AC vs DC: Differences Explained

AC and DC are the two ways electrical current flows. DC moves in one steady direction at a constant voltage and powers batteries, electronics, solar panels, and automotive systems. AC reverses direction many times per second (60 Hz in North America, 50 Hz in most of the world) and is the standard for power distribution because transformers can step its voltage up or down efficiently. The "120V" on your outlet is the RMS value, the effective voltage that does the same work as 120V DC would; the actual peak voltage of that sine wave is about 170V. This page covers the peak / RMS conversion, why RMS is what your meter and Ohm's Law see, and when each type is used.

Formulas

V_peak = V_rms × √2 ≈ V_rms × 1.414

V_rms = V_peak ÷ √2 ≈ V_peak × 0.707

AC (alternating current) reverses direction 60 times per second (60 Hz in the US). The voltage oscillates between positive and negative peaks. The "120V" on your outlet is the RMS (root mean square) value. The effective voltage that does the same work as 120V DC. The actual peak voltage is 170V. DC (direct current) flows in one direction at a constant voltage. Used in batteries, solar panels, and electronics.

Worked Examples

Example 1: 120V AC outlet: what is the peak voltage?

V_peak = 120 × 1.414 = 169.7V

Example 2: 240V AC outlet

V_peak = 240 × 1.414 = 339.4V

Example 3: 12V car battery (DC)

Peak = RMS = 12V (DC has no oscillation)

Common Mistakes

Treating AC voltage as if it were the peak. 120V AC is the RMS value, not the peak.
Assuming DC and AC formulas are interchangeable for reactive loads.
Forgetting that most electronics internally convert AC to DC. The power supply handles the conversion.

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This is a formula reference page for educational use. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before applying any of these formulas to real installations.

Standards & References

This page cites the following electrical codes and standards. Always consult the current edition of your local adopted standard for authoritative requirements.

IEC 60038. IEC standard voltages. Defines 230/400V as the nominal low-voltage supply for 50Hz systems, which harmonized European residential supply in 1995.
International Electrotechnical Commission. Reference →
ANSI C84.1. American National Standard for Electric Power Systems and Equipment: Voltage Ratings (60 Hz). Defines the nominal system voltages (120, 208, 240, 277, 480V) and their allowable ranges at the service entrance and utilization point.
American National Standards Institute / NEMA. Reference →
NIST SP 811: SI Unit Definitions. Definitions of the volt, ampere, watt, ohm, coulomb, and joule under the International System of Units (SI). Authoritative reference for unit conversions and precision.
National Institute of Standards and Technology. Reference →

Disclaimer: The information on this page is provided for reference. Always consult a licensed electrician and the current edition of your local adopted electrical code before performing electrical work.