Molded Case Vs. TM Breakers

[mstrlucky74]

Whats the difference between molded case and 100% thermal magnetic breakers? Price higher on 100% thermal magnetic I believe. Thanks.

 

[ron]

A 100% breaker can be loaded with 100% continuous load forever - and a regular molded case or non-100% thermal magnetic for that matter can be loaded to 80% continuous forever or 100% non-continuous forever or 100% continuous for 2 hours and 59 minutes (which makes it non-continuous because it didn't stay on for 3 hours )

 

[Jraef]

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.

 

[wwhitney]

This seems like a good place for a question I've been wondering about for a while:

Is there any technological reason a manufacturer couldn't produce a MCCB that uses an electronic trip, that is 100% rated, and that can be used in standard panelboards without extra clearance to other circuit breakers? I'm assuming here that an electronic trip's trip curve wouldn't be affected by ambient temperature.

Thanks,
Wayne

 

[ron]

Is there any technological reason a manufacturer couldn't produce a MCCB that uses an electronic trip, that is 100% rated, and that can be used in standard panelboards without extra clearance to other circuit breakers? I'm assuming here that an electronic trip's trip curve wouldn't be affected by ambient temperature.

Nope, except it would have to be a tested assembly, so the breaker only goes with that tested panelboard, not just any standard panelboard in their line.

 

[Jraef]

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.

 

[jim dungar]

Is there any technological reason a manufacturer couldn't produce a MCCB that uses an electronic trip, that is 100% rated, and that can be used in standard panelboards without extra clearance to other circuit breakers? I'm assuming here that an electronic trip's trip curve wouldn't be affected by ambient temperature.

The Square D I-Line panel, from Schneider Electric, has the ability to be ordered with 100% rated branch breakers. But they are rarely worth the cost until you start getting into circuits over 400A (about the point where savings in material and labor outweigh the cost of the special panel construction).

 

[Jraef]

Interesting, didn’t know that. Maybe because the I-Line bus stabs are designed to carry away the heat?

 

[mbrooke]

I just don't follow the 125% rule

 

[wwhitney]

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

 

[wwhitney]

The Square D I-Line panel, from Schneider Electric, has the ability to be ordered with 100% rated branch breakers. But they are rarely worth the cost until you start getting into circuits over 400A (about the point where savings in material and labor outweigh the cost of the special panel construction).

So are these standard thermal-magnetic breakers, and does the the special panel construction mean more space between breakers?

Cheers, Wayne

 

[wwhitney]

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

 

[jim dungar]

So are these standard thermal-magnetic breakers, and does the the special panel construction mean more space between breakers?

No. Square D does not have 100% thermal magnetic breakers. The I-Line version are electronic trip, which was why I said they are not cost effective at lower amperages.

The panel probably requires more ventilation slots in the cover.

 

[mbrooke]

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.

Correct- and then some. Wire carrying 90*C of current listed in table 310.16 will not even approach 60*C.

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).

Correct. Typically time current curves start at 125% at 40*C, and UL does not require tripping at 134%.

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.

In theory. Most loads listed as continuous will do fine being treated as none continuous. Residential electric heat, water heaters, ect is a perfect example. Other loads like commercial lighting will also do well provided that every branch circuit isn't so. Most "general circuit" panels have enough diversity.

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%.

Part that erks me is the wire also have to be increased.

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

IMO the breaker was up-sized first- but for some reason I can not understand code choose to require that the conductor be up sized indiscriminately. Unless you have receptacles on the circuit, there is simply no point.