Proper conductor sizing for motor MCP+OL circuit?

[tumbleweed1]

Canadian application here, but should not be different than NEC. I'm in the middle of a tussle between two very experienced guys here about conductor sizing on a motor branch circuit fed from a MCP and OL.

Example:
On a 50HP/600V motor- FLA=52A as per code table.
Simple Calcs:
52A*1.25=65A
52A*2.5=130

Overload= Max of 1.25*FLA
Conductor size= Min of 1.25*FLA
MCP size= Max of 2.5*FLA

Eng 1: Conductor needs to be sized for overload. Conductor is thermally protected by overload set at same 125% rating; Does not need to be sized for ampacity of MCP since MCP is only magnetic trip
OL= 65A
Conductor= 65A
MCP= 100A

Cable is sized for 65A minimum. This 125% is much less than 100A. Do not upsize to match frame of MCP. For trip settings, this keeps us in the middle of the available range on the MCP trip settings (1300% max trip dial)


Eng 2: Conductor needs to be sized for MCP frame size. MCP is 100A, conductor must be sized at least 100A even if overload is set at 65A. Wants to save money on cables so chooses MCP=70A and conductor=70A.
OL= 65A
Conductor= 100A
MCP= 100A

My thoughts:
Agree with Eng 1. Have looked at MCP trip curve and even with a MCP with 70AF, this MCP will operate indefinitely at 200A. Therefore, overload protects cables at lower currents, and MCP steps in at higher currents. Therefore, protect conductor with overload, and MCP is besides the point.

Your thoughts???

Closest thing I found for a past thread was this:

http://www.eng-tips.com/viewthread.cfm?qid=193220

 

[jraef]

Here, ENG 1 gets the prize. The motor cable is sized at 125% of the FLA table, nothing else.

The OL relay protects for running current overload but not short circuits, the MCP only protects from short circuit current.

Separate issue based on how you worded this: don't make the common mistake of immediately setting/selecting the OL at 125% of the motor FLA. MANY of the OL relays on the market ALREADY factor that into the trip point and if you read the instructions you are to set/select based on the FLA period. If you then set/select at 125% of FLA, you are now at 125% of 125% and will lose the motor.




"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

A little point that both engineers may have missed.
Under the Canadian code, an overload setting of 125% is only allowed for 1.15 or greater service factor motors.
Motors with a service factor of 1.0 or with no service factor shown, must have overloads set at 115%.

"28-306 Rating or trip selection of overload devices (see Appendix B)
(1) Overload devices responsive to motor current, if of the fixed type, shall be selected or rated or, if of the
adjustable type, shall be set to trip at not more than the following:
(a) 125% of the full load current rating of a motor having a marked service factor of 1.15 or greater; or
(b) 115% of the full load current rating of a motor that does not have a marked service factor or where
the marked service factor is less than 1.15."

Take a look at Appendix B, notes on rules, for section 28. This may give you the authority needed to resolve the discussion.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I agree with jraef, at least per NEC. For a motor feeder the conductor is sized to AT LEAST 125% of motor FLA as given in the NEC (not actual FLA from nameplate). Nothing else matters. Not the size of the MCP or the rating of the OL.

 

[tumbleweed1]

Duly noted Waross- I left that point out just to compare apples here.

And yes jraef, I understand you set the overload to the amperage of the motor, while it incorporates the 125%.

I suppose where Eng 2 is trying to make a point is perhaps with rule 14-104 and Table 13. 1c is not explicitly overruled in Section 28, other than 28-106(1) saying motor branch circuit conductors must be at minimum 125% FLA.

I'm really just trying to find the part of the code that contradicts Eng 2, however it may be a bit of reading between the lines.

"
14-104 Rating of overcurrent devices (see Appendix B)
(1) The rating or setting of overcurrent devices shall not exceed the allowable ampacity of the conductors that
they protect, except
(a) where a fuse or circuit breaker having a rating or setting of the same value as the ampacity of the
conductor is not available, the ratings or settings given in Table 13 shall be permitted to be used
within the maximum value of 600 A;
(b) in the case of equipment wire, flexible cord in sizes Nos. 16, 18, and 20 AWG copper, and tinsel cord,
which are considered protected by 15 A overcurrent devices; or
(c) as provided for by other Rules of this Code.
"

 

[waross]

except
...
(c) as provided for by other Rules of this Code.

Motor circuit and feeder conductors are covered under Section 28-Motors and Generators.
See
Rule 28-200 Branch circuit overcurrent protection (see Appendix B)
Each ungrounded conductor of a motor branch circuit shall be protected by an overcurrent device complying
with the following:
(a) a branch circuit supplying a single motor shall be protected, except as permitted by Item (c), by using an
overcurrent device of rating not to exceed the values in Table 29 using the rated full load current of the
motor;
However, Table 29 does not list MCPs.
Notes to Table 29:
(2) For the use of instantaneous-trip (magnetic only) circuit breakers in motor branch circuits, see
Rule 28-210.
28-210 Instantaneous-trip circuit breakers (see Appendix B)
When used for branch circuit protection, instantaneous-trip circuit breakers shall be
(a) part of a combination motor starter or controller that also provides overload protection; and either
(b) rated or adjusted, for an ac motor, to trip at not more than 1300% of the motor full load current or at not
more than 215% of the motor locked rotor current, where given, except that ratings or settings for trip
currents need not be less than 15 A;

Appendix B
See pages 465, 466, 467, 468 and 469.
There are some good examples.
Note also:
Under the NEC you use the table for the motor amperage. (I think)
Under the CEC you use the actual motor amperage. Unless it is not available, then revert to the table.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[noside]

voltage drop calculation should also be considered in sizing of conductor
, at least not more than 15% upon starting