20 amp motor on a 20 amp breaker?

[burrellski]

I just received my new Grizzly G0478 Hybrid (stealth gloat) and am working on incorporating it into my shop. The motor is spec'd at 20 amps and the manual suggests plugging it into a 30 amp circuit. Is this thing going to be choked / constantly tripping the breaker on a dedicated 20 amp circuit?

If yes, then I guess I have 2 options.
Install a 220V 20 amp circuit (motor draws 10 amps @ 220V)
Install a 110V 30 amp circuit

If I have to choose one of the above options, I would prefer to install a 220V circuit (I already have wire, don't really want to run 10ga line, etc...) but I believe the magnetic switch on the tablesaw will need to be replaced with a 220V compatible model, an expense that I'd like to avoid.

What's everyone think?

Thanks
Matt

[HarmsWay]

If the saw is rated for 20 amps and you have a dedicated 20 amp circuit, then definitely use that first. If it's tripping the breaker regularly, re-jumper for 220 and replace the magnetic switch. Don't forget to send me the old 110 switch. Nice (thinly disguised) tool gloat!

Bob

[LarryG]

In theory this works, but it'll be marginal. As Bob says, there would need to be nothing else on the circuit -- even a single 100W light bulb will draw about one amp. Remember too that you'll get at least a small amount of voltage drop due to the resistance of the wiring.

The biggest potential gotcha is that a 20A breaker will not deliver 20A indefinitely. Breakers are designed to deliver only 80% of their rated capacity (in this case, 16A) continuously; more than that and they will gradually overheat and trip. So if you're working the saw long and hard, as when ripping a lot of thick, dense stock, the breaker may start tripping annoyingly often.

It will cost you nothing to use the saw for a while and see what happens. FWIW, if it were me, I'd make plans to rewire to 240V more or less immediately.

[ksum]

I may be wrong, but isn't the motor rating a max draw that would happen at start-up and under a heavy load? On a dedicated line, my experience has been that I have not tripped a breaker in the same scenario mentioned.

I would try it and plan on switching to 220 at some point in the future. Another option would be to switch the breaker out for a higher amp rating if the 20 keeps tripping. Do watch wire size on that, and return it back to 20 when you switch to 220.

Karl

[LCHIEN]

personally I think it would be OK on a 20A line.
Larry says that they intend for you to size breakers for loads that run 80% of the breaker load.
But I don't really think you will be violating that as 80% of the time (or more) you will be below 20 Amps. In addition, 20A breakers should sustain 20A for a really long time, almost indefinately. And allow short excursions above that for brief loads.

I'd give it a try, as Harm'sWay suggests anyway.

[Knottscott]

Not much harm in trying it once or twice to see if it pops the breaker. The amp rating is a nominal rating AFAIK, so it'll likely draw more than that when turned on, then will settle to some number below 20.

If you need to do some wiring, don't bother with the 30 amp circuit, just go straight for the 220v. Are you sure you need to replace that switch? If so, you could probably sell the current one...looks like a nice switch.

[HarmsWay]

If it's like every magnetic switch I've seen, the relay coil will be either 110v or 220v. They're not cheap so keep that in mind.

Bob

[LarryG]

In addition, 20A breakers should sustain 20A for a really long time, almost indefinately.

Nope -- that is perhaps the most common misperception about breakers. Our electrical consultant explained to me that circuit breakers are designed such that they will deliver amperage in excess of 80% of the rated capacity for a guaranteed maximum of either one hour, or three hours (can't recall which he told me). After that, they are subject to tripping.

IOW, they will deliver 80% of the rated capacity 24/7, but NOT the rated capacity 24/7.

[LCHIEN]

technical details on Circuit breakers (long-winded engineer talk)

Nope -- that is perhaps the most common misperception about breakers. Our electrical consultant explained to me that circuit breakers are designed such that they will deliver amperage in excess of 80% of the rated capacity for a guaranteed maximum of either one hour, or three hours (can't recall which he told me). After that, they are subject to tripping.

IOW, they will deliver 80% of the rated capacity 24/7, but NOT the rated capacity 24/7.

Larry, we discussed this before.

your reference is what a electrical contractor told you verbally.

My reference is a Square-D circuit breaker specification sheet from Square D. It's pretty technical but refer for example to a ordinary breaker like the one on page 22 of this document. It's attached but too small to read. You'll have to print it out from the linked document, to read the actual numbers.

http://ecatalog.squared.com/techlib/...01&action=view

You will note this is a TRrip curve for a square D breaker, model QO 1-25A models from 1 to 3 poles, your basic household breaker. All other breakers will be very similar to this.

You will note that the horizontal axis is the normalized current, e.g. the multiples of the rated current. If the unit is a 20A breaker, then 20 Amps is 1.0 on the scale, 40 amps is 2.0 etc. It's logarithmic so they can show detail from .1 of rated curren to 1000x rated current.
The vertical scale is time in seconds. It's logarithmic so they can show detail from a few milliseconds to 1000's of seconds.
The white area to the left are conditions where the breaker will not trip.
The white area to the right is conditions where the breaker must trip. The gray area in the middle is the range of conditions where the breaker may or may not trip.

Its a funny looking curve because these breakers incorporate both electromagnetic tripping and thermal tripping. The EM tripping is to prevent gross overload and the thermal tripping allows the motor to start at some overcurrent and handle brief overcurrent but if its sustained then it will trip. The more overcurrent it is, the less time it takes to trip.

The bottom portion , time below 1 cycle or 16ms, shows that the device will not trip under 12x rated current (240A for a 2A breaker). After 1 cycle, the breaker may trip anywhere from about 12x to 60x.
After that, by 1 seconds, the breaker will trip on 4X to 9X of rated current. As it transitions from electromagnetic trip to thermal trip.

By ten seconds the breaker will trip from 1.8x to 3x the rated current.

You will notice the curve goes straight up and becomes asmyptotic at 1.0X for the minimum trip and about 1.3x for the maximum trip which says that the manufacturer specs say that every one of the QO breakers they make will carry at least full rated amps for 1000 seconds - typically I would guess about 110%-120%, but no less than 100% of rating - and probably well beyond because this represents thermal equilibrium (at this point the thermal heat sense element is cooling off as fast as it heats up). Although they don't spec beyond 1000 seconds, I would expect it to last for days and days, if not forever (or until the temperature exceeded the 40°C or 104°F ambient rating).


I didn't look at every other commercially sold breaker spec sheet, I just picked this one at random, but it says to me that the breaker will hold at least 100% of its rated trip current under normal operating conditions.

As an engineer, I'm actually impressed at the tolerances they hold and the combination of thermal and electromagnetic trip that work pretty much ideally with motors and appliances.

Now, that stated, I believe the NEC suggests sizing a branch circuit so that the expected load is not more than 80% of the breaker. This is because people generally add stuff, and its for continuous load. In the case of a saw, you will be careful not to have more than the single saw load, and the saw will typcially NOT be