Re: breaker size
buzzlectric,
This may be a LONG post but this is how I look at it and maybe it will help put the pieces to the puzzle together for you also.
You have to define what type of protection you intend to provide and understand what the NEC is referring to. 670 talks about overcurrent protection and not simply short circuit protection. It also talks about a machine and not a single motor so the entire machine is to be considered. You are supplying the load of a machine and must provide conductors to service the load and an overcurrent protective device (OCPD) to protect the conductor. In addition, if a OCPD is specifies as referred to in 670.3(B) then is must be supplied as the manufacturer specifies. That OCPD may or may not be the same as the OCPD protecting the cable feeding the machine.
It is of my opinion that art 240 is a place to start for branch circuit protection and art 430-52 is the mother load for motor circuit protection which one can learn a lot from. Between the two articles I have found that where OCP is referred in other placed in the NEC it amkes more sense.
I find NEC art 430-52 to be very interesting. The way I see it when you really break it down what are you trying to do when a breaker is applied, either thermal magnetic(TM) or magnetic (MO) only (instantaneous).
Remember that TM breakers are UL listed and are stand-alone devices where MO, motor circuit protectors (MCP) are not. They have a reverse UR and can only be used as part of a listed assembly such as a part of a combination starter.
So, in the application you had referred to will the MO (instantaneous) MCP breaker be an approved application? Probably not because the MCP in reverse UR. The machine manufacturer is looking for an OCPD at the machine terminals as you have implied be your reference to art 670.
Now for that 250% you brought up. I may assume that the 250% may partially be in reference to a TM breaker in the first part of 430-52 where if also talks about fuses. If you read 430-52 farther it then talks about MO (MCP)breakers. Note the allowable mag. adjustment allowed there. It certainly isn't 250% but 800 or 1100 to 1700% maximum.
This isn't just a bunch of smoke and mirrors but it does have a purpose.
One has to decide what type of protection is to be provided by the breaker. Are you providing overload protection and short circuit protection? Remember TM breakers are intended to protect cable unless the device that they are feeding states that their application is to be used otherwise such a A/C that requires a given size HACR rated breaker or a machine manufacture may specify.
With a motor starter you have and overload (OL) relay which heaters that are specifically selected for application with the motor being controlled. Thus, close motor overload protection that a TM breaker can not provide. The cable is also protected by OL if the cable has been sized properly. But the OLR does not provide short circuit protection.
So, what would a TM breaker bring to the party if OL protection had already provided? Nothing except that it has the magnetic element that the OLR does not have. An OLR will not pick up faulted motor windings or a cable fault, as there would be spikes of current that the thermal element would never see. That?s the job of a properly size and set MCP.
If you look at the motor ratings it will have FLA and LRA. The LRA probably will be in the neighborhood or 7x the FLA as a rule of thumb. So a motor with 100FLA may have an LRA of 700a.
Now take a look at the magnetic settings as allowed by the NEC in Art 430-52, 800-1100% with a maximum of 1700% (it used to be 1300% max.) of the FLA of the motor. That 800% for a 100FLA motor is 800a, which is greater than an anticipated 700LRA. But that doesn't take into account that the LRA doesn?t represent the actual inrush current peak when the motor is first energized which can be much greater than the LRA especially with energy efficient motors. That MO breaker is looking at the motor circuit from the starter to the motor for short circuits but mainly it is looking at the motor for winding failure which, again, the OLR will not respond to.
You will also the NEC reference to ground fault protection as it relates to motors. The magnetic or instantaneous element of a breaker will provide this if sized or adjusted properly. Personally I advise that the MCP be set as low as possible but not nuisance trip when the motor starts. Should there be a current higher the MCP would look at it as a fault and trip.
Finally goi9ng back to that 250% referred to with TM breakers in Art 430-52. Again, the fixed mag. calibration of an industrial breaker is 10x the breaker rating with 5-10x commonly for adjustable. Take that motor at 100FLA and you would be allowed to apply a TM breaker rated 250% or 250a. Remember that OL protection is already provided with the OLR so the thermal element of the breaker is basically useless but the mag. element will be 2500a which is over twice that of the 800-1100% (1700% max} as allowed for n MCP. Go figure!!
The only saving grace that I have found with using a TM breaker is on a rare occasion that you have an energy efficient motor where the inrush may be greater than 1700% which has happened. At least 250% rated TM breaker is my backup plan.
Dave
