Earthing and Bonding of motor frames, Junction Box
- Thread starter winter
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Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
My question is, is it installed yet? Whats taking so long :grin:
Yes, based on your post, 4 AWG is the correct size for a 300A overcurrent device - 250.122.
Earthing/grounding -Table 250.66
Bonding - Table 250.122 (load side of the service disconnecting means)
Yes, based on your post, 4 AWG is the correct size for a 300A overcurrent device - 250.122.
Earthing/grounding -Table 250.66
Bonding - Table 250.122 (load side of the service disconnecting means)
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
But for a motor, the size of the EGC is based on the size of motor overload protective device see 250.122(D). Motors will typically have the short circuit device (CB or fuse) higher than that required in 240.4 (IE protect a conductor at its ampacity) to allow the motor to start.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
tom baker said:
250.122(D) mentions a motor short circuit protector, what is a motor short circuit protector? Doesn't sound like a fuse or inverse time CB.
wirenut1980
Senior Member
- Location
- Plainfield, IN
Bluelight Special on EGC's
Bluelight Special on EGC's
IMO, 250.122(D) is giving us a break in allowing us to size the EGC based on the "small" overcurrent device (overload) of the motor rather than the "large" overcurrent device (motor short circuit protector) of the circuit, because like Tom said, these short circuit protectors are allowed and aften are oversized to allow motor to start.
Bluelight Special on EGC's
IMO, 250.122(D) is giving us a break in allowing us to size the EGC based on the "small" overcurrent device (overload) of the motor rather than the "large" overcurrent device (motor short circuit protector) of the circuit, because like Tom said, these short circuit protectors are allowed and aften are oversized to allow motor to start.
- Location
- Illinois
- Occupation
- retired electrician
250.122(C) only applies when using an MCP (this device is only permitted as part of a listed combination motor starter). It does not apply when the OCPD on the line side of the controller is a circuit breaker or fuse. In these cases the EGC must be sized based on the rating of the circuit breaker or fuse. However if that results in an EGC larger than the motor conductors, 250.122(A) says it can be reduced to the size of the circuit conductors.
- Location
- Illinois
- Occupation
- retired electrician
That is an instantaneous trip device. It is not an overcurrent protective device because it does not provide protection from overloads. It only provides protection from short circuit and ground faults and can only be used as part of a listed combination motor starter.infinity said:
- Location
- New Jersey
- Occupation
- Journeyman Electrician
don_resqcapt19 said:
So it isn't an instantaneous trip circuit breaker but another device that operates in the same fashion.
- Location
- Illinois
- Occupation
- retired electrician
The instantaneous trip breaker and the motor circuit protector are the same thing...a magnetic only device. No thermal device for overloads.infinity said:
Don
Man this is not my speciality but I believe the instant trip operates like and arc fault. When a circuit shorts it has an instant voltage spike of the order of 3 times the RMS voltage of the circuit. I know those thing can't operate on current overload because the could handle the ampacity. For my own knowledge someone please correct me if I'm wrong.
- Location
- Illinois
- Occupation
- retired electrician
E.C.
I has nothing to do with a voltage spike. It looks at the current if it exceeds the trip point the device opens.
I has nothing to do with a voltage spike. It looks at the current if it exceeds the trip point the device opens.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
don_resqcapt19 said:
That makes perfect sense, thanks Don. I don't recall ever seeing an MCP.
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