Amperage for 5HP table saw?

Hello Ralph, the big point in using an inverter on the TS is not the speed (except you want to cut steel on your saw), but the inherent motor break in the inverter. My saw was spinning for close to a minute with a heavy dado stack on after switching it off. With the inverter, it comes to an instant stop.

Regards Matthias

schrieb im Newsbeitrag news:1078204646.761648@smirk...

"No more than 80%", for permanently attached loads, yup.

Yup. 30A is "more than sufficient".

At 120V, yup. not at 240.

They, and even bigger ones, do exist. You *don't* want to see the price tag, however.

Using Ohm's law this is a relatively simple question to answer...

Power (watts) = I (amperage) x E (voltage)

When you have two values for the variables in the equation you can figure out the third.

P 3728.5 (watts 1 HP x 5) Therefore I = --- or I = ------ E 230 volts

Using the "80% rule", a 20 amp breaker should not exceed a continuous load above 16 amps. It seems to me, that with the proper type of "slow" breaker, you should be OK with a 20 amp breaker.

Joe

PS

I am not an electrician or an electrical engineer. In fact, I have never even played one on TV.

That only works for an ideal motor. Unfortunately, they do not exist in real life. You need to adjust for power factor and efficiency.

P = (HP * 746) / (pf * eff)

So with an efficiency of 80% (.8) and a Power factor of 90% (.9), 1 horsepower = 1036 Watts

For the 5 HP example I = (5 * 746) / (.9* .8) / 230 = 22.5. amps

Using the 80 % rule on breaker sizing, 22.52/.8 = 28.23, so a 30 amp breaker will be required.

-- Al Reid

"It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so." --- Mark Twain

Alas, this assumes 100% efficiency, which saws are not. For example, my

Paul K

A 30 Amp breaker should be O.K. for a table saw, I had real problems when I was trying to run a 5HP 24" planer/thicknesser because the mass is so large it takes that much longer to get up to speed. Your 20 Amp running load is right, but the startup Amps are round 2 to 2 1/2 times that.

Bernard R

you guys are starting to get there, but whats so hard about looking on the motor for the current rating or calling the manufacturer and getting a power factor curve for it if you insist on going that route?

randy

i didnt mean to come off as an ass, but now that i read it, i kind of do..

all i mean is that if you want to start doing the math, there are many factors involved. the load conditions on startup. the load conditions during running. and none of these are simple to calculate but most motor manufactures will provide charts.

but in any case, if you cant run your 5horse motor on a 30 amp circuit, something is wrong.

randy

I just finished wrestling with that very issue. I became the unexpected owner of a 5 HP Single Phase Unisaw and had to figure out how to wire it. In a phone call to Delta's technical people, they told me:

1. Delta's 5 HP motor draws 100 Amps for about 1.5 seconds at startup.
2. A 30 Amp "slow trip" breaker will hold it, but they are more expensive and hard to find. (Home Depot never heard of 'em.)
3. A 40 Amp breaker will hold it fine, and that is what Delta recommends.
4. The 5 HP saw comes without a power cord because they expect it to be hard-wired by someone who knows what he's doing.

I sought advice from a couple of Electrical Engineers at work. They assured me that:

1. The NEC allows a 40 Amp breaker on #10 wire IF the wire is only feeding an electric motor that is hard-wired, AND there are no other loads on the circuit.
2. It's not unsafe to run such a setup, because the motor's internal overload switch will protect the motor itself. The only other catastrophe would be a direct short, and the #10 wire will carry enough amps to trip the 40 Amp breaker before the wire overheats.

So, you'll need to ditch the idea of twist-lock plugs for a 5 HP motor. Plan to use either a 30 Amp slow trip or a 40 Amp breaker, along with whatever wire size makes you happy (as long as it's #10 or bigger).

The information presented above is hearsay. I am not a licensed electrician, nor an electrical engineer. Check local codes before installation. Void where prohibited. Your mileage may vary. Free advice is worth what you pay for it.

DonkeyHody Even an old blind hog finds an acorn every now and then.

Are you sure about that one. All of the references I am aware of limit the overcurrent device to 30A on a #10 wire.

Anyone know for sure?

-- Al Reid

"Al Reid" asks:

the overcurrent device to 30A on a #10 wire.

It is all a function of the insulation used.

The function of the c'bkr is to protect the insulation on the wire, not the wire itself and most definitely not the motor.

Don't have my code book handy to verify; however, the use of a #10 conductor on a 5HP/240V/1PH motor even if legal, would at best be considered a bad installation.

The difference between the I^2*R heat losses of that #10 vs the lower heat losses of a #8 would soon consume any material savings made during installation.

After that, the power company would be laughing all the way to the bank.

Personally, I'd use #6 wire /w/ a 2P-40A c'bkr for a 5HP/240V/1PH motor.

SFWIW, have designed and sold the equipment to build more than a few electrical distribution systems during my career.

HTH

John

No, it's not. Theampacity may be higher than 30 A, but code limits the evercurrent device to 30A. See the tables in section 310. Section 310 states that #14 = 15A, #12=20A and #10=30A. I agree that the rated ampacities may, however, be greater than those limits.

My question is, can anyone cite the section of the NEC that allows a larger overprotection that is stated in section 310?

I have never ran across that exception and I have been an Electrical Engineer for 24 years working in, among other areas, electrical construction engineering. I am still looking at the code book.

overcurrent device to 30A on a #10 wire.

See the disclaimer attached to my post. No, I'm not sure, because I didn't look it up myself; but I'm satisfied those guys knew what they were talking about. I'm comfortable with it. But then, I'm known for taking risks and living on the ragged edge. I used to hang off cliffs on ropes and ride motorcycles really fast. I don't have a guard on my tablesaw blade. And I used PVC pipe for airline. Don't do what I do. Do what makes YOU comfortable.

DonkeyHody If ignorance is bliss, why aren't more people happy?

Comfort is not the point. If you run an undersized wire, you're risking problems with the system that might become critical. If that system is part of your house wiring, failure could be fatal. I very much doubt the NEC allows a

Another point: most electrical engineers of my acquaintance know very little about NEC and building codes in general. They have no need to know, unless their speciality is in some area of construction.

I've seen guys who can do a superb job of designing transport wiring systems for production lines that I would let wire the last motorcycle I had...and that was a bike without a headlight or taillight.

Charlie Self "There is nothing wrong with America that cannot be cured with what is right in America." William J. Clinton

I'm anything but an electrician, but I learned early that the breaker is there to protect the wire, 'cause when wires fail houses burn.

djb

I have a Left Tilt Unisaw with a 5 HP, single phase 230V, TEFC motor. It uses the full voltage motor starter (GPE). I have the saw connected to a 30 A breaker. The saw starts right up and has never tripped the breaker when starting or in use. While waiting for delivery of the saw, I talked to Delta technical support. They said I would need a 40 Ampere Slo-Blow Fuse. But as noted in earlier email, there is no such beast at Home Depot. Once I had the saw, I looked at rating of the motor starter. I was rated for 18-25 A devices. And I did verify that the motor was rate at 5 HP.

I have a true 5 HP motor that is rated at 25 amps @ 230 volt. I did a quick check through a Grainger catalog and found only one 5 HP motor under 20 amps @ 230 volt. Most were 22-23 amps. Greg

Delta states a requirement of a 40 amp slow blow fuse for protection on their 5 HP Unisaw. It may run on a 30 amp circuit, but I would not bet on it! Greg