Molded Case Vs. TM Breakers - Mike Holt's Forum

What Ron said notwithstanding, there is a problem with your question as worded. You are asking for a comparison between what you perceive to be two options, but in fact you have mixed up three related, but not comparable things, because they are three ASPECTS of circuit breakers that could potentially exist in the SAME breaker.

Molded case relates to the CONSTRUCTION of the breaker, officially a “Molded Case Circuit Breaker or “MCCB” as opposed to Insulated Case Circuit Breaker (ICCB) or Power Circuit Breaker (PCB) or Miniature Circuit Breaker (MCB).

Thermal Magnetic has to do with the TRIP SYSTEM of the breaker, as opposed to Magnetic Only or Electronic Trip or Hydraulic Trip.

100% has to do with the USE RATING of an MCCB, as opposed to a “normal” rating. Ron describes that well except there is no such thing as an “80% rated” MCCB. There are “normal” MCCBs and there are 100% rated MCCBs. All MCCBs that are in a panel next to other breakers are normal breakers and all breakers are tested and listed at 100% of their rated value. But you cannot USE them that way. Breakers protect the conductors. Conductors are to be sized for 125% of their continuous load. The inverse of 125% is 80%, meaning that since the conductors are rated for 125% of the load, the maximum load is 80% of the rating of the conductors and since the breakers are sized to protect the conductors, you would never see more that 80% of the load on those breakers; continuously, which is where the “80% rated” concept comes from. Then BECAUSE of that, panel mfrs take advantage of that fact in how the panel and MCCBs are designed with regard to heat dissipation, which is why MCCBs in a panel can be mounted next to each other, touching.

But it if you need to use an MCCB at 100% of its rating continuously, it CANNOT be used in a panel like that, it must have free air around it to dissipate the added heat. So because of THAT, breakers intended to be used at 100% continuously must be specifically LABELED as such, which will mean it cannot be used in a panel, except as a main. If it is not a Main, it must be separately enclosed by itself.

Then when you start discussing POWER Circuit Breakers (PCBs), where each PCB has its own totally separate cubicle in switchgear, those are ALL rated 100%, so it’s unnecessary to mention it.

SO back to your original question; you CAN have a Molded Case Thermal Magnetic 100% rated breaker. So that’s why asking for a comparison is technically spurious.

I just don't follow the 125% rule

Well, here's my understanding, in case it helps you (and so someone can correct me if I'm wrong). It's all about limitations of standard breakers; a wire can carry current at its ampacity indefinitely without damage.

A standard thermal-magnetic MCCB tested by itself (in an enclosure? in open air?) at 40C ambient is supposed to hold its rated current indefinitely. That's a spatial arrangement that maximizes the breaker's ability to cool off by rejecting heat to its environment. Under these conditions, the thermal trip unit will reach some steady-state temperature, due to an equilibrium between the self-heating from the current through the breaker and the breaker's heat rejection to the environment. The thermal trip unit is calibrated to not trip at this steady-state temperature (but presumably to trip at a temperature just a bit higher).

Now take that same breaker and put it in a panel full of other breakers. The nearby breakers inhibit any convective cooling off the case of the breaker, plus the nearby breakers are themselves heat sources, so their temperature may be higher than ambient. The result is a lower ability of the breaker to reject heat to its environment. That means that if you run the full rated current through the breaker, the steady-state temperature of the thermal trip unit will be higher than previously; the equilibrium has shifted. That temperature may now be above the trip point, causing the breaker to trip when you don't want it to.

A work-around for this is to prohibit continuously running the full rated current through the breaker. Somehow the factor of 80% was arrived at, it was judged that reducing the current to at most 80% of the rating would shift the equilibrium temperature back sufficiently to avoid unintended tripping. In other words, you need to oversize the breaker by a factor of 125%.

Code-wise, that factor of 125% requires upsizing the conductors by 125% as well; otherwise the conductors would be underprotected by the breaker. [And upsizing the conductors has the further benefit of improving the breaker's ability to reject heat via conduction through the conductors.] The code writers chose to put the requirements in terms of upsizing the conductors as the first matter, with the upsized breaker as a side effect. I don't find that particular enlightening, since the limitations of thermal trip units are the whole reason for the requirement, so I prefer to think of it as upsizing the breaker as the first matter, with the upsized conductors as a side effect.

Cheers, Wayne

It’s not necessarily just the trip unit that’s the issue though, it’s the ability of the entire breaker to dissipate heat. Most (if not all) breakers with ETUs can be ordered as 100% rated for the same price, but once it has that 100% rated label on it, your use of it becomes restricted. One of those restrictions will be that it must be mounted with limitations on what is next to it. Other breakers on either side will violate that.

