If you are asking 3 phase how many volts, the direct answer is 208V, 480V, or 600V in North America, and 400V in IEC regions (Europe, UK, AU), when measured Line-to-Line (L-L). If you measure Line-to-Neutral (L-N) on a standard Wye-configured system, those voltages drop to 120V, 277V, 347V, and 230V respectively. There is no single universal '3-phase voltage'; the exact number is fixed entirely by your regional grid standard, the transformer configuration (Wye vs. Delta), and where you place your multimeter probes.
The Core Assumptions: Line-to-Line vs. Line-to-Neutral
The assumption that fixes your voltage answer is the measurement topology. In a 3-phase Wye (Y) system, you have two distinct voltage potentials available simultaneously. The relationship between them is governed by the square root of 3 (approximately 1.732).
The formula to convert Line-to-Neutral (L-N) voltage to Line-to-Line (L-L) voltage is:
VL-L = VL-N × √3
Worked Example (US Industrial 480V System):
If your transformer secondary outputs 277V from any single phase to the neutral/ground bus, the voltage between any two hot phases is:
277V × 1.732 = 479.76V (Rounded to the nominal 480V).
This mathematical relationship is why a standard US commercial building can run 208V 3-phase HVAC compressors and 120V single-phase receptacles from the exact same panelboard. You are simply utilizing different tap points on the same transformer secondary.
Regional Voltage Standards and Tolerance Ranges
Grid voltages are nominal targets, not absolute constants. According to NFPA 70 (NEC) and IEC 60038 standards, utilities are permitted a variance, typically ±10% under normal conditions, though equipment must often tolerate up to ±20% during brownouts or heavy loading.
| System Type | Nominal L-L | -20% (Min) | -10% (Low) | +10% (High) | +20% (Max) |
|---|---|---|---|---|---|
| US Commercial (Wye) | 208V | 166.4V | 187.2V | 228.8V | 249.6V |
| US Industrial (Wye) | 480V | 384.0V | 432.0V | 528.0V | 576.0V |
| EU / IEC Standard (Wye) | 400V | 320.0V | 360.0V | 440.0V | 480.0V |
| US Heavy Industrial | 600V | 480.0V | 540.0V | 660.0V | 720.0V |
How the Shift Happens: 120V vs 230V vs 3-Phase
Understanding how single-phase voltages scale into 3-phase systems clears up massive confusion for DIYers and junior techs cross-wiring imported machinery.
- The 120V to 208V Shift: In North America, residential single-phase is 120V L-N. In commercial 3-phase Wye systems, that exact same 120V L-N potential yields 208V L-L. If you plug a 240V single-phase heater into a 208V 3-phase circuit, it will run, but it will only produce 75% of its rated heat output (since power scales with the square of the voltage:
(208/240)^2 = 0.75). - The 230V to 400V Shift: In IEC regions, the wall outlet is 230V L-N. When shifted to a 3-phase Wye system, the L-L voltage becomes 400V (
230V × 1.732 = 398V). This is why European 3-phase motors are rated for 400V, not 480V. - The 240V Delta Exception: Older US industrial sites often use 240V Delta. Here, L-L is 240V, but there is no standard L-N voltage for the whole system. One phase (the 'high leg' or 'wild leg') will read 208V to neutral, while the other two read 120V. You cannot derive 120V single-phase from the high leg.
Decision Path: Identifying Your 3-Phase System Voltage
Use this decision tree to determine the exact voltage you are working with and select the correct breaker or component rating.
| If Your Measurement / Context Is... | Then Your System Is... | Concrete Pick / Action |
|---|---|---|
| Measuring ~208V L-L and ~120V L-N in a US office/retail space. | 208Y/120V Wye | Use standard 120V L-N receptacles; size 3-pole breakers for 208V L-L loads. |
| Measuring ~480V L-L and ~277V L-N in a US factory/warehouse. | 480Y/277V Wye | Use 277V L-N for lighting; use 480V L-L for motors. Do NOT use 120V devices without a step-down transformer. |
| Measuring ~400V L-L and ~230V L-N in Europe/UK/AU. | 400Y/230V Wye | Standard IEC 230V single-phase and 400V 3-phase equipment applies directly. |
| Measuring 240V L-L, but L-N reads 120V, 120V, and 208V. | 240V High-Leg Delta | Identify the high leg (often orange wire per NEC). Never connect L-N loads to the high leg. |
When Voltage Conversions Become Meaningless
There are two scenarios where trying to convert or calculate 3-phase voltages will lead you to a dead end or a dangerous mistake:
- When Power Factor (PF) is Unknown for Power Calculations: If you are trying to convert 3-phase voltage and measured amperage into real power (Watts) using the formula
P = V × I × √3 × PF, the conversion is meaningless if you assume a PF of 1.0 on an inductive load. A 480V motor drawing 10A at 0.8 PF is consuming 6.65 kW, not the 8.31 kW you would calculate if you ignored the PF. Always use a clamp meter with true power measurement for inductive loads. - Assuming Delta Systems Have a Neutral: If you are working on an ungrounded Delta or a corner-grounded Delta system, asking 'what is the Line-to-Neutral voltage' is a meaningless question. There is no neutral. Attempting to wire a 120V L-N control circuit to a 480V Delta system without an isolation transformer will result in a dead short or a floating, unpredictable voltage reference that can destroy PLC logic boards.
FAQ: 3-Phase Voltage Quick Answers
Can I run a 208V motor on a 240V supply?
Generally, no. While NEMA standards allow a ±10% voltage variation, 240V is 15.3% higher than 208V. This will cause the motor to draw excessive magnetizing current, overheat, and trip its overload relay. Use a buck-boost transformer to drop the 240V down to 208V.
Why does my multimeter read 0V L-N on a 480V Delta system?
Standard 480V Delta systems are often ungrounded or corner-grounded. In an ungrounded Delta, there is no physical neutral, so a L-N reading will float or read 0V depending on capacitive coupling. You must measure L-L to verify the 480V potential.
Is 3-phase 415V the same as 400V?
Yes, practically speaking. 415V was the older UK/Australian standard. The harmonized IEC 60038 standard updated the nominal voltage to 400V, but the tolerance bands overlap entirely. Equipment rated for 415V will run perfectly fine on a modern 400V nominal supply.
