Horsepower to current conversion

Old 12-31-01, 01:58 AM
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Question Horsepower to current conversion

As a follow up on my previous post, thankfully answered by Ron, how does one determine current draw based on electric motor horsepower rating.

Strictly speaking, 1 HP is approximately 746 watts. In a 110V circuit and assuming 100% efficiency, this would mean the motor would pull about 6.8 amps. At say 60% efficiency, this would mean about 11.3 amps for a 1 HP motor.

So, the 1.5 HP motor I mentioned in my previous post would seem to pull 17 amps - assuming 60% efficiency. Consequently, it should run on a 20A circuit.

What am I missing here? Is this just too close to the breaker limit? Are motors less than 60% efficient?
Old 12-31-01, 02:06 PM
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Normally, a 20 amp breaker should be ok for a 1.5 hp motor. Your calculations are correct. The only possible issue is if the motor draws more then 17 amps once in a while due to varying load conditions, then you may have a nuisance, occasionnal tripping problem. A 1.5 hp rating is for full load conditions, and will have a duty cycle rating as well (usually continuous for such motors). If this motor is powering a tool like a table saw where you might be ripping long, green lumber, then it could bog down and draw over 20 amps, and trip the breaker after a few seconds. a jointer could do the same thing.
Old 12-31-01, 04:35 PM
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The information I refered to in the previous post, 115V, 1.5 HP motor draws 20 amps was from the National Electric Code 1999 Table 430-148.
If the actual manufacturers information is not available, the code says go by the 430-148 table which give average data.
Code allows circuit rbeakers upto 250% of the full load amperes as required to start the motor. The small the breaker the better, but matching the breaker to the full load current will almost never work. Starting current could be 6-10 times the full load amperes for several seconds.
Old 01-01-02, 02:48 PM
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HandyRon is right on the money. Your problem is called the inrush amps of your motor. When a motor starts up it pulls many more times in amps than the full load current rating of a motor. A breaker or non time delay fuse is not designed to handle this high in rush of power. Handy Ron gave the short version and he is correct in what he is saying. If you want the long version and how this motor and wiring design works then click on the link below.

Hope this helps

Old 01-02-02, 01:33 AM
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Thumbs up

Thanks to all for the information! I knew motors pulled extra current during start up or under load, but was not aware it could be so much extra. Your posts clarified this somewhat confusing topic.

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