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Amps for 240V table saw circuit?

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Forum topic by Damian Penney posted 2175 days ago 12447 views 0 times favorited 38 replies Add to Favorites Watch
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Damian Penney

1140 posts in 2626 days


2175 days ago

How many amps should a 240V circuit be rated to run a 3HP 240V saw?

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


38 replies so far

View EEngineer's profile

EEngineer

890 posts in 2248 days


#1 posted 2175 days ago

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!"

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Damian Penney

1140 posts in 2626 days


#2 posted 2175 days ago

Perfect, thanks :)

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

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DaveH

400 posts in 2413 days


#3 posted 2175 days ago

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.

-- DaveH - Boise, Idaho - “How hard can it be? It's only wood!”

View Damian Penney's profile

Damian Penney

1140 posts in 2626 days


#4 posted 2175 days ago

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

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SCOTSMAN

5352 posts in 2220 days


#5 posted 2175 days ago

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

View niki's profile

niki

426 posts in 2714 days


#6 posted 2175 days ago

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

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tooldad

657 posts in 2350 days


#7 posted 2175 days ago

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.

View Bob #2's profile

Bob #2

3808 posts in 2656 days


#8 posted 2175 days ago

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

View GMman's profile

GMman

3902 posts in 2332 days


#9 posted 2175 days ago

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

View tmj's profile

tmj

2 posts in 2175 days


#10 posted 2175 days ago

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…

View Damian Penney's profile

Damian Penney

1140 posts in 2626 days


#11 posted 2174 days ago

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

View CedarFreakCarl's profile

CedarFreakCarl

594 posts in 2688 days


#12 posted 2174 days ago

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

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CedarFreakCarl

594 posts in 2688 days


#13 posted 2174 days ago

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

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GMman

3902 posts in 2332 days


#14 posted 2174 days ago

15 amps I would rather reset a breaker than damage something

View EEngineer's profile

EEngineer

890 posts in 2248 days


#15 posted 2174 days ago

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!"

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