Short answer: In the United States, AC power alternates at 60 cycles per second. The 120 V you measure is an RMS value; the waveform actually peaks at about 1.414 × 120 ≈ 170 V. A house service is usually 120/240 V split-phase: two ungrounded conductors 240 V apart, each 120 V to the neutral. On a 3-wire circuit with resistive loads, the neutral carries only the difference between the two line-to-neutral currents, and a load connected line to line at 240 V adds no neutral current. Real power in watts equals volts × amps × power factor.
PSI lists theory in two subject areas of the Texas Journeyman exam: Definitions, Theory and Plans has 3 scored NEC Knowledge questions, and Calculations and Theory has 2 scored Calculations questions. PSI does not say how many cover AC basics alone. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. The examples here use ideal values to teach the ideas. Real loads, voltages and power factors vary, so use nameplate data and measured values on the job, and follow your supervisor.
The words you need
| Term | Plain meaning |
|---|---|
| Frequency | How many complete cycles happen each second. US utility power is 60 Hz, and the current reverses direction twice in every cycle. |
| RMS voltage | The AC value that heats a resistor the same as that much DC. AC voltage ratings and standard AC meter readings use RMS values. |
| Peak voltage | The top of the sine wave. For a sine wave, peak = RMS × 1.414. |
| Split-phase | A 120/240 V system from a center-tapped transformer winding, used in most houses. |
| Volt-amperes (VA) | Apparent power: RMS volts × RMS amps, before power factor. Conductors are sized by the current, not by VA alone. |
| Watts (W) | Real power that does work or makes heat. Watts = VA × power factor. |
| Power factor (PF) | Watts ÷ VA. 1.0 for resistive loads like heaters, less than 1.0 for motors and many electronic loads. |
For a deeper look at the voltage, current and resistance relationship, see our Ohm’s law guide.
RMS and peak
A 120 V receptacle does not sit at 120 V. The voltage swings from about +170 V to about −170 V and back, 60 times a second. The 120 V figure is the RMS value. Insulation, surge ratings and some electronic components have to handle the peak, which is why the difference matters.
How 120/240 V split-phase works
The utility transformer’s secondary winding has a center tap that is grounded and becomes the neutral. The two ends of the winding are the two ungrounded conductors, often called L1 and L2.
- L1 to neutral: 120 V.
- L2 to neutral: 120 V.
- L1 to L2: 240 V, because the two halves are in series and 180 degrees apart relative to the neutral.
120 V loads connect from one line to the neutral. 240 V loads, such as a water heater or a dryer heating element, connect from L1 to L2.
Why the neutral carries only the difference
On a 3-wire 120/240 V circuit with a shared neutral, the line-to-neutral currents from the two lines flow in opposite directions in the neutral and partly cancel. With resistive line-to-neutral loads on opposite lines, the neutral carries only the difference between the two currents. Motors, electronic loads and other loads with a different power factor or a distorted current need a more detailed calculation. That is why a multiwire branch circuit must have its two ungrounded conductors on opposite lines; on the same line, the neutral currents would add instead of cancel. A 240 V line-to-line load does not use the neutral at all.
Worked example 1: neutral current
A 3-wire 120/240 V circuit has 15 A on L1 to neutral and 9 A on L2 to neutral, both resistive. A 4,500 W, 240 V water heater is also connected L1 to L2.
- Neutral current from the 120 V loads: 15 − 9 = 6 A.
- Water heater current: 4,500 ÷ 240 = 18.75 A. It flows L1 to L2 and adds 0 A to the neutral.
- Line currents: L1 = 15 + 18.75 = 33.75 A. L2 = 9 + 18.75 = 27.75 A.
- Check: if both 120 V loads were 15 A, the neutral would carry 15 − 15 = 0 A.
Watts, VA and power factor
A resistive heater has a power factor of 1.0, so watts equal VA. A motor or other inductive load has a lower power factor, so it draws more current than its watts alone suggest. For example, a 120 V load drawing 8 A at a power factor of 0.8 uses 120 × 8 = 960 VA, but only 960 × 0.8 = 768 W of real power. The wiring still carries the full 8 A. Our power factor guide explains why.
Worked example 2: a 240 V heater on 208 V
A 4,500 W water heater element is rated at 240 V. It is installed on a 208 V supply, which you find in buildings with a three-phase 208Y/120 V system. How much power does it make?
- A resistive element keeps the same resistance. Power changes with the square of the voltage.
- Ratio: (208 ÷ 240)² = 0.8667² = 0.7511.
- Power at 208 V: 4,500 × 0.7511 = about 3,380 W.
- Check: the current also drops. At 240 V it is 18.75 A. At 208 V it is 3,380 ÷ 208 = about 16.25 A.
The element makes about 25% less heat than at its 240 V rating. Whether the water heater is suitable for 208 V depends on its rating and the manufacturer’s instructions. In an ideal calculation that ignores heat losses, heating the same water takes about a third longer. This is a useful example of how voltage changes the output of a resistive load.
Check yourself
- Peak voltage of a 120 V RMS sine wave? About 170 V.
- 3-wire circuit, 12 A on L1 and 12 A on L2 to neutral. Neutral current? 0 A.
- Does a 240 V line-to-line load add current to the neutral? No.
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Practice theory and calculation questions on paper
Texas Journeyman Electrician Exam Prep 2026 NEC (Tsai Electro, paperback): 327 pages and 486 practice questions, 316 across 20 chapters plus 2 full two-part mock exams of 85 questions each (59 questions in 130 minutes, then 26 in 110 minutes). It includes calculation lessons and code lookup paths. Use it alongside your own code book.
FAQ
Why is house voltage 120/240 V?
Most homes are fed by a transformer with a center-tapped secondary. The center tap is the neutral. Each end of the winding is 120 V to the neutral, and the two ends are 240 V apart.
What is the difference between RMS and peak voltage?
RMS is the effective value that meters and nameplates show. For a sine wave, the peak is about 1.414 times the RMS value, so 120 V RMS peaks at about 170 V.
How much current is on the neutral?
On a 3-wire 120/240 V circuit with resistive line-to-neutral loads, the neutral carries the difference between the two line-to-neutral currents. Other load types need a more detailed calculation. Loads connected line to line at 240 V add nothing to the neutral.
Is AC theory on the Texas Journeyman exam?
PSI lists theory in the Definitions, Theory and Plans subject area (3 scored NEC Knowledge questions) and the Calculations and Theory subject area (2 scored Calculations questions). PSI does not say how many cover AC basics alone.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
