MCP vs Circuit Breaker

[kody916]

Based on what I know, MCP is only used for short circuit protection and Circuit breaker is used for overload as well as short circuit protection.
what factor actually determines if we have to use an MCP or Circuit Breaker?
I recently saw a drawing where an MCP was protecting a motor. How would that MCP protect the cable/motor incase there is an overload condition.

Thanks

 

[big john]

Check out 430.40 and 430.52.

If you want the protection afforded by a good overload relay, then you don't need to have a thermal-magnetic breaker in the circuit and you are allowed to protect the conductors with a MCP. They share protection ability with the relay covering the "thermal" functions, and the MCP covering the "magnetic" trip functions.

 

[Jraef]

"MCP" is actually a trade name (from Westinghouse, now Eaton) for what is officially called an Instantaneous Trip (IT) Circuit Breaker. Another common term is "Magnetic Only" or "Mag-Only" circuit breaker. All it means is that the circuit breaker has no thermal trip sensing elements. They can ONLY be used as part of a FACTORY assembled tested and listed assembly, specifically a "Combination Motor Starter". The reason is, as per those NEC sections, it can ONLY be used when the motor starter circuit contains a separate Thermal Over Load Relay (TOL). So the TOL protects the entire circuit against an overload, the MCP protects everything only from a short circuit. And MCP (IT) breaker is NOT UL listed, it is only UL Recognized as a component, because it can ONLY be used as part of a larger assembly and needs another device (ie. the TOL) to make it function correctly in a circuit protection scheme.

YOU can never "decide" whether or not to use an MCP/IT CB, other than indirectly by buying a factory built, tested and listed combo starter that uses one.

Historically, Thermal-Magnetic (TM) Cicuit Breakers used to come with a fixed and sealed Magnetic Trip based on the thermal rating. So if you had a motor starter, it was sometimes a problem to get a magnetic trip that could be set to correctly protect the motor windings. This was especially true of small motors, because the smallest CB you could buy was 15A, but if you had a 1/2HP 480V motor with a FLC of 1.1A, and the mag trip of the 15A CB was 150A (10x the rating), it was too high to protect that motor from a short circuit. So Westinghouse came out with that MCP because it had ADJUSTABLE magnetic trips that could be turned down (or up) to closely match the motor needs.

Since then, many, if not most, TM breaker mfrs have gone to making the mag trips adjustable now, so the real NEED for an IT breaker has actually gone away. Starter mfrs still use them however because for them, it saves money not having to include the thermal trips.

If you saw a motor connected to an IT/MCP breaker with no motor starter and TOL, that would be illegal. But... when the Europeans came on the scene here in North America, they use a breaker that has adjustable THERMAL trips. Technically, that was not something that could be used as a "Circuit Breaker" as defined by UL489 here in North America until relatively recently, but you could use it as a "Manual Motor Starter" under UL508, as long as there was a circuit breaker or fuses somewhere ahead of it. Sometime recently (when I wasn't looking), UL changed to allow adjustable thermal trips and still call it a "Circuit Breaker", as long as it is ONLY used on a motor circuit. Officially, that is now called a Motor Protection Circuit Breaker (MPCB). But in Europe, they call them "Motor Circuit Protectors" because the copyright laws that Westinghouse and Eaton had did not apply there. So you sometimes see MPCBs called out as "MCP"s in EU equipment.

Got that?

 

[templdl]

Based on what I know, MCP is only used for short circuit protection and Circuit breaker is used for overload as well as short circuit protection.
what factor actually determines if we have to use an MCP or Circuit Breaker?
I recently saw a drawing where an MCP was protecting a motor. How would that MCP protect the cable/motor incase there is an overload condition.

Thanks

Have you reviewed NEC art 430-52? It covers both.

 

[don_resqcapt19]

... But in Europe, they call them "Motor Circuit Protectors" because the copyright laws that Westinghouse and Eaton had did not apply there. So you sometimes see MPCBs called out as "MCP"s in EU equipment.

Got that?

And to add a bit more to any possible confusion there is also a fuse device called a "motor short-circuit protector". 430.53(C)(7).

Informational Note: A motor short-circuit protector, as used in this section, is a fused device and is not an instantaneous trip circuit breaker.

 

[Haji]

Circuit breaker is used for overload as well as short circuit protection.

A thermal magnetic circuit breaker actually senses over current and not over load, strictly speaking, because there is a definite operation of circuit breaker over 135% of current setting only.

 

[don_resqcapt19]

A thermal magnetic circuit breaker actually senses over current and not over load, strictly speaking, because there is a definite operation of circuit breaker over 135% of current setting only.

By definition, in the NEC, an over current protective device provides overload protection.

Overcurrent. Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

Overcurrent Protective Device, Branch-Circuit. A device capable of providing protection for service, feeder, and branch circuits and equipment over the full range of overcurrents between its rated current and its interrupting rating. Such devices are provided with interrupting ratings appropriate for the intended use but no less than 5000 amperes.

 

[adamscb]

There are two main types of rises in current: short-circuits, and overloads. Overloads arise when electrical equipment draw only slightly more than rated current, which can increase over time. At first, overloads do not harm your system. However, if left unchecked, damage can be caused. On the other hand, short-circuits happen very quickly, and produce currents that are several magnitudes larger than full load. Short-circuits must be eliminated very quickly.

MCP-style breakers only offer protection from short-circuits. These are very common in MCC buckets. Inside MCC buckets, you have your breaker and then your overloads beneath the starter. Why do you need both? The answer is to provide protection for both short-circuits and overloads. The MCP circuit breaker provides short-circuit protection, while the overloads provide the overload protection.

So then this question arises: why aren't overloads used, let's say in 480v switchgear breakers? The breakers commonly found in low-voltage switchgear sections are called thermal-magnetic breakers, because they provide both overload (thermal) and short-circuit (magnetic) protection. You don't need to install overloads in this case because the circuit breaker already provides overload protection.

When you try to differentiate between an MCP and a "circuit breaker", in reality an MCP is a special type of circuit breaker (MCP stands for Motor Circuit Protector). MCPs are used almost exclusively in branch circuits, while thermal-mags are used in feeder circuits.

Hope this helps.

 

[GoldDigger]

There are several reasons for using separate overload instead of relying on the thermal trip of a breaker:
1. The overload current needs to be set precisely to allow full load operation but trip just above that point. A fixed value breaker usually will not allow you to get close enough.
2. A good motor overload will have a time characteristic that fits the motor temperature rise and damage curves better than the breaker. Especially over the time scale involved with too frequent restarts of the motor.