# Amps for 240V table saw circuit?



## damianpenney (Jun 22, 2007)

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


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## EEngineer (Jul 4, 2008)

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.


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## damianpenney (Jun 22, 2007)

Perfect, thanks


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## DaveHerron (Jan 21, 2008)

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.


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## damianpenney (Jun 22, 2007)

20A it is then.


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## SCOTSMAN (Aug 1, 2008)

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


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## niki (Mar 26, 2007)

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 (Mar 24, 2008)

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.


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## boboswin (May 23, 2007)

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 .


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## GMman (Apr 11, 2008)

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


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## tmj (Sep 15, 2008)

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…


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## damianpenney (Jun 22, 2007)

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.


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## CedarFreakCarl (Apr 21, 2007)

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.


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## CedarFreakCarl (Apr 21, 2007)

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.


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## GMman (Apr 11, 2008)

15 amps I would rather reset a breaker than damage something


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## EEngineer (Jul 4, 2008)

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.


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## sIKE (Feb 14, 2008)

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.


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## CedarFreakCarl (Apr 21, 2007)

Sorry, I couldn't help myself.


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## EEngineer (Jul 4, 2008)

*Carl* - ya know, there's still room on that panel for a couple more controls…


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## CedarFreakCarl (Apr 21, 2007)

EE - Nice to know we've all got a sense of humor. Thanks!


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## sel (Jun 9, 2009)

sIke had a good point 266 days ago and I have a question now regarding distance. I hope that some of you are still paying attention. I have to make a run of about 100 feet from the panel to the garage where my 3HP Unisaw is located. Does this distance dictate a heavier gauge wire and breaker than the 12AWG/20amp suggested? Do I need to run 10AWG? 
By the way, I am running the wire through conduit in a trench. Since I am putting the conduit in, I figure I should allow room for a future wire pull AND/OR should pull what is necessary for some additional circuits right now. 2 additional questions: what size conduit (ABS I presume) and how many wires? The 240 doesn't require a neutral. Can I somehow get a 120 off of the 2 wires pulled for the 240 or do I need to pull 3 more for 120s?
Finally, should I run all into a new sub panel in the garage and distribute from there? Thanks.


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## kerflesss (Mar 7, 2009)

Carl, you forgot a few toggle switches and a volume control LOL.

Go with the 20amp circuit provided you are dedicating this line for only you TS. If you are running other things such as a DC, then do your calculations to figure the entire load and wire accordingly.
Breakers are wire protectors. Over load devices protect motors.
I would check your motor plate and be sure that you have your circuit capable of your motors needs.
Starving your motor will create excessive heat and thusly shorten the life of your motor.

Sel, assuming that you are trying to provide for all your shop tools and perhaps lighting I'd suggest a 60-100 sub panel. For example Schd 40 1 1/4 run would be more than enough with a #6 wire (Blk,Red,Wht) and #10(Grn) with a 60 amp breaker to matching subpanel and give you future room to add a circut. I'd leave a pull cord in the pipe if you intend to do so…


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## JoeLyddon (Apr 22, 2007)

CedarFreakCarl,

You forgot a large Variac knob… the Key to overall control…


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## Schummie (Feb 3, 2008)

Hello Damian,

We use in Europe on 220v a 16 amp for a 3hp table saw.

Success Schummie.


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## sel (Jun 9, 2009)

I am in Los Angeles. Is Schd 40 code compliant for subterranean electrical runs?
Also, when you say a "#6 wire" do you mean 6 AWG or does that refer to a 3-wire cable like Romex? Same question with respect to the #10 grn. Do you suggest a 10 AWG wire. If yes to 6 and 10 AWGs, is that becasue of the distance from the main panel?


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## kerflesss (Mar 7, 2009)

The Schd 40 is a gray electrical pipe in the electrical dept of your borg store. It should be buried a minimum 18". 
Yes 6AWG THHN which is also common there. Romex can not be run in conduit. Its used in free air (attics or in walls). The #10 green wire is your ground wire. The Black and Red would be you hot and the White neutral.
You might get the primer "Simplified wiring" from Depot/Lowes. Also a Pocket wire spec publication called "Ugly's" 
which gives Wire capacities among other things; ie: #14-15 amps; #12-20A; #10-30A etc. 
The distance from the panel is not at issue, but the capability to carry the amperage to your sub panel requires the #6 wire. Distance would be of concern if you were to have a exceedingly long run where you would experience voltage drop. You'd than have to increase your wire size to #4. You should be fine at 100'.

Think of electricity as water. The longer the run, due to friction, the pressure drops off. The bigger the pipe the greater the pressure. With electricity the friction is resistance. Hopefully (keeping it plain as I can) this helps. Good luck.


