min. size motor circuit protector required

[winslowfam]

Hi...I would like some clarification on motor circuit protector sizing. The code often refers to Art. 240.6 for standard breaker sizes, to be used for sizing circuit overcurrent protection. The smallest standard size breaker on that list is 15A, so I've always thought that for motor short circuit protection, the smallest breaker rating I needed to go to was 15A, even for small motors (like a 1/2 HP at 480V with 1.1FLA). In my recent reading of Art. 430 for Motor Circuit Protectors, there is no reference to Art. 240.6 for the MCP rating...just directions on how to calculate the MCP rating based on motor FLA. There are even some handy motor protection charts from vendors (Cutler Hammer, etc.) that specify a 15A breaker to protect a 1/2 HP motor, but their chart call for a inverse time breaker...not an MCP. Do you have any comments on this, on whether or not 15A is the smallest rating I would need to go to with an MCP to be code-complaint, or not? thanks.

 

[jumper]

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

When the short-circuit and ground-fault protection device value that you find in Table 430.52 doesn't correspond to the standard rating or setting of overcurrent protection devices as listed in 240.6(A), use the next higher protection device size [430.52(C)(1) Ex. 1].

NEC 2011:

430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and
ground-fault protective device shall comply with 430.52(B)
and either 430.52(C) or (D), as applicable.
(B) All Motors. The motor branch-circuit short-circuit and
ground-fault protective device shall be capable of carrying
the starting current of the motor.
(C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device
that has a rating or setting not exceeding the value calculated
according to the values given in Table 430.52 shall be used.

Exception No. 1: Where the values for branch-circuit shortcircuit
and ground-fault protective devices determined by
Table 430.52 do not correspond to the standard sizes or ratings
of fuses, nonadjustable circuit breakers, thermal protective
devices, or possible settings of adjustable circuit breakers,
a higher size, rating, or possible setting that does not exceed
the next higher standard ampere rating shall be permitted.

 

[winslowfam]

great...thanks for your help.

 

[iceworm]

Hi...I would like some clarification on motor circuit protector sizing. The code often refers to Art. 240.6 for standard breaker sizes, to be used for sizing circuit overcurrent protection. The smallest standard size breaker on that list is 15A, so I've always thought that for motor short circuit protection, the smallest breaker rating I needed to go to was 15A, even for small motors (like a 1/2 HP at 480V with 1.1FLA). In my recent reading of Art. 430 for Motor Circuit Protectors, there is no reference to Art. 240.6 for the MCP rating...just directions on how to calculate the MCP rating based on motor FLA. There are even some handy motor protection charts from vendors (Cutler Hammer, etc.) that specify a 15A breaker to protect a 1/2 HP motor, but their chart call for a inverse time breaker...not an MCP. Do you have any comments on this, on whether or not 15A is the smallest rating I would need to go to with an MCP to be code-complaint, or not? thanks.

MCPs are essentially a Mag-only CB. They have no thermal element. MCPs are always part of a listed combination controller. So if you are using an MCP forget 240.6. It does not apply.

The number on the MCP handle has very little to do with continuous current capability of the CB. It is an indicator of the range of the instantaneous trip current. For example an SQD GLJ 3A has an adjustable trip range of 9A - 33A. Yes, one could run 32A continuous through the CB marked 3A and it won't trip - ever.

Wild guess on my part (not connected with your question):
All the small ones are built on the 100A frame. I suspect (this means I don't know), that all have the same contacts and I'm guessing they all have 100A contacts, and will pass 100A just fine.

Back to your question:
Generally, one sets the instantaneous trip to 800% to 1300% FLA - as detailed in 430.52.C.3. And for small motors, such as 1/2 hp, you can't get there with the normal factory offerings - hence the exception for 15A ratings in 430.52..C.3.

All the mfg have application guides for thier listed combination controllers. Definitely recommend reading the ones for the brand you like.

ice

 

[templdl]

Remember that the MCP is a reverse UR component listed device with no "stand alone ratings." It must be used wthe a combination motor starter. MCPs start at 3a continuous, then 7a, 15, 30, 50, 70, 100, and (2) at 150a with different mag trip ranges. The max mag settings are 30 for a 3a, 70a for a 7a, 150a for a 15a etc. This is based upon the CH MCP breaker.
It is EXTREMELY IMPORTANT TO NOTE that you MUST never exceed the continuous rating of MCPs of sizes up to 150a this is because of the magnetic structure. The current actiuaslly flows them a coil which forms a solenoid which pulls a bar againt the core when enough current flows. It is an instantaneous current that it responds to. The gap between the coil and the bar is adjust by changing the setting on the breaker. Again, 100% of the current goons through that coil. The solenoid of the 3a breaker consists of many turns of very find wire as compared to the 150a which consists of a very heavy conductor of few turns.
Bottom line? If you exceed the current rating of an MCP it will almost with all certainty be destroided as you will fry the coil. A customer of mine had equipment at Cape Kennedy where Cape Kennedy decided that they could test the calibration of these breakers and promptly smoked them when they did. By simply holding the MCP to your nose you could smell the smoke.
An MCP relies on being part of a listed combination motor starter and the integral OLR of overload protection. When properly set the MCP responds to ground fault/instantaneous currents as a result of a failing or failed motor reducing or eliminating the cause of motor fires while protecting the motor circuit.