It would be possible (though rare) for a fault condition that drew for example 10A at 120V = 1200W to occur, which would be a fire hazard, without tripping the breaker.

While the equipment can only use perhaps 2.5A, the AC wall outlet will provide at least 15A to 20A continuously without tripping the circuit breaker or blowing the fuse. A 1A, 5V supply will require much less than 1A at 120VAC input. It is usually safe to assume at least 70% efficiency at full load (higher for more modern supplies), so the power supply will only need perhaps 1.5A at 120VAC input. Power supplies are not 100% efficient, thus it will require 5% to 30% more than 120W of input power to produce 120W of output power. As (for example) a motor load such as a refrigerator or saw briefly draws a much higher starting current, in order to avoid 'nuisance trips' a 20A rating does not instantly remove power when that load is exceeded.Ī V BB power supply rated (for example) 12V at 10A can provide up to 12V x 10A = 120W (Watts) of DC power. A typical US AC wall outlet provides 110V to 120V and is protected by a fuse or circuit breaker with a 15A or 20A rating.

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