Designer69
Senior Member
if you guys would be so kind, what are some of the differences/benefits/disadvantages of using a circuit breaker over a motor circuit protector or vice versa.
CB vs. MCP
- Thread starter Designer69
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zog
Senior Member
- Location
- Charlotte, NC
What is the application? Your question is much to vauge, I could do a 4 hour seminar on that question.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
There are traditional US style MCP's which are not much more than a circuit breaker with a magnetic trip function only. But, there are also devices which Europeans call MCP's and the traditional US name is Manual Motor Starter. Which one are you asking about?
Designer69
Senior Member
the US style mcp and the application is to protect a 25 HP fan motor for a cooling coil which is used to cool a bus duct. thanks
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
An MCP in that context is NOT something that you can decide to use unless you are a motor control equipment manufacturer. They are not UL listed, they are only UL Recognized (UR) because they must be used in conjunction with an overload relay in the circuit. So they can ONLY be used as a part of a listed motor starter assembly. Even most UL508 panel shops do not go to the extent of bothering with the extreme expense of testing and listing a motor starter using an MCP; its cheaper to just use a Thermal Mag breaker.
Designer69
Senior Member
so if you use a thermal magnetic CB in a combination starter would you still use a thermal overload relay or is the thermal element of the CB sufficient?
BAHTAH
Senior Member
- Location
- United States
Thermal Overload
Thermal Overload
You would still use the thermal overload in the starter which is adjustable. The setting of the thermal overload is based on the current on the motor nameplate.
Thermal Overload
You would still use the thermal overload in the starter which is adjustable. The setting of the thermal overload is based on the current on the motor nameplate.
Based on NEC requirements the overloads at the starter will protect the motor from overloads and the upstream CB is used for short circuit protection. If the CB is sized per NEC the thermal portion of the breaker will be too large to offer any overload protection for the motor, hence the overloads in the starter. If the upstream breaker were in a combination starter or an MCC you could specify an MCP.
Designer69
Senior Member
ok so they are sized smaller than the CB, what would prevent them from blowing on motor start? is there a time delay or something?
weressl
Esteemed Member
There are circuit breakers with adjustable thermal elements that satisfy the functional requirements of both disconnecting means, short circuit and overload protection. But many of those are IEC equipment w/o UL listing. Standard ANSI MCB's are not suitable for such use but their tripping characteristics are tailored for wire transformer SC and OL protection.
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