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#217935 - 01/12/17 05:34 PM 100% breakers?  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,069
Reading the Schnieder Electric page they talk about 100% breakers and say these would relieve the installer from applying the 125% rule on continuous loads. I guess I do not understand all I know about these things.

Greg Fretwell

Work Gear for Electricians and the Trades

#217936 - 01/12/17 05:58 PM Re: 100% breakers? [Re: gfretwell]  
ghost307  Offline
Joined: May 2005
Posts: 900
Chicago Illinois USA
Most circuit breakers have to be derated to 80% for continuous loads. That's why a normal 200A breaker can only feed a 160A continuous load. 100% Breakers are rated such that they can carry 100% of their nameplate rating continuously; thus a 200A breaker that's rated by the manufacturer at 100% can carry 200A continuously.

Hope this helps.


#217939 - 01/13/17 01:15 AM Re: 100% breakers? [Re: gfretwell]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,069
That is not exactly the way I read that. It says 100% of the non continuous load and 80% of the continuous load.
The examples actually say you can use the conductors at 100% on a feeder if you have a 100% breaker.

If the conductors are sized using the 80%/100% rule, you should be able to install a breaker that protects that conductor at 100% if I read the trip curve right.

When you read the Schnieder page it says "regular" breakers will carry 100% of their rating and goes off talking about enclosure size etc.

Last edited by gfretwell; 01/13/17 01:24 AM. Reason: Add link

Greg Fretwell

#217940 - 01/13/17 12:35 PM Re: 100% breakers? [Re: gfretwell]  
ghost307  Offline
Joined: May 2005
Posts: 900
Chicago Illinois USA
A 100% rated breaker can carry its full rating continuously. That means that the continuous load can be held at 100% as well as the non-continuous load.

Conductor sizing isn't covered here, but there is no requirement to derate the ampacity of current-carrying conductors except for temperature or voltage drop considerations. If the table says that a 500kcmil will carry 380 Amps, you can load it all the way to 380 Amps.


#217943 - 01/14/17 07:11 PM Re: 100% breakers? [Re: gfretwell]  
jraef  Offline
Joined: Jul 2004
Posts: 95
San francisco, CA, USA
This is perhaps one of the most misunderstood aspects of circuit breakers.

The so called "80% rule" is actually not stated anywhere in circuit breaker listing and labeling rules. UL489, the standard for Molded Case Circuit Breakers, requires that all MCCBs are tested at 100% load.

YOU must size CONDUCTORS at 125% of the continuous load you are feeding per the NEC. You then size the breakers for protecting those CONDUCTORS. So INDIRECTLY the breaker will never carry more than 80% of the load that you selected the conductors for. Ergo people refer to them as being rated for 80%, but that's just because of the way the NEC is worded.

You cannot use a 100% rated breaker unless you have also met all of the conditions for the CONDUCTORS that allow you to size the CONDUCTOR at 100% of the load. One of those conditions will end up being that the conductors are rated for 90C. In order for you to use conductors rated and sized for 90C, all components in the circuit must also be rated for 90C. Lugs on standard circuit breakers are NOT rated for 90C, they are rated at best, 75C. So one thing you get when you buy a 100% rated breaker is that the lugs will be 90C (usually copper, not Al). The other thing is that panelboards are NEVER rated for 90C conductor use. So the only way to use a 100% rated breaker is to buy it as a separately enclosed stand-alone breaker, or as an open breaker that YOU will build into an assembly, such as a switchboard or MCC, that you will have listed for use with 90C rated conductors at 100% load.

So for example where you see this done most often is that the MAIN breaker in a switchboard or MCC is selected to be rated 100%, because the load side of that breaker is going to bolt directly to the bus bar, not cables, so that connection can be rated for 90C. The incoming cables to that breaker can then be sized, selected and rated based on 100% load as well.

But if you have a breaker in a panelboard and you think you can simply upgrade it to 100% rated to get more current out of it, you are going to be disappointed because you will find that the 100% rated breaker cannot be used that way.

Last edited by jraef; 01/14/17 07:14 PM.


#217948 - 01/15/17 12:10 AM Re: 100% breakers? [Re: gfretwell]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,069
Thanks, that is sort of what I thought but this was concisely worded.

Greg Fretwell

#217951 - 01/16/17 10:13 AM Re: 100% breakers? [Re: gfretwell]  
Texas_Ranger  Offline
Joined: Dec 2001
Posts: 2,393
Vienna, Austria
Another interesting difference between the US and any other country I've worked with so far - in Europe all tables refer to continuous loads so 100% is always 100% unless you need to derate for bundling, higher ambient temperature, voltage drop etc. The only rule is that the nominal current of the overcurrent protection must be greater or equal than the load current and smaller or equal to the conductors' rating. And all conductors commonly used in fixed wiring are 70C.

Last edited by Texas_Ranger; 01/16/17 10:14 AM.

#218174 - 02/04/17 01:53 PM Re: 100% breakers? [Re: gfretwell]  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,205
Chesapeake, VA
NEC requires the cabling & OCP to be sized 125% but immediately grants an exception if the breaker is rated for 100%. My understanding is that this is due to breaker construction and heat tolerance and the tendency of simple thermal trip units to nuisance trip. If you look at the actual trip curves for molded case breakers, you'll see it's not a clean line, but a wide shaded area where the breaker may trip at different times in different conditions (ambient temp, how hot the breakers above/below it are, manufacturing tolerances, etc). Heat soak can shift the thermal trip portion of these curves well to the left. You can safely run an 80% rated breaker to 100% with non-continuous load, but if you keep them at 100% too long, the thermal trip unit could trip below the OCP rating.

Typical trip curves:

Virtually all the inexpensive molded case breakers I see for residential and commercial use are 80% rated, but once you get into the big industrial breakers with adjustable electronic trip units, they're all 100% rated. I don't think I've ever seen a 100% rated 20A breaker, but I also don't think I've ever seen a 1200A+ breaker that wasn't 100% rated. Which makes sense: the small breakers rely on internal heat buildup to tell if it's overloaded, but the high-end trip units are using CTs and actual current measurements. The same physics applies to overseas breakers whether the local codes require it or not- it's not like there's a difference; they all use the same thermal trip principals. Actually, perfect example: Schneider will take the same breaker and slap an MGE or SquareD label on it depending where it's being sold, but it's literally identical.

Last edited by SteveFehr; 02/04/17 01:56 PM.

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