Three-phase generator voltage is normally quoted as a single figure: 400V line-to-line with 230V line-to-neutral on a 50 Hz system, or 480V with 277V on 60 Hz. Those numbers are correct. On their own they’re also misleading, because they’re labels rather than limits.
There is no such thing as a 400V supply. Nobody generates at 400V, and nobody ever has.
You have probably measured a voltage that did not match the plate and wondered whether something was wrong. Usually nothing is. This article sets out the band each of these voltages may legitimately move within, where those bands come from, and how to tell a normal reading from a fault. Three of the four figures in the title are European harmonisation labels; the fourth is North America’s standard.
We build three-phase sets up to 3,000 kW and specify them against these standards every week. Send us your site’s measured voltage alongside the equipment nameplate, and we will tell you whether the reading is normal. Talk to our engineering team →
Key Takeaways
- 400V is a label, not a supply. It was created to cover Europe’s older 380V and the UK’s 415V systems, by widening tolerances until the bands overlapped.
- IEC 60038 permits ±10% at 230/400V, which is 360V to 440V. A UK 415V supply may reach 439.9V.
- ANSI C84.1 sets 480V Range A service voltage between 456V and 504V; utilization goes down to 432V.
- A motor nameplated 460V on a 480V system is not an error. Nameplate voltage is a utilization voltage, set roughly 4% below system nominal.
- The test is the band, not the label. A reading that differs from nominal is usually a normal supply.
Three-Phase Generator Voltage: The Two Numbers on Every Plate
A set that can supply both voltages is marked with both, separated by a slash. The order never changes: line-to-line first, line-to-neutral second. A European plate reads 400/230V, a North American one 480/277V, a UK one 415/240V. The higher figure is the one most specifications quote, and the one that sizes switchgear and cable, which our guide to how much current a three-phase generator delivers works through.
Which Number Is the Line Voltage
Line voltage is the potential between any two line conductors; phase voltage is the potential between one line conductor and neutral. The two are related by the √3 constant, which our three-phase diesel generator guide derives in full.
For reading a plate, the rule is simple: the larger number is line-to-line, the smaller is line-to-neutral, and the smaller is the figure a single-phase tap uses. Weighing a single-phase set against a three-phase one is a separate comparison.
What the Slash Format Hides
A slash tells you the alternator is wye-connected and brings out a neutral, so both voltages are available. A plate carrying only one figure often means a delta connection, where the two coincide. A machine marked simply “415V” isn’t necessarily single-phase.
It also matters whose plate you are reading: a set’s plate names what the machine produces, a load’s plate what it requires.
Why 400V Is a Label, Not a Supply
Before European harmonisation the continent ran two supplies. Mainland Western Europe used 380V three-phase at 50 Hz; the UK used 415V.
The single figure of 400V was introduced to give both one classification. What makes it interesting is how. The two systems were declared to overlap by widening their tolerances until the bands met:
| Supply | Tolerance | Range it may legitimately reach |
|---|---|---|
| UK 415V | +6% / −10% | 373.5V to 439.9V |
| Mainland Europe 380V | +10% / −6% | 357.2V to 418V |
| The overlap, sold as “400V” | n/a | 373.5V to 418V |
The consequence is written into equipment ratings. A device rated for a 380V supply is assured only up to 418V; one rated for 415V only down to 373.5V. A device rated at 400V must tolerate everything in between.
Schneider Electric’s published guidance is blunt about the arithmetic. A 415V supply “can rise to as high as 439.9V AC and still be within tolerance,” while “the maximum assured rated voltage for a 380V AC product is only 418V AC.” So 380V and 415V are both correct and neither is wrong: two real supplies sharing one name.
Lena met the boundary case on a packaging line in the Netherlands. The drive was rated for a 380V supply; the site was quoted as 400V. It ran for two years, until a high-tolerance period pushed the supply past the 418V the drive had been assured to and it tripped twice in a week. Nothing had failed: the supply was inside its band and the drive was inside its own, but the two bands did not quite cover each other.
What the Standards Actually Permit
Two frameworks set the bands, one per continent. How voltage pairs with frequency is covered in our 50 Hz vs 60 Hz generator guide; here the question is how far a voltage may move.
IEC 60038 and the 50 Hz Band
For 50 Hz systems, IEC 60038 names 230/400V as the standard nominal voltage and permits ±10% under normal operating conditions. In three-phase terms that is 360V to 440V. Per phase it is 207V to 253V.
The 230/400V figure was introduced in 1983 with migration targeted for 2003, and 220/380V and 240/415V systems still exist; a narrower −10%/+6% tolerance applied during the transition.
Annex A does something useful. It gives the highest and lowest voltage at the supply terminals and at the utilization terminals separately, and assumes a maximum voltage drop of 4% inside the consumer’s installation. Hold on to that 4%: it reappears on the other continent. Sizing conductors around it is a separate calculation, covered in our generator set electrical connection guide.
ANSI C84.1 and the 60 Hz Bands
For 60 Hz systems, ANSI C84.1 does the same work in a different vocabulary. It separates service voltage, what the utility delivers, from utilization voltage, what the equipment sees. The gap is where the installation’s own losses live.
At 480V nominal, the ranges are:
| 480V system | Range A | Range B |
|---|---|---|
| Service voltage | 456V to 504V | 440V to 508V |
| Utilization voltage, lighting circuits | 440V to 504V | 424V to 508V |
| Utilization voltage, other loads | 432V to 504V | 416V to 508V |
Range A is normal operation. Range B excursions must be infrequent and short, and anything outside Range B needs prompt correction.
