Electric & Safety - How do I convert watts to amps and amps to watts?

How do I convert watts to amps and amps to watts?

The formula for converting Watts to Amps is : Watts = Amps * Volts 

The formula for converting Amps to Watts is : Amps = Watts / Volts 

SAFTEY WARNING : Only use 80% of the circuit breaker capacity! 

Common Conversions North American and other 110 volt countries 

Lights : 
1000W / 110V = 9.1A 
600W / 110V = 5.4A 
400W / 110V = 3.6A 
250W / 110V = 2.3A 

Circuits : 
10A * 110V = 1100W and 80% safe usage is 880W 
15A * 110V = 1650W and 80% safe usage is 1320W 
25A * 110V = 2750W and 80% safe usage is 2200W 
30A * 110V = 3300W and 80% safe usage is 2640W 

Common Conversions British Commonwealth and other 240 volt countries 

Lights : 
1000W / 240V = 4.1A 
600W / 240V = 2.5A 
400W / 240V = 1.7A 
250W / 240V = 1.1A 

Circuits : 
10A * 240V = 2400W and 80% safe usage is 1920W 
15A * 240V = 3600W and 80% safe usage is 2880W 
25A * 240V = 6000W and 80% safe usage is 4800W 
30A * 240V = 7200W and 80% safe usage is 5760W 

Remember that a circuit services more than one power outlet. 
A two outlet wall plug will use only one circuit. 
There will be more outlets on 240V systems per circuit than there will be in 110V systems. 

  • December 13, 2006, 8:22 pm - Harrad

    Nominal line voltage in the US is 120 (One Hundred Twenty) Volts.

    110V has never been a standard, and has not been used since WW-II.

    The discrepancy makes a significant difference in your calculations.

    The correct calculations as follows:

    Lights :
    1000W / 120V = 9.1A
    600W / 120V = 5.4A
    400W / 120V = 3.9A*
    250W / 120V = 2.5A*

    Halve current demand for 240V.
    *Note corrections for standard current draw.

    Circuits :
    10A * 120V = 1200W and 80% safe usage is 960W *
    15A * 120V = 1800W and 80% safe usage is 1440W
    25A * 120V = 3000W and 80% safe usage is 2400W * **
    30A * 120V = 3600W and 80% safe usage is 2640W **

    *Non standard US residential breakers.
    **Not safe to power cord connected devices with cords smaller than 12 Ga, Nor with NEMA5-15 (standard 120V) sockets/plugs.

    Remember that a circuit services more than one power outlet. (sometimes)

    A two outlet wall plug will use only one circuit. (sometimes)

    There will be more outlets on 240V systems per circuit than there will be in 110V systems.
    (Not necessarily)

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