that all other rules also comply.
 

kwired

Electron manager
Location
NE Nebraska
It mostly only effects how you deal with determining conductor ampacity and overcurrent device selection when you have continuous loads.

If you don't have any continuous loads you don't even pay any attention to whether or not the breaker is 100% rated or not.

The main issue is the termination rating.

We need to increase conductor ampacity by a factor of 1.25 for the continuous portion of the load because the terminations of a non 100% rated device depend on the conductor as a heat sink for the device.

This 125% factor is mentioned in 210.19 for branch circuit conductors and 215.2 for feeder conductors and is mentioned again in 210.20 and 215.3 for overcurrent protection.
 

kwired

Electron manager
Location
NE Nebraska
Using a 100A standard rated breaker, one would need greater than 90.1A worth of wiring to be compliant (presuiming all other rules also comply). But do you still need 90.1A worth of wiring to be compliant with an 80A/100% rated breaker? Or is it 70.1A worth of wiring?

If you are wondering where I'm getting 90.1A and 70.1A, this is from 240.4(B), the "next size up" rule. And like I said, assume that all other rules also comply.

The proper way of determining conductor size is to determine the load then the size conductor needed, similar when selecting overcurrent protection.

Stating you need 90.1 amps of conductor for a 100 amp breaker is putting the cart before the horse in some ways. You need to determine what the load is, select the ampacity of conductor needed, during that process you need to determine if the load is continuous, or if a portion of it is continuous as well as determine if a 100% rated breaker is being used or not. Then select the conductor size and overcurrent protection setting necessary.
 

Carultch

Senior Member
Location
Massachusetts
The proper way of determining conductor size is to determine the load then the size conductor needed, similar when selecting overcurrent protection.

Stating you need 90.1 amps of conductor for a 100 amp breaker is putting the cart before the horse in some ways. You need to determine what the load is, select the ampacity of conductor needed, during that process you need to determine if the load is continuous, or if a portion of it is continuous as well as determine if a 100% rated breaker is being used or not. Then select the conductor size and overcurrent protection setting necessary.


The reason why I posed the question that way, is that I've already solved the other conditions that govern conductor sizing, and I wanted you not to focus on those.

The remaining condition for conductor sizing that I wanted you to focus upon, is for the OCPD to protect the wire at its ampacity. And "protect" implies the next size up rule, in this instance.
 

Carultch

Senior Member
Location
Massachusetts
The proper way of determining conductor size is to determine the load then the size conductor needed, similar when selecting overcurrent protection.

Stating you need 90.1 amps of conductor for a 100 amp breaker is putting the cart before the horse in some ways. You need to determine what the load is, select the ampacity of conductor needed, during that process you need to determine if the load is continuous, or if a portion of it is continuous as well as determine if a 100% rated breaker is being used or not. Then select the conductor size and overcurrent protection setting necessary.

Maybe I need to give you the full example.

Given:
316.48A of total ISC
395.6A of max continuous current (1.25*Isc)
493.7A worth of 1.56*Isc
400A continuous duty rated breaker
Alternative: 500A standard duty breaker

1. 75C Terminal amps needed at 1.56*Isc = 493.7A, which is 2 sets of 350 kcmil
2. 90C wire amps needed with 0.8 total derate at 1.25*Isc, which is 2 sets of 300 kcmil
3. 75C terminal amps needed to be protected by 500A standard breaker, 350kcmil
4. 90C wire amps needed to be protected by 500A standard breaker, 450/.8/2 = 281.25. 2 sets of 400 kcmil AL

Observe that part 4 is governed by the selection of a 500A standard rated breaker. But with a 400A/100% rated breaker, the 2 sets of 350 kcmil would appear to be sufficient, unless I'm not understanding this correctly.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
My understanding is that the primary difference is the lugs. In fact, I have been told that in many cases there is no difference at all other than the labeling.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Finding 100% rated breakers with a high DC rating may be difficult.
One design factor is that a normal breaker rating does not take into account possible heating from adjacent fully loaded breakers.
The combination of 100% rated breakers and 100% rated enclosure does.
 

Carultch

Senior Member
Location
Massachusetts
Ground sizing with 100% rated breakers

Ground sizing with 100% rated breakers

How does one determine the EGC associated with a 100% rated breaker?

Do you pick one directly off the rating indicated on the breaker, and Table 250.122?
Or do you need to multiply its rating by 125%, and then go to Table 250.122?
 

Carultch

Senior Member
Location
Massachusetts
Finding 100% rated breakers with a high DC rating may be difficult.
One design factor is that a normal breaker rating does not take into account possible heating from adjacent fully loaded breakers.
The combination of 100% rated breakers and 100% rated enclosure does.

That's my understanding. That the entire enclosure containing the breakers needs to be a listed assembly to use them at continuous duty.
 

pv_n00b

Senior Member
Location
CA, USA
The conductor calculations do not change based on the breaker being 80% or 100% rated. Just use the breaker rating you have selected as you normally would. Same for selecting the EGC.
 

Carultch

Senior Member
Location
Massachusetts
The conductor calculations do not change based on the breaker being 80% or 100% rated. Just use the breaker rating you have selected as you normally would. Same for selecting the EGC.

I don't understand what you are trying to tell me. Which one are you trying to tell me?
A: A breaker that is 400A continuous duty rated should be treated as if it were a 500A standard breaker with another label.
B: A breaker that is 400A continuous duty rated should be treated precisely as a 400A breaker, when it comes to selecting an EGC, and evaluating it for whether or not it protects the wire at its ampacity.
 

iwire

Moderator
Staff member
Location
Massachusetts
I don't understand what you are trying to tell me. Which one are you trying to tell me?
A: A breaker that is 400A continuous duty rated should be treated as if it were a 500A standard breaker with another label.
B: A breaker that is 400A continuous duty rated should be treated precisely as a 400A breaker, when it comes to selecting an EGC, and evaluating it for whether or not it protects the wire at its ampacity.

B
 

kwired

Electron manager
Location
NE Nebraska
Thanks iWire. Exactly what I was looking for.
Was EGC one of your major concerns in OP, or was that just another question that arose later in the thread?

250.122 has no mention of continuous or non continuous, everything is based on the OCPD setting, and leaves it wide open to how that setting is determined. We all know different devices have different trip curves as well yet 250.122 doesn't take that into consideration either.
 

Carultch

Senior Member
Location
Massachusetts
Was EGC one of your major concerns in OP, or was that just another question that arose later in the thread?

250.122 has no mention of continuous or non continuous, everything is based on the OCPD setting, and leaves it wide open to how that setting is determined. We all know different devices have different trip curves as well yet 250.122 doesn't take that into consideration either.

That, and the conductor sizing algorithm in general. It may seem like it is a question of just a few conductors, but it is a question that propagates across an entire project.

I was under the illusion that a continuous rated device is the same "gizmo" as the equivalent standard rated device that had been re-labelled with a different rating. And I wanted to understand if/how it really was any different.
 

pv_n00b

Senior Member
Location
CA, USA
I don't understand what you are trying to tell me. Which one are you trying to tell me?
A: A breaker that is 400A continuous duty rated should be treated as if it were a 500A standard breaker with another label.
B: A breaker that is 400A continuous duty rated should be treated precisely as a 400A breaker, when it comes to selecting an EGC, and evaluating it for whether or not it protects the wire at its ampacity.

B

Use the rating you selected, you do not have to "correct" the 100% rating to 80% for use in selecting any other components.
 
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