120 V, 240 V and 208 V: Understanding the Difference

Two supply arrangements that can both provide 120 volts.

A 120/240 V split-phase system uses a center-tapped single-phase source. The nominal voltage from either line to neutral is 120 V, while the nominal voltage between the two lines is 240 V.

A 208Y/120 V system is a three-phase wye arrangement. Each line to neutral is nominally 120 V. The phase relationships make the nominal line-to-line voltage about √3 × 120, or 208 V, rather than 240 V.

These are descriptions of system arrangements. They do not establish which supply exists at a particular property. Other configurations also exist, and a commercial address alone does not prove that equipment receives three-phase power.

Further reading: FAQ FA112573: Meter-main use on 208Y/120 V systems · Short-Circuit Current Calculations and electrical formulas

The phase relationship explains the difference.

In the split-phase description, the two line-to-neutral voltages are opposite in polarity at a given instant. Their difference gives the 240 V line-to-line value. This is a single-phase, three-wire supply arrangement.

In a balanced three-phase wye model, the phase voltages are separated by 120 electrical degrees. The line-to-line voltage is the difference between two phase-voltage waveforms. You cannot get that difference by simply adding their RMS magnitudes.

The word “phase” and the number of wires answer different questions. Two phase conductors selected from a wye supply can feed a single-phase load; that load is not automatically three-phase equipment.

Further reading: FAQ FA112573: Meter-main use on 208Y/120 V systems · Short-Circuit Current Calculations and electrical formulas

Why a single-phase load may receive 208 volts.

Equipment supplied between two lines of a 208Y/120 V system can receive 208 V single-phase power. If the application also uses neutral, the arrangement can provide 120/208 V single-phase, three-wire service to suitable equipment.

Schneider Electric describes such applications for specified meter-main equipment. The example shows why the actual supply and exact product rating matter. It does not establish compatibility for every panel, appliance or connector.

Consequently, 208 V is not automatically a symptom of a failed 240 V supply. Unexpected voltage at an existing installation still needs interpretation in the context of its intended system and equipment.

Further reading: FAQ FA112573: Meter-main use on 208Y/120 V systems

Match the equipment’s complete rating.

Some equipment is rated for both 208 V and 240 V, with different performance at each voltage. Other equipment has a narrower rating. Look for the actual voltage range, phase, frequency and manufacturer instructions.

For one fictional fixed-resistance heating element, power at 208 V would be (208 ÷ 240)², or about 75.1%, of its power at 240 V if resistance remained unchanged. This example illustrates a relationship; it does not predict the behavior or approve the use of a particular appliance.

Motors, regulated electronics and controlled heating systems do not all follow that simple fixed-resistance comparison. Do not extend it to equipment whose operating behavior has not been established.

Further reading: 9.5 Electrical Energy and Power

A different plug is not a supply conversion.

A physical adapter does not by itself change voltage, phase or frequency. Equipment suitability also involves the installation, protection and manufacturer requirements.

For a project, gather the equipment model and any nameplate or installation information already available. Your Mister Safety professional can help organize the scope around the actual property supply and the equipment being added. There is no need to open energized equipment to gather those initial details.

Questions you may be asking.