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## marcb (May 5, 2008)

My basic rule of thumb is to wire for a lot of capacity and use NEMA starters with heaters to protect the motors.


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## sel (Jun 9, 2009)

Thanks. My concern was the drop of ampacity due to the length of the run and to the number of wires in the conduit. I guess you all feel that 6AWG THHN is of sufficient size that even with 4 wires in 1 1/2" conduit the amount of heat generated will be insignificant. At the risk of pressing my luck here, whadda you all think of 8AWG? Safe, or should I stick with 6? I do have the Wiring Made Simple book (green cover), very old, but electricity hasn't changed that much.

Carl,
I understand the utility of each and every dial, switch and light on both panels with the exception of the little green one above the orange dial on panel EE. I can't for the life of me figure out what that one is for.


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## BarryW (Sep 15, 2007)

duh, what "on"...?


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## JoeLyddon (Apr 22, 2007)

Don't know how LONG your run is…

#6 would definitely be SAFE…

#8 would also be safe depending on the run…

The longer the run, the bigger the pipe wants to be…

That's my guess anyway…


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## sel (Jun 9, 2009)

My run is about 100 feet


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## TopamaxSurvivor (May 2, 2008)

My handy dandy Sq D motor calculator shows a 3 hp at 230v single phase at 17 amp, #10 wire, 35 amp breaker. Voltage drop is an issure for length of run and higher loads. At 100 feet, I'd go at least #8. National Electrical Code requires a single disconnecting means to disconnect al the power in a seperate building. Using a subpanel is the proper way to do it.


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## ajosephg (Aug 25, 2008)

Carl, the EE panel needs some meters, at least voltage and current, and frequency would also be cool. Maybe you could do it with an oscilloscope so we could check each phase as well. Oh, I forgot, knowing the power factor is also interesting.


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## JMatt (Mar 2, 2009)

I just had an electrician install a sub in my shop. He pulled #6 through 1 inch PVC about 50 feet. No problem. All of them were single conductors. No Romex or anything like that. In my shop, I've run 30amp breakers with #10 wire for the 240v outlets. I've got #12 and 20amp breakers for everything else. I ran the 30amp breakers and bigger wires 'just in case'. A little more expensive but I won't have to worry much about power needs. If you can afford it, get a 60-100amp subpanel out in that shop. You won't regret it one bit.


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## JoeLyddon (Apr 22, 2007)

Sounds good!


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## JimmieD (Sep 27, 2009)

Hey Lumberjocks,

I'm new to this forum and not sure i know what I'm doing or am in the right forum to even ask a question but I'll give it a try. Tell me if I am wiring my PM 2000 right. 3hp 13amp motor, 240 volt, plug has 3 wires, black, white and green for ground. I put on a 100 amp panel in my shop to run all my toys. I ran a 10/2 wire on a 30amp 2 pole breaker to the PM 2000, black to black, white to red and taped the white wire red to indicate hot, green to ground. Do I have it right? This saw was given to me 1 year ago by an old man who bought it new in 07 for tearing down an old barn and doing some chores around his place. The saw was still in the crate when he showed it to me in his garage. We did a swap for me tearing down his barn he gave me the saw. He passed away a few months ago and i want to treat this saw like a piece of gold and not burn it up wiring it wrong. Any help would be greatly appreciated.

Thanks
Jimmie D


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## TopamaxSurvivor (May 2, 2008)

You got it ;-))


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## ondablade (Aug 23, 2009)

One bit of hard information. My 220V/50Hz/3hp (Irish) Robland X-260 saw motor was fitted with 16A slow blow fuses, and these seemed to be fine in that i never blew one.

On the other hand i (unthinkingly) ran it from a 13A fused domestic socket for a while at first. It would run off that if you were careful starting it, but would pop the 13A fuse at the drop of a hat. In addition it would produce light arcing occasionally between the plug pins and the socket, although i only found out about this when i noticed light burning on a plug pin.

Don't skimp on the cable, because the voltage drop and effect on the motor will increase the amps drawn.

I can't remember for sure, but think a 3hp single phase motor like this is rated at about 14A at full load, although this could be 10% out either way.

I'm not sure, but i think you'd probably find that too that it would spike quite a bit above this on start up for a few seconds. (hence the slow blow fuses)...

ian


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## TopamaxSurvivor (May 2, 2008)

The code says 17 amp full load for 3 hp at 230v, reality, probably 15.5-16. #10 cu wire, 35 amp breaker. Probably have to go to 40 if the 30 doesn't hold. Standard fuse will never ever hold a motor starting unless it is way over sized. Motors starting requires slo-blo type fuses. Dual elemnt fuse if you are using them is 25 amp.


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