How many watts can a 15 amp breaker handle?

1,800 volt-amperes at 120 volts. If the load is continuous, meaning it runs three hours or more, NEC 210.20(A) limits you to 1,440 volt-amperes.

Why do LED fixtures trip a breaker even when the wattage is low?

LED drivers draw a large capacitor charging surge at switch-on. Enough drivers energized at once can exceed the instantaneous trip point of the breaker even though the running current is only a few amperes.

Does the manufacturer fixture count override the wattage math?

Yes. NEC 110.3(B) requires listed equipment to be installed per its listing instructions, so a published maximum number of luminaires per circuit is enforceable.

How many 10 watt light fixtures can you put on a 15 amp breaker?

On paper, 144. In the field, the number that matters is usually much lower, and it is set by the LED driver inrush current rather than by wattage. Here is the math and then the part that actually trips breakers.

The wattage math

A 15 ampere circuit at 120 volts carries 1,800 volt-amperes. Section 210.20(A) requires the overcurrent device to be rated not less than 100 percent of the noncontinuous load plus 125 percent of the continuous load. Lighting that operates three hours or more is a continuous load, which is normal for a store, an office, a shop, or an exterior building light. That drops your usable capacity to 1,800 divided by 1.25, or 1,440 volt-amperes.

1,440 divided by 10 watts is 144 fixtures. If the lighting is genuinely noncontinuous, such as a closet or a residential hallway, you get the full 1,800 volt-amperes and 180 fixtures. Note that you must use the input volt-amperes from the fixture label, not the LED chip wattage. A luminaire marked "10 W" frequently draws 11 or 12 volt-amperes at the input because of driver losses and power factor, so size from the nameplate. Section 220.14(D) tells you to compute a luminaire outlet at the maximum volt-ampere rating of the equipment and lamps for which the luminaire is rated.

What really sets the limit: inrush

Electronic LED drivers charge an input capacitor at the instant of switch-on. That surge can be 20 to 60 amperes per driver for a few hundred microseconds. Twenty small drivers energized by one relay or one switch can produce a peak that trips a standard thermal-magnetic breaker even though the steady-state draw is under two amperes.

Every reputable fixture manufacturer publishes a maximum number of units per 15 or 20 ampere circuit for exactly this reason. Section 110.3(B) makes that instruction enforceable, since listed equipment must be installed in accordance with the instructions included in the listing. Many drivers land somewhere between 30 and 60 units on a 15 ampere circuit. If the spec sheet gives you a number, that number wins over the wattage calculation.

Dwelling units are calculated differently

In a house you do not count fixtures at all for the load calculation. Section 220.12 assigns 3 volt-amperes per square foot for general lighting and receptacle outlets, and 210.11(A) tells you how many circuits that requires. Individual fixtures are then distributed across those circuits. The wattage cap still applies to any one circuit, but you are sizing the service and feeders from area, not from a fixture count.

Field takeaway

Use 1,440 volt-amperes as the working ceiling for a continuous 15 ampere lighting circuit and 1,800 volt-amperes when it is not continuous. Divide by the nameplate input volt-amperes, not the marketed wattage. Then check the driver spec sheet for a maximum fixture count and use the smaller of the two. If the run is long, verify voltage drop as well, because the informational note to 210.19(A) recommends holding branch circuits to 3 percent.