Molded Case Breaker & Continuous Loads

[ron]

Standard 400A molded case breaker with 400A trip, seems obvious that maximum continuous current is 320A.

Standard 800A molded case breaker with adjustable solid state trip unit with LTPU dialed down to 400A (800A sensor x 0.5 LTPU), is the maximum continuous current still 320A? Is the 80% nature of a standard molded case breaker now not an issue since the LTPU is dialed down below 1?

 

[Jraef]

There is no 80% rule for circuit breakers. ALL circuit breakers are tested and listed for 100% of their rated current continuously.

But you cannot USE them that way. The breaker is sized to protect the cable, the cable is sized at 125% of the current. So INDIRECTLY, the breaker is never used at more than 80% of it's rating (80% is the inverse of 125%), that's where this comes from. Then since that is the way they are used, the breaker mfrs use that in the design of breakers when mounted in switchboards and panelboards with regard to the heat rise expected from the breakers and the effects of them being mounted next to each other.

If you buy breakers marked as "100% rated", you will find that to get that, they will NOT be available in a switchboard or panelboard, they must be stand-alone (or used as Main Breakers in some circumstances) and you will be required to use 90C rated conductors and 90C rated terminals. So even if you buy a 800A breaker with an ETU set for 400A, you can use it for the full 400A only if you follow the same rules you must always follow with regards to 100% rating of conductors and temperature ratings of terminals.

 

[ron]

Jraef, Thanks for the info. The extra stuff about 100% rated breaker is helpful, but I'm not sure I understand the answer to my OP.

In the case of an 800A breaker dialed down to 400A, you are suggesting that it is no different than a regular 400A breaker with regard to it ability to carry continuous current, yes?

It would seem weird because with the turn of a dial, we know this same breaker and lugs can accommodate 80% of 800A of continuous current (640A), so why wouldn't it handle 400A of continuous current?

 

[wwhitney]

So even if you buy a 800A breaker with an ETU set for 400A, you can use it for the full 400A only if you follow the same rules you must always follow with regards to 100% rating of conductors and temperature ratings of terminals.

This is the part I don't understand:

I get that with thermal magnetic breakers adjusting the thermal mechanism to have a trip curve that holds indefinitely at 100% of rated current, in a panelboard at 40C ambient with 40 other heat generating breakers would lead to excessively long trip times at higher currents. That's just the nature of the thermal environment in a panelboard. So for a continuous load breakers are allowed to trip at 80% of their rated current. [Which leads to oversizing the breaker by a factor of 125% for continuous loads, which leads to oversizing the conductors by a factor of 125% so the conductors are still properly protected. That is, I see the rules as driven by the limitations of available breakers.]

But with an ETU I would think a manufacturer could program in whatever trip curve is desired. As the OP points out, clearly with a 800A breaker and an ETU set for 400A, the equipment can thermally handle 400A continuously. So why couldn't you do what the OP provides? Is it just that the products don't exist, because every manufacturer programs their ETUs to mimic thermal magnetic breakers, including all of their limitations?

Cheers, Wayne

 

[Jraef]

What I was saying was that the issue is NOT the rating of the breaker, it's how you use it.

 

[ron]

What I was saying was that the issue is NOT the rating of the breaker, it's how you use it.

I understand the words you are saying but I do not understand how it applies.

I'll try an example:

If I have such a standard 800A frame breaker with the LTPU dialed down to 400A, and I apply 400A of conductor (600kcmils), I can't get the 400A continuous out of it because it isn't a 100% rated breaker, but that if I applied 500A worth of conductor to the same breaker, then I could get 400A continuous load out?

 

[Jraef]

80% rating of breakers is never mentioned in the NEC. Nor is there any mention of LTPUs or ETUs, etc.

210.20

(A) Continuous and Noncontinuous Loads. Where a
branch circuit 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.
Exception: Where the assembly, including the overcurrent
devices protecting the branch circuit(s), is listed for operation
at 100 percent of its rating, the ampere rating of the
overcurrent device shall be permitted to be not less than the
Sum of the continuous load plus the noncontinuous load.

(emphasis added)

So what is an 800A breaker with a 400A LTPU listed as? 800A or 400A?

Answer: 400A. So if that self same breaker does NOT have a label on it identifying it as "100% rated", you cannot use it as such.

Now to that point, here is an excerpt from UL 489 for Molded case Circuit Breakers:

9.1.4.4 A circuit breaker, having a frame size of 250 A or greater, or a multi-pole type of any ampere rating rated over 250 V; and intended for continuous operation at 100 percent of rating, shall be marked: “Suitable for continuous operation at 100 percent of rating only if used in a circuit breaker enclosure Type (Cat. No.) ____ or in a cubicle space ___ by ___ by ___ mm (inches).” Equivalent wording shall be permitted. Location Category C. The blanks are to be filled in with the minimum dimensions.

That's what a "100% rated breaker" must have on it. No label, no "listing", no good for the exception in 210.20.

Can you get an AHJ to look at it differently? Maybe. Can you rely on that? Not in my opinion. How did I come to that conclusion? I tried and failed...

 

[victor.cherkashi]

I don't know much about listing, but I know that breakers with electronic overcurrent module doesn't have temperature rating problem and probably can be used in an enclosure or in panel. they cost a little more than thermo magnetic.

Sent from my Nexus 5X using Tapatalk

 

[ron]

80% rating of breakers is never mentioned in the NEC. Nor is there any mention of LTPUs or ETUs, etc.

210.20

(emphasis added)

So what is an 800A breaker with a 400A LTPU listed as? 800A or 400A?

Answer: 400A. So if that self same breaker does NOT have a label on it identifying it as "100% rated", you cannot use it as such.

I like the use of 210.20, thanks

 

[wwhitney]

I'm still curious if there is any technical (physics) issue with what the OP proposed, as opposed to a regulatory issue.

Cheers, Wayne

 

[Jraef]

I'm still curious if there is any technical (physics) issue with what the OP proposed, as opposed to a regulatory issue.

Cheers, Wayne

I wouldn't think so. But that could be said of a number of things in the NEC...
Most (arguably all) archaic sounding rules in the NEC are there for a reason, some lost in time and/or changes in technology.

But... I can almost guarantee that an 800A frame electronic breaker with a 400A LTPU is going to cost WAY more than a 400A Thermal-Mag breaker rated for 100%, so it might be a rule that's pointless to change. You can always propose it though, we have a forum for that you know because there are people here who are on the committees.
http://forums.mikeholt.com/forumdisplay.php?f=72