Amps for 240V table saw circuit?

1HP ~ 750 watts
watts = Volts X Current
motors are about 70% efficient

So:
3HP X 750watts = 2250watts
2250watts / .7 eff = 3214watts
3214watts / 240 Volts = 13.4 Amps

Now, you can argue about motor efficiency, but a 15 Amp circuit at 240V should support a 3 HP motor under load. Make it a 15 Amp slow-blow fuse to support higher motor starting currents. Circuit breakers are inherently slow-blow.

-- "Find out what you cannot do and then go do it!"

Perfect, thanks :)

-- I am always doing that which I can not do, in order that I may learn how to do it. - Pablo Picasso

I’d go 20amp or 30amp. Circuits should not be loaded for more than 80% of rated breaker amps for continuous load.

You might want to consider pulling 10guage wire and using a 30amp breaker for it. Some 3hp motors pull 19amps and require a 30amp circuit. I run my 3hp grizzly cabinet saw on a 30 amp breaker. Remember the breakers main purpose is to restrict how much juice you can pull from the power company through the circuit and the wire size, breaker, and receptacles need to be matched up properly to prevent electrical fires.

-- Dave Herron, Boise, ID -- “That which does not crash the browser, makes it stronger”

20A it is then.

-- I am always doing that which I can not do, in order that I may learn how to do it. - Pablo Picasso

normally I would have said 13 amp is a bit too low and16 amp might be more the thing but if you want to take it to twenty then so be it.Alistair

-- excuse my typing as I have a form of parkinsons disease

Hi Damian

It’s very difficult to answer your question.

You see, to produce a 3HP that is 750×3 = 2250W (actually, 1HP = 746W but you are not planing to design the motor, don’t you…so 750 is good enough)

Now, for those 3HP OUTPUT the input is always bigger and it depends on the motor efficiency to convert electric power to mechanical work and always it will be less than 1 (or less than 100%)

There is a PDF file “comparison of 10 table saws”, all of them with 3HP but the Ampere is varying from 12A (high end saws…and expensive) to 18A (low end saws…and cheap)

Usually, the Amp is written on the name-plate but if its missing, I would take DaveH’s advise and go with 20A….to be on the safe side

Regards
niki

I am running a 3hp shaper off a 20amp 220v outlet. Typically from my experience, 2hp can be 15amp, but wire for 20 to be safe, 3hp 18 amp startup, so go 20, 30 if you want to be safe but more costly, 5hp requires 30amp 10ga wire.

Just stick in a 20 amp breaker assuming your wiring is correct for the load.

20 amps wont hurt anything as I doubt if you will be running other than your DC at the same time.

Commercial shops—- please ignore my advice .

-- A mind, like a home, is furnished by its owner

Be safe use 15 amps I have 35 years of electrical work

if i recall electrical motors of this capacity, there may be a capacitor on it…. start up is the usual breaker tripper, so if the motor plate indicates 15 amp, it may or may not trip the breaker…

GMan, why do you think 15A is safest? I thought that if your circuit can supply what the motor wants then it doesn’t matter how far and above that you go because the breaker is there to protect your wiring, not your appliance.

-- I am always doing that which I can not do, in order that I may learn how to do it. - Pablo Picasso

I think wiring size vs. amperage is the critical link here. Here are what I consider safe wiring sizes.
14 ga. = 15 amps max.
12 ga. = 20 amps max.
10 ga. = 30 amps max.
The motor will only pull the amperage necessary to make it run efficiently. I’ve got a 3hp motor on my dust collector on a 30 amp circuit that I also use for my welder. (never at the same time, it’s only got one plug).
The dc only pulls 15 amps. These are the numbers I’ve always used and my house hasn’t burned down yet.

-- Carl Rast, Pelion, SC

Damien:
Here’s a couple of websites that you can use for reference.
Great Inspector
Renovation Headquarters
I don’t know if you’re running a new circuit or using an existing one, but if running a new one, I would run 12 ga. wire and use a 20 amp circuit if your breaker box will support it. If you’re using an existing circuit, you can go to the big box store and get a wire gauge and figure out what you have. Probably the best thing you can do is hire a licensed electrician and remove all doubt. Hope this helps.

-- Carl Rast, Pelion, SC

I used a 20A breaker, and 12 ga. wiring, for my 3HP General 650.

Remember… A circuit breaker protects the line wiring, not necessarily the powered object. We don’t swap breakers as we plug in different items around the house, do we? This is a BIG, often repeated misunderstanding.

There is normally a fuse or breaker in the powered object itself for protection of the device.

-- - Real men read directions

15 amps I would rather reset a breaker than damage something

My, my, lots of comments on this one.

In my initial reply, the numbers were all conservative. 70% is actually rather low efficiency for a motor; many sources use as high as 85% for RI motors. Brush-type universal motors run considerably less efficient, initially and after lotsa brush wear. 70% is very conservative, even for a universal motor.

1 horsepower is actually 745.6999 watts if you wanna be precise. 750 is only +0.13% error, why quibble? And, again, it leads to a more conservative answer.

But I haven’t heard anyone question the most basic assumption; voltage! At my humble abode, the line-to-line voltage actually reads about 238V. I have seen as low as 220 in some locations. Now, subbing 220 V into my original equations yields 14.6 A which might be running close to the limit on a 15 Amp circuit.

Basically, if you already have a 15A, 240V circuit, use it and put in a slow-blow fuse, you’ll be fine. If you are running a new line, by all means, use 12 AWG and fuse it for 20A. You’ll be plugging something else in for sure.

-- "Find out what you cannot do and then go do it!"

I recommend that you go to one of the box stores and in the electrical section of that store you will find a book called “Wiring Simplified”. It will give you everything you (an average layperson) should need to know about wiring. The gauge of wire is determined by two factors: distance and load. The shorter the run of copper the greater the load it can carry this is due to resistance on the wire (aka the longer the load runs on the wire the hotter it will get). The larger the gauge of wire (remember the number gets smaller the larger the wire gets) the more heat it can handle and therefore the larger the load. I built my16×24 shop this last year and I put in wiring to handle 240volts at 20 amps with ~30ft runs. I dropped two circuits one for dust collection and one for operating machinery (you know the cuttin stuffs). From what you have described a twenty amp circuit should be fine. Once again this depends upon the distance you are from the breaker box and how much amperage you will draw.

-- //FC - Round Rock, TX - "Experience is what you get just after you need it"

Sorry, I couldn’t help myself.

-- Carl Rast, Pelion, SC

Carl – ya know, there’s still room on that panel for a couple more controls… :)

-- "Find out what you cannot do and then go do it!"

EE – Nice to know we’ve all got a sense of humor. Thanks!

-- Carl Rast, Pelion, SC