Motor Protection?

[samir]

I am new to my job so having problem understanding this:
The 60HP motor is fed from a 200Amp breaker and cable size is #2AWG (115Amps).

Question: Why is the cable rated ampacity less than the circuit breaker? This will not protect the cable.
And why is the design of the cable and breaker different for motors than the other loads?

I checked NEC was not clear with the motor protection Article 430.

 

[Smart $]

First thing to understand about motor circuits is you have two separate requirements for protection: 1) ground fault; short circuit, and 2) overload. In motor circuits, the overload protection protects both motor and its conductors. The supplying OCPD (in this case a breaker) must have a high enough rating to not nuisance trip on startup (motors have high current inrush on startup).

 

[samir]

First thing to understand about motor circuits is you have two separate requirements for protection: 1) ground fault; short circuit, and 2) overload. In motor circuits, the overload protection protects both motor and its conductors. The supplying OCPD (in this case a breaker) must have a high enough rating to not nuisance trip on startup (motors have high current inrush on startup).

Thanks Smart $ for a prompt reply.

So for instance if there is a 60HP motor, FLA= 69Amps, LRA=470 Amps.
How to go about sizing the wire and breaker?

If there was an VFD would that make a difference in the sizing of both?

 

[brother]

Thanks Smart $ for a prompt reply.

So for instance if there is a 60HP motor, FLA= 69Amps, LRA=470 Amps.
How to go about sizing the wire and breaker?

If there was an VFD would that make a difference in the sizing of both?

Basically, wire size, breaker (short circuit and ground fault protection) is based off the table in 430.250, thats called FLC full load current.

Name plate amps better known as FLA is used to size the overloads.

Assuming this 60hp is a 3 phase 480 volt motor,

art. 430.22(A) 430.6 Wire size has to be at least 125% of the FLC (table amps) 77 X 1.25=96.25

art. 430.52 Table 430.52
Short circuit Ground fault protection can be a maximum of 250% times FLC. 77 X 2.50=192.5 Breaker size UP 200 amp. art 430.32 Overloads are 125% of the NAME Plate amps FLA 69 X 1.25 = 86.25 you cannot size up on overloads they must be sized down.

This is not all inclusive but it will get you started on motors. Mike Holt got some good books on them as well. Hope this helps.

 

[Smart $]

Thanks Smart $ for a prompt reply.

So for instance if there is a 60HP motor, FLA= 69Amps, LRA=470 Amps.
How to go about sizing the wire and breaker?

If there was an VFD would that make a difference in the sizing of both?

There are a good many details involved with appropriate sizing. Typically, for a single 60HP motor (HP value on namplate), 600V or less, you find the FLA in the appropriate table towards end of Article 430 {2011 NEC} (Table 430.250, for example). Conductors are sized at a minimum for 125% of that value. Then see 430.52 and its table for GF/SS maximums to determine a breaker rating. A VFD's supply conductors and OCPD must be based on its rated input current.

Keep re-reading Article 430 and, though it still confuses the experienced, it will become clearer with time and experience.

 

[DARUSA]

Basically, wire size, breaker (short circuit and ground fault protection) is based off the table in 430.250, thats called FLC full load current.

Name plate amps better known as FLA is used to size the overloads.

Assuming this 60hp is a 3 phase 480 volt motor,

art. 430.22(A) 430.6 Wire size has to be at least 125% of the FLC (table amps) 77 X 1.25=96.25

art. 430.52 Table 430.52
Short circuit Ground fault protection can be a maximum of 250% times FLC. 77 X 2.50=192.5 Breaker size UP 200 amp. art 430.32 Overloads are 125% of the NAME Plate amps FLA 69 X 1.25 = 86.25 you cannot size up on overloads they must be sized down.

This is not all inclusive but it will get you started on motors. Mike Holt got some good books on them as well. Hope this helps.

Most of the time the overload protection will be set as 1.15 % of the FLA based on the PF and temperature rated of the motor.

 

[jumper]

Here are two articles that deal with motors. Along with the info already posted they may helpful also.

The first is branch circuit sizing with OL and OCPD.

The second is for feeders and OCPD.

http://ecmweb.com/design/motor-calculations-part-1-motors-and-branch-circuit-conductors

http://ecmweb.com/content/motor-calculations-part-2-feeders

 

[samir]

Basically, wire size, breaker (short circuit and ground fault protection) is based off the table in 430.250, thats called FLC full load current.

Name plate amps better known as FLA is used to size the overloads.

Assuming this 60hp is a 3 phase 480 volt motor,

art. 430.22(A) 430.6 Wire size has to be at least 125% of the FLC (table amps) 77 X 1.25=96.25

art. 430.52 Table 430.52
Short circuit Ground fault protection can be a maximum of 250% times FLC. 77 X 2.50=192.5 Breaker size UP 200 amp. art 430.32 Overloads are 125% of the NAME Plate amps FLA 69 X 1.25 = 86.25 you cannot size up on overloads they must be sized down.

This is not all inclusive but it will get you started on motors. Mike Holt got some good books on them as well. Hope this helps.

Thank you. Really appreciate the reply.

 

[samir]

Thank you everyone.

 

[samir]

Here are two articles that deal with motors. Along with the info already posted they may helpful also.

The first is branch circuit sizing with OL and OCPD.

The second is for feeders and OCPD.

http://ecmweb.com/design/motor-calculations-part-1-motors-and-branch-circuit-conductors

http://ecmweb.com/content/motor-calculations-part-2-feeders

Thanks Jumper for the two articles. As you are the moderator, I want to know is there a forum here, where I can get some help on the beginning PE preparation.
Also, some training opportunities updates by engineers to improve my skills and broaden my expertise.

 

[jumper]

Thanks Jumper for the two articles. As you are the moderator, I want to know is there a forum here, where I can get some help on the beginning PE preparation.
Also, some training opportunities updates by engineers to improve my skills and broaden my expertise.

We have two forums that could post such questions in: Exam Prep or Electrical Calculations/Engineering

I would start a new thread in either of those if I were you.