Is there a formula for sizing the amps needed for a breaker?

[hpg4815]

HI,

I have a small shop space with single phase run to a 100amp breaker main, and I bought a few items such as a welder, plasma cutter and a compressor. In order to ensure I select the correct breaker for these devices and any other ones in the future, I was wonder if there is a formula or table/chart that one can use to determine the correct breaker size to use for each said item? For instance the comprossor states 240v/ 60 Hz/15amp. IT has a 3.7 HP motor. What formula or process does one use to determine the the correct breaker/wire/outlet size needed? I've read that for each 1 HP = 745.7w, and in the case of the compressor specs, I multiplied 745.7 x 3.7 = 2759.09 and then divide that by the 240v mentioned on the compressor in order to determine the amps. But that only gives me 11.49 amps. Which to me makes no sense on why one would need a 30 amp dual pole breaker. Basically I want to know what the a rule of thumb is which allows one to determine the breaker size, outlet size, and wire size for a device based on the specs printed on the device.

 

[DefyG]

Consult the manufacturers data, you should be able to find something online for the equipment.
Its not so much the rating but the type needs to be considered too, typically C type MCB's for motors or even D type for equipment with particularly heavy starting currents, or HRC type fuses?

 

[Lucien Nunes]

[Note the OP is in the USA where breaker types and ratings are different to EU/UK]

The sizing of breakers and ampacity of branch circuits needs to both comply with the NEC and meet the needs of the equipment as specified by the manufacturer. Calculating these yourself from motor hp is possible but a bit more complex than your trial calcuation above. For multiple reasons, it is better to get the actual nameplate line current of the motor if possible and to size equipment using the NEC tables.

As to why you got a surprisingly low figure for current: The motor hp rating is the mechanical output power. The electrical input power is greater because motors are not 100% efficient. Then, the current is greater than simply P/V because the power factor is less than unity due to the winding inductance. Then you have to allow for the starting current which is usually much higher, up to 5x the running current, and often dictates the minimum breaker rating. Often the breaker will protect the wiring but not the machine itself, where a separate overload relay matched to the motor is incorporated.

 

[Megawatt]

HI,

I have a small shop space with single phase run to a 100amp breaker main, and I bought a few items such as a welder, plasma cutter and a compressor. In order to ensure I select the correct breaker for these devices and any other ones in the future, I was wonder if there is a formula or table/chart that one can use to determine the correct breaker size to use for each said item? For instance the comprossor states 240v/ 60 Hz/15amp. IT has a 3.7 HP motor. What formula or process does one use to determine the the correct breaker/wire/outlet size needed? I've read that for each 1 HP = 745.7w, and in the case of the compressor specs, I multiplied 745.7 x 3.7 = 2759.09 and then divide that by the 240v mentioned on the compressor in order to determine the amps. But that only gives me 11.49 amps. Which to me makes no sense on why one would need a 30 amp dual pole breaker. Basically I want to know what the a rule of thumb is which allows one to determine the breaker size, outlet size, and wire size for a device based on the specs printed on the device.

Each price of equipment should have a label on them stating the voltage and amperage. That is what your breaker size and wire is determined. Sorry for the delay on helping you out.