OK, but I'm trying to understand the technological limitations, as opposed to the listing limitations. It sounds like the listing limitations are based on the behavior of thermal trip units. My questions:

1) Am I correct in my understanding that an ETU's trip curve is independent of ambient temperature (to first order)?

2) Can the electronics in an ETU be made to withstand the internal temperature of a breaker carrying its full rated load continuously, when packed into a load center with adjacent breakers?

3) If so, is there any other reason a manufacturer couldn't make ETU MCCBs for regular small panelboards that function as 100% breakers, without limitation on breaker spacing?

I understand, of course, that even if technologically possible, there's little incentive or market for manufacturers to develop this product and get the UL listing standards adjusted to accommodate them.

Thanks,
Wayne

The National Electrical Manufacturers Association has just what you need to answer your circuitbreaker application and preventive maintenance questions. NEMA publishes two standards that provide a wealth of information to help users and specifiers select and maintain circuit breakers.

Goto Sager to know more.

NEMA Standards Publications AB 3- and AB 4- can answer application questions and preventive maintenance questions for molded case circuit breakers. Proper use of these standards can help prevent fire and shock hazards, down-time due to systems failures, and other operations mishaps.

The two NEMA standards serve a different purpose than the standards used to certify circuit breakers. Molded case circuit breaker manufacturers in the US certify their products to the safety and performance requirements defined in the Underwriters Laboratories Inc. UL 489 standard. When circuit breakers meet or exceed these requirements UL permits their listing Mark to be placed on the circuit breaker. Electrical inspectors that enforce electrical codes look for this listing Mark when approving electrical installations.

While knowledge of the construction and performance requirements is important to manufacturers, information regarding the application, field testing, and maintenance of circuit breakers is more important to specifying engineers, installers, and maintenance personnel. That information is found in these two NEMA standards.

If you specify molded case breakers:
The NEMA AB3- standards publication, Molded Case Circuit Breakers and Their Application, describes the types of circuit breakers available and many of their specific applications. It also lists circuit breaker accessories and ratings, and provides guidance on the selection of circuit breakers. Finally it defines much of the terminology associated with circuit breaker products. In addition to molded case circuit breakers, NEMA AB3 covers molded case switches and their accessories.

Section 5 of NEMA AB3 is especially useful for those who select circuit breakers. This section covers the following topics:

• electrical system parameters environmental conditions (extreme temperatures, humidity, corrosive atmospheres)
• vibration
• shock
• harmonics
• feeder and branch circuit breakers
• load considerations
• temperature ratings of conductors and conductor ampacity

Section 5 also includes a detailed explanation on the following:

For more mold case circuit breaker supplierinformation, please contact us. We will provide professional answers.

• time-current curves
• series-combination ratings
• capacitor switching considerations
• motor loads
• ground-fault protection
• circuit breakers used in DC voltage systems

Finally, Section 5 provides a basic overview of the UL standards requirements for the Listing of circuit breakers.

If you inspect or maintain molded case breakers:
The NEMA AB4 standards publication, Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers used in Commercial and Industrial Applications, provides guidance for maintenance and field testing of circuit breakers and explains how to evaluate the condition of a circuit breaker. NEMA AB4 provides basic procedures that may be used to inspect and maintain molded case circuit breakers rated up to and including volts, 50/60 Hz ac or ac/dc. The test methods described in NEMA AB4 may be used to verify certain characteristics of circuit breakers originally built and tested to the requirements of the UL 489 and NEMA AB1 standards. However, it is not intended, nor is it adequate, to verify proper electrical performance of a molded case circuit breaker which has been disassembled, modified, rebuilt, refurbished, or handled in any manner not intended or authorized by the original manufacturer.

These test methods are intended for field use only and are non-destructive. They cannot be used to verify all performance capabilities of a molded case circuit breaker, since verification of certain capabilities requires destructive testing.

Some industrial users have indicated that they are required to conduct periodic operational tests of their circuit breakers. The non-destructive tests outlined in Section 5 of NEMA AB4 may be used to verify certain operational characteristics of molded case circuit breakers including:

• mechanical operation
• insulation resistance
• individual-pole resistance
• inverse-time overcurrent tripping
• instantaneous overcurrent tripping
• rated hold-in current

Section 6 describes tests for the proper use of devices such as shunt trip and undervoltage trip releases, electrical operators that allow opening and closing the circuit breaker from a remote location, auxiliary switches that operate when the circuit breaker opens or closes, and alarm switches that operate when the circuit breaker trips automatically.

The NEMA AB3 and NEMA AB4 standards publications are valuable tools for the safe and efficient operation of molded case circuit breakers. They are indispensable to contractors, specifiers, plant engineers, and maintenance personnel. Keep these NEMA standards in mind the next time you have an application or maintenance question concerning molded case circuit breakers. For a copy of AB3 or AB4 call NEMA at 703/841- or IHS at 800/854-.