Help me save money without electrocuting myself or burning down my workshop

My brother and I are in the final stages of our workshop-building saga.  We've found our location and we're getting ready to move the tools.  At the current time, we have one tool that requires big power and there may be a desire to have some other things down the road (heater, bigger welder, that sort of stuff).  We've asked the landlord to run a NEMA 10-30 outlet for us (240v, 30 amp same as a clothes dryer) and the cost on that is fairly high.  Our use will be somewhat occasional -- it's for hobby work, not commercial production -- so we're willing to make a compromise here and there and we'd really only ever run one thing at a time on the circuit.  The saw calls for 1ph 230V only (it's a Jet JTAS-10XL-1) and has been used for the last decade or so on that type of circuit.

I had this crazy thought:  Since we're not running this all the time, could we use a portable generator instead?  The Predator 8,750 from The Hammer Store is about half the cost of the electrician's install bill and it has a NEMA L14-30 plug that we could utilize for 240V to the table saw.  The rated running wattage is 7,000 and a 30amp 240V is maxed out at 7200 watts, right?  So, in theory, that should be plenty of power to run any one appliance that would ordinarily run on a 30 amp 240V circuit (assuming that there's nothing we would use that would use literally 100% of its rating, right?).  We'd need to build a power cable with the correct NEMA 10-30 outlet of course, but would that work or is that a terrible idea?

Short answer, probably not.

Long answer:

Single phase, capacitor start motors draw about 3x their full-load amps for a couple seconds on startup, a generator, even if it has enough output current will still take a couple seconds to throttle up and make that power. It can't supply power fast enough to satisfy the demand from the electric motor.

If the 230V motor draws 15 amps, the start-up current will be around 45 amps for a couple seconds until the motor is up to speed. This load is for such a short time that it won't trip a normal circuit breaker in your house but it will overload the generator while the generator tries to speed up to meet demand and will, in turn, starve the motor.

This will either burn out your saw or your generator.

The number you need to consider is the starting current, or LRA on the motor.  This will typically be much higher than the running amp draw.  Regardless, the voltage produced by your generator will drop significantly when the motor starts, and will cause even more current flow through the start windings.  Might work, might toast your motor.  Can you post an image of the motor label on the saw? 

Edit:  Ah, Trans_Maro beat me to it, but yeah, what he said! 

HA! In before the help! :P

Kinda related but recently I ran a Lincoln 140 welder off a 5kw generator. In theory I wouldn't be pushing it at all. I'm reality I worried cause it really loaded down everytime I was welding. Lucky for me it was a small job so I didn't have to worry about killing the poor generator. 

That's kind of what I'm worried about - hate to burn out the motor in the saw!  Unfortunately, there is no tag on the outside of the saw.  I probably need to take the cabinet apart to check the tag on the motor itself.  If the generator's starting Watts (8750 in this case) are greater than the start-up wattage of the saw motor, though, I should be okay?  If I can't find anything on the tool itself I'll call Jet on Monday and see if they have any further information.  If I'm doing the math right, 3hp is about 2700 Watts or 10ish amps, no?  3x would be about 30 amps or 7200 Watts which gives about a 1500 Watt cushion on the startup rating.  

I swear I'm not trying to be the guy who asks for advice and then argues about it!  Just want to make sure I understand the math!

I would think a table saw, starting with very little load and all wouldn't care about a brief voltage dip. Also, this is only a 3hp 14.5 amp motor, so more like 3400 watts full load.  I bet it'd be fine, reading the manual (page 14) it tells you to just double the wattage on motors for the start figure, you'd be well within spec. 

EDIT: If you wanted to go all out isn't this also one of the things a "soft starter" is for? Or will that not work on a single phase capacitor start motor?

Nevermind.

Yes you absolutely can run a table saw off a generator. It's done every day on construction sites all across the country. 

As long as your generator is big enough. Sounds like it is. 

All electric motors do have a heavy draw on startup, but a table saw spins extremely freely. Should be fine. 

If you want to test the theory before buying a generator, just rent one for a day. 

I recommend the rental idea. You'll be surprised how annoying the noise from that generator is with it running all day. 

The numbers look to me like you would be fine. I did exactly this for a year while I had no power in my shop and had no problems.

This is similar to the setup the guy who works on my Samurai has. His shop is off-grid, most of it runs on renewable power and a EV battery pack sections. For big tools like the compressor, he fires up a generator. It's a fairly big one though, by size it's similar to a 9kw unit I've seen.

If you have the option of running too much electricity to the shop, do it.  The bitter taste of the expense will fade every time you walk into a well let, warm shop with all the proper electrical outlets.

It's only money. And, you will spend most of the money wiring now, plus you will buy a generator, plus you will get fed up with the inconvenience and rewire eventually anyway.

Any chance of changing the saw to 110 volt?  Call Jet and see what they say.  I have a smaller Jet table saw that I love.  Yes, it'd be nice to have a 3 horse saw, but as you're finding out, they need special power that isn't readily available.  Save the 3 horse motor for later.  

In reply to Kramer :

That's a good idea. 

Read the motor plate. A lot of motors can be rewired to be either voltage. 

Actually, if the generator has a 230-volt outlet, you'd be better off keeping the 230-volt motor on the saw.  A 115-volt motor of similar output is going to draw approximately twice as many amps.

A 3hp motor wired for 120 volts would pull ~30 amps full load, a 120 volt outlet that big is a real oddball. I'd think with a smaller motor you'd also need to change the pulley ratio to keep from bogging the motor down and overheating it. If you did that I don't think you'd want to go bigger than 1hp, maybe 1.5 with 20 amp circuits.

I knew I could count on yous guys!  Thanks for all the info and the education.

The motor player does not appear to support rewiring to 120.  And even if that's the case, Watts is Watts.  We could swap out the motor, but at that point we'd just sell the thing and buy a different saw.  And apparently it's a really nice saw.  I'm not really into woodworking but that's what I'm told.

After looking at the motor plate it appears that this is a motor that could require up to 5x its running amperage to start (class E).  A generator that would push that many Watts would be way more spendy than just having the electric run.  

The sounds and inconvenience of the generator wouldn't be a big deal since we already have a 120 circuit.  But it would be nice to plug in a heater go get some food and come back to a warm workshop without worrying about a generator.

So in the end, we're going to spend the 1400 to have the circuit run.  That just sounds like the most prudent course of action.

wae said:

So in the end, we're going to spend the 1400 to have the circuit run.  That just sounds like the most prudent course of action.

I concur!