MCP sizing - MCC 1

[cherry2000]

Recently our MCC vendor has come back to us saying that they need to know if the motors in our plant are High efficiency. It seems that High efficienly motors have higher starting currents, which leads to a different size of MCP as compared to that for a normal eff. motor.
If this is true, does this mean that we have to indicate in our MCC list which motors are high efficiency? This will also mean that till the time we get all the data sheets ( though our specs calls for High efficiency motor, some mech. packages have given standard efficiency motor ) we will not be able to finalise the MCC schedule. Is there any way out of this?

 

[jraef]

The issue is valid, albeit not universal. By that I mean that there are a number of different ways in which motor manufacturers achieve their efficiency ratings. ONE of those ways is to reduce the winding resistance, so as to reduce the copper losses. The side effect is that the instantaneous inrush current, that 1-2 cycles which initially creates the magnetic field in the motor, can be significantly higher than what you may be used to and this is the current that is seen by the Mag trips on a circuit breaker. So, normally on an MCP circuit breaker the Mag trip range is between 700 and 1300% of rated amps. On some EE motors, the instantaneous inrush can be as high as 2000%, which means you may not be able to turn those breakers up high enough to avoid nuisance tripping. Because of this, some electronic trip breakers are now coming with higher trip adjustment ranges, but it is difficult to attain a wide range on a standard Mag trip. So if your MCC supplier is providing standard Mag trip MCPs, they will need to know in advance if the motors are going to need the higher ranges. the reason why it isn't universal is that some motors sold as "energy efficient" do not use this tactic so they don't have the nuisance tripping issue.

By the way, most of this info is alluded to in article 430.52 of the NEC by reading the exceptions to the Magnetic Trip Settings for paragraph 3).

Exception No. 1:
Where the setting specified in Table 430.52 is not sufficient for the starting current of the motor, the setting of an instantaneous trip circuit breaker shall be permitted to be increased but shall in no case exceed 1300 percent of the motor full-load current for other than Design B energy-efficient motors and no more than 1700 percent of full-load motor current for Design B energy-efficient motors. Trip settings above 800 percent for other than Design B energy-efficient motors and above 1100 percent for Design B energy-efficient motors shall be permitted where the need has been demonstrated by engineering evaluation. In such cases, it shall not be necessary to first apply an instantaneous-trip circuit breaker at 800 percent or 1100 percent.

So if you notice that last sentence, it says "...the need has been demonstrated..." which is what your MCC supplier is asking you for. Otherwise, without the engineering information up front, someone will have to install the normal breakers and demonstrate to an inspector that they won't work, then change them. This is an expensive approach. The 1700% is a compromise and even though some are as high as 2000%, the duration is very short so (hopefully) the 1700% will work. I have not heard of it not working anywhere yet.

 

[dpc]

There is also a trade-off between efficiency and power factor in motor design. Higher efficiency motors can have a lower power factor with resulting higher current draw for steady-state current as well as the inrush issue.

If it were me, I'd tell the MCC vendor that all motors will be high-efficiency. I doubt that you will have any problems if "standard" motors are supplied.

I would also specify high-efficiency for all motors unless there is some compelling reason to not do so.