Note the two utilization rows. A lighting circuit gets a narrower band than a motor, because lamps tolerate undervoltage less well than a motor winding does. “The equipment” is not one thing.
Why Your NEMA Motor Says 460V on a 480V System
A motor on a 480V system is normally nameplated 460V, not 480V. Buyers see the mismatch and assume a mistake. It isn’t one.
Nameplate voltage on North American motors is a utilization voltage, not a system voltage. The same pattern runs across every standard class:
| System nominal | Motor nameplate |
|---|---|
| 480V | 460V |
| 600V | 575V |
| 240V | 230V |
| 208V | 200V |
| 120V | 115V |
The roughly 4% gap between the columns is the terminal voltage expected once the run to the machine is accounted for. NEMA’s ±10% tolerance then covers the difference comfortably: a 460V motor spans about 414V to 506V, and 480V sits inside that. The system voltage describes the supply; the nameplate describes what arrives.
The older ratings cause real trouble. 440V and 220V are superseded. A motor ordered today as 220/440V or 240/480V is furnished as 230/460V, and machines built before that change still carry the old figure.
Pieter was sourcing a secondhand 315 kVA set for a quarry in Spain when he found 440V on the alternator plate. That figure appears in no current standard for the region, and it took a call to the manufacturer to confirm the machine was a legacy-rated unit rather than something mislabelled. The unit was sound; what he nearly did was order a second set to match a number obsolete for years.
Here is where the two continents meet. IEC 60038’s Annex A assumes a 4% drop between the supply and the utilization terminals; ANSI C84.1 draws the same split; and the 480V-to-460V nameplate gap is that same 4%, seen from the machine end. Two standards, written decades apart, arriving at the same mechanism.
Is Your Voltage Reading Normal, or Is It a Fault?
Start by measuring in the right place. Take the reading at the machine terminals rather than at the generator, and take it under load: a set reads high with nothing connected and settles once load is applied.
Then apply the band test. Ask whether the reading sits inside the range for its nominal class, not whether it matches the number on the drawing.
- On a 400V nominal supply, IEC 60038 permits 360V to 440V. A reading of 429V is high but legal.
- On a 480V nominal service, ANSI C84.1 Range A runs from 456V to 504V. A reading of 460V is comfortably inside it.
- 415V on a 400V nominal system is a normal UK supply, not a fault, and so is a 460V motor on 480V.
What points to a fault is a reading outside Range B, or a phase-to-phase imbalance, which isn’t a voltage-class question and isn’t treated here.
Ruth measured 429V on a site whose drawings called for 400V and treated it as a problem. The team was one signature away from ordering a set to correct a supply condition that needed no correcting. The reading was inside the standard; the drawing was the fault.
Getting the Voltage Right on the Enquiry
“400V” on its own is ambiguous, and that ambiguity is where expensive mistakes begin. The unambiguous form names all four things:
400/230V, 3-phase, 50 Hz
Written that way, the enquiry cannot be read two ways. Leave out the phase-to-neutral figure and you may be quoted a set with no neutral brought out; leave out the frequency and you may be quoted a machine wound for the wrong market. Specifying the two as a pair is what our generator set specification guide covers in full.
One refinement worth knowing about: a dual-voltage alternator can be reconnected on site to deliver either figure.
Send us the four figures and the load list, and we will confirm the configuration before anything is built.
Frequently Asked Questions
What voltage is a three-phase generator?
A three-phase generator is quoted by its line-to-line voltage, most commonly 400V on a 50 Hz system or 480V on a 60 Hz system. The matching line-to-neutral voltages are 230V and 277V. These are nominal, not fixed: IEC 60038 permits 230/400V systems to vary by ±10%, which is 360V to 440V.
What does 400V 50Hz mean on a generator?
It names the two things the set is built to produce: 400V line-to-line at the alternator terminals, and 50 Hz output frequency. The voltage is nominal, so a real supply may sit anywhere in the 360V to 440V band IEC 60038 permits.
Why does my 400V generator read 415V?
Because a UK 415V supply is classified as 400V, and it may rise to 439.9V and still be within tolerance. A reading there is normal.
What does 400V/230V mean on a nameplate?
It means the alternator is wye connected and supplies both voltages. The larger figure, 400V, is the potential between any two line conductors; the smaller, 230V, is the potential between one line conductor and neutral.
Can a 480V generator run 400V equipment?
Not directly. 480V sits outside the ±10% band IEC 60038 permits around 400V, so a 400V supply rating should not be connected to it. A reconnectable alternator or a transformer is the usual route.
Is 440V the same as 415V?
No. 440V is a superseded nameplate rating, replaced by 460V on motors, while 415V remains a live UK supply classification.
Conclusion
Three of the figures in this article’s title are European labels; the fourth is North America’s standard. 400V covers Europe’s 380V and the UK’s 415V by overlap. 480V is real, and the 460V motor built against it is not an error. Both standards assume a few per cent of the supply is lost before it reaches the machine.
Checking three phase generator voltage needs neither standard on the desk. Measure at the machine, measure under load, and ask whether the reading sits inside the band for its nominal class. Inside the band is normal. Chasing a reading toward the label can push a healthy installation toward a genuine fault.
Our diesel generator range is configured to the supply standard of the destination market rather than to a nominal figure alone. If a site reading is worrying you, send it to us with the nameplate of the load. Talk to our engineering team →
