489V to 466V Buck-Boost Transformers
489 volts is a meter reading, not a system voltage. Choose the system phase and equipment nameplate amps below to see an available, buyable unit.
Load current (amps)
Quick answer
- Measured input
- 489V
- Required output
- 466V
- Correction
- Buck (lower voltage) · 4.7%
- Size from
- System phase and equipment nameplate amps
Technical details
489V to 466V technical overview
489 volts is a meter reading, not a system voltage. Nothing is nominally 489, and the pair exists because what a buck-boost transformer applies is a ratio rather than a pair of labels. A 489V to 466V unit multiplies whatever arrives at its input by about 0.953, so a supply holding at 489 volts leaves the transformer at 466 volts, roughly 1 percent above a 460 volt nameplate instead of 6 percent above it. Selecting at this granularity is worth doing only when the supply has been measured and is known to sit near 489. Single-phase and three-phase, 10 through 60 amps.
Where 489V to 466V correction is used
A 4.7 percent trim is a small change, and whether it earns the hardware depends less on what the equipment is than on how many hours a year it spends energized.
Continuously energized loads make the strongest case. Refrigeration racks and cold storage compressors, chilled and condenser water pumps, exhaust and make-up air fans, and process air compressors on 460 volt motors run most of the year. A few percent of surplus voltage is a small penalty per hour and a large one per decade, because the heat it makes never stops being made.
Equipment that idles energized is the case people miss. Standby pumps, backup compressors, machine tools between cycles and conveyors waiting on product draw magnetizing current the whole time they are on. Magnetizing current climbs faster than voltage does, so an idling motor on a high supply runs warm while producing nothing.
Electronics that are never switched off are the third group. Drive front ends, control transformers, PLC and instrument supplies and LED drivers hold a fixed offset with no duty cycle in which to recover.
Short-duty equipment rarely justifies it. A hoist or a machine that runs an hour a week accumulates too little exposure to repay the unit and the labor to install it.
Why this 489V to 466V voltage pair matters
Treat the selection as choosing where inside the equipment's tolerance band to sit, rather than as matching two numbers.
The multiplier is fixed at roughly 0.953 and follows the input wherever it goes. At 489 volts in, the output is 466. At 495 it is 472, and at 480 it is 457. All three figures fall inside the plus or minus 10 percent NEMA MG-1 allows around a 460 volt nameplate, a band running from 414 to 506 volts, so a supply wandering across the normal 480 volt spread stays in specification at both ends after correction. That is the real test for a small trim: not whether it lands on 460 exactly, but whether both ends of the measured range come to rest somewhere comfortable.
The reason to bother is that the uncorrected reading holds at one edge of that band permanently. 489 volts is a compliant service voltage, since ANSI C84.1 Range A for a 480 volt system runs from 456 to 504 volts, and the utility is under no obligation to move it. Compliance describes the supply, not the operating point of what is connected to it. Correcting to 466 volts shifts the equipment from about 6 percent above nameplate to about 1 percent above it, which is the middle of the band instead of the top.
Installation notes
Sizing guidance
Record a range rather than a number. One reading describes the minute it was taken. Log line-to-line voltage at the intended connection point across a working week where the tools allow, or at minimum at shift start, mid-shift and after hours. Apply the ratio to the highest and lowest figures recorded and check both answers against the equipment's stated input window. A fixed-ratio device cannot narrow a supply that swings, only move the whole swing down.
Current comes from the load side, since the rating is what passes through the transformer. Work from full-load amps at the equipment nameplate voltage and take the next step up from the available 10, 15, 20, 30, 40, 50 and 60 amp ratings. Where several loads share one correction, size on the current that runs concurrently rather than on the total of every nameplate present. Where a drive is involved, use its rated input current in place of motor full-load amps.
Phase follows the equipment, worked line to line for three-phase loads. Because only the 23 volt difference passes through the windings, the unit correcting a substantial motor load here is a physically small one.
Installation notes
Confirm the result after energizing. Measure the output with normal load running and compare it against what the ratio predicted; on a trim this small that reading is the only evidence the right unit was chosen. Where a supply drifts with the season, repeat the check at the other end of the year.
Remember what the device is. Its windings are those of an insulating transformer, connected so that one winding carries both input and output. That is an autotransformer connection: the 466 volt side shares a conductive path with the 489 volt side, galvanic isolation is not present, and no separately derived system results, so grounding and bonding stay referenced to the existing supply. Three-phase corrections are normally made with two units in open delta, which shifts line-to-line voltage and produces no neutral.
Current differs on the two sides of a buck connection, the load side carrying slightly more than the line side because its voltage is lower for the same load power. Follow the NEC for the current actually present at each connection rather than applying one figure to both. Work inside 480 volt gear is qualified-person work under NFPA 70E. XFMR Direct is an independent online retailer carrying buck-boost transformers from several manufacturers.
Common questions
- Why is there a 489 to 466 volt pair when the nominal system is 480 to 460?
Because a buck-boost transformer applies a fixed ratio to whatever is present instead of regulating to a target. A 480 to 460 volt selection is a 4.2 percent trim, and applied to a supply actually sitting at 489 volts it delivers about 469 volts, leaving part of the surplus in place. The 489 to 466 pair is a 4.7 percent trim chosen against the measured figure. The gap between the two selections is small, which is the point: pairs exist at this spacing so the correction can be matched to a measurement rather than to a label.
- Does the output have to land exactly on my equipment's nameplate voltage?
No, and an exact match is the wrong target. NEMA MG-1 covers a 460 volt motor from 414 to 506 volts, and most 460 volt control and electronic equipment publishes an input window of its own. What matters is where inside that window the equipment operates and how much of the window is left over for supply variation and for voltage drop in the branch circuit feeding it. At 466 volts a 460 volt nameplate is about 1.3 percent high, near the center of the band, with room remaining on both sides.
- Will correcting from 489 to 466 volts lower my energy bill?
Not by an amount most sites can measure. The correction reduces core loss and magnetizing current, and both reductions are real, but they are small and they are largest on motors that are lightly loaded or idling rather than on machines doing full work. The mechanical work a loaded motor performs does not change, so most of the energy it draws does not change either. The reason to make this correction is thermal, meaning lower winding and component temperature and the equipment life that follows from it. Treat any change in consumption as incidental.
- Is the amp rating taken from the 489 volt side or the 466 volt side?
From the current passing through the transformer, which on a buck connection is higher on the 466 volt side. For a given load power, lower voltage means more current, so selecting from load current at 466 volts is both the correct and the conservative approach. Take full-load amps from the equipment nameplate rather than from the breaker, since motor branch circuit protection is deliberately sized above running current and will overstate what the transformer has to carry.