Lights on a circuit, how many fit.

Short answer

A 15 A, 120 V circuit carries 1,800 W at most and 1,440 W continuously (80%). With 9 W LED lamps that is up to 160 lamps by the math, though the practical limit is wiring and box space. With 60 W incandescent lamps it is 24.

Worked example

A 15 A, 120 V circuit with the 80% limit gives 1,440 W. If it already carries 300 W, 1,140 W remain: 126 lamps at 9 W, or 19 lamps at 60 W. At 20 A the limit is 1,920 W.

Formula, assumptions and sources

Usable watts = volts x breaker amps x 0.8 for loads of three hours or more (x 1.0 for shorter use). Fixtures = whole number of (usable watts minus existing watts) / watts per fixture. The 80% factor comes from UL's breaker guide: a breaker not marked for 100% continuous use is for no more than 80% of its rating when the load runs three hours or more. Lighting that stays on three hours or more is a continuous load, so the default is 80%. Use the lamp's actual input watts including the driver. This ignores the circuit's other receptacle loads, LED inrush on large counts, dimmer ratings, and box fill or wire-count limits.

Table values here follow the NEC as reproduced in the sources below (editions 2008 to 2020 era); fill percentages, tables and allowances can change between editions, so confirm against the edition your jurisdiction has adopted.

• UL, Circuit Breaker Guide (80% of rating for loads of three hours or more) (retrieved 2026-10)
• HyperPhysics (Georgia State University), Electric Power: P = VI (retrieved 2026-10)

Code and safety notes

This tool follows the U.S. National Electrical Code (NFPA 70). The Canadian Electrical Code (CSA C22.1) and local amendments differ in several tables and rules, so Canadian installations must be checked against the CEC and the authority having jurisdiction.

Planning aid only, not electrical advice. Electrical work can cause fire and shock; the adopted code edition, the equipment listing and a licensed electrician or inspector decide what is allowed.

FAQ

How many recessed lights can be on a 15 amp circuit?

At 120 V and the 80% limit, a 15 A circuit carries 1,440 W: 120 lights at 12 W each. Most electricians cap the count well below the math for practical wiring reasons.

How many watts is a 20 amp lighting circuit?

20 A x 120 V = 2,400 W, and 1,920 W when loaded continuously (80%).

Why use 80% for lights?

Lighting often runs three hours or more, and a standard breaker is meant for no more than 80% of its rating under such continuous loads.

Is there a maximum number of lights per circuit?

The limit this tool checks is the load in watts. Practical limits such as wire connections and box fill, and local code rules, may be stricter, so check with your inspector.