Motor/HVAC equipment voltage requirements

Status
Not open for further replies.
J

John Paul

Member
Location
Norfolk, VA
Questions:

Why do mechanical manufacturers specify 460V when common distribution is 480V?

If equipment is rated at 460V what is the standard tolerance to voltage fluctuation, %10 over and under seems normal? Does the NEC have anything to say about % regulation?
 
The IEEE standard (Gray Book) deals with this. 480V is the nominal system voltage, 460 is the nominal utilization voltage. Tolerance would be based on 460V, manufacturers design the equipment so optimal performance is seen in the middle of the tolerance range. With a higher nominal system voltage, in case of voltage drops, you still are in the range to power the motor efficiently.
 
"With a higher nominal system voltage, in case of voltage drops, you still are in the range to power the motor efficiently."

* Makes sense. If you had say 2% voltage drop on a 477V supply (always seems like the switchgear voltmeter reads 477), then the equipment would see 468V.

After looking in the NEC 2002 (2008 is on order) I did see that sec 110.4 VOLTAGES states that "The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected."

Therefore, if we're hooking up HVAC equipment to 480V power shouldn't it be rated at 480V instead of 460V as stated on the nameplate? I know I'm splitting hairs but it's fun and I'd like to give an intelligent informed answer to a Mechanical Engineer.
 
check the tables for motors in article 430 and you will find the ampacities for motor conductors listed according to the voltages you seek (nema)
 
John Paul said:
"With a higher nominal system voltage, in case of voltage drops, you still are in the range to power the motor efficiently."

* Makes sense. If you had say 2% voltage drop on a 477V supply (always seems like the switchgear voltmeter reads 477), then the equipment would see 468V.

After looking in the NEC 2002 (2008 is on order) I did see that sec 110.4 VOLTAGES states that "The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected."

Therefore, if we're hooking up HVAC equipment to 480V power shouldn't it be rated at 480V instead of 460V as stated on the nameplate? I know I'm splitting hairs but it's fun and I'd like to give an intelligent informed answer to a Mechanical Engineer.
Click to expand...

I hope this excerpt from the IEEE Gray Book will help
"The difference between the nameplate rating of utilization equipment and the system nominal voltage sometimes
causes confusion. A recurring request is to make two voltage ratings identical. However, the difference in voltage
ratings is necessary because the performance guarantee for utilization equipment is based on the nameplate rating and
not on the system nominal voltage. For utilization equipment, such as motors where the performance peaks in the
middle of the tolerance range of the equipment, better performance can be obtained over the tolerance range speci?ed
in ANSI C84.1-1989 [2] by selecting a nameplate rating closer to the middle of this tolerance range."

Based on what gathered from the history of power distribution, before the 1960's motors were rated for 440V on a nominal 480V system, the intent being that most motors were used where you had long circuits so to compensate for the voltage drop (and limited utility capacity at that time) the utilization voltage rating was less than the nominal so that the equipment would still function respectively in spite of any significant voltage drop.
Since then, the utility distribution system has reduced the voltage drop in the distribution system, NEMA recommended improvements in motor design, and surveys showed by the mid 60s the average voltage supplied to motors on a 480 nominal system was 460V.
 
More info:

NEMA recommends that all electrical appliances and motors operate at nameplate +/- 10%.

Nominal voltage per ANSI C84 descibes voltages we're used to, 120,208, 240, 277, 480. And sets utility servies variation to be +/- 5%
 
Status
Not open for further replies.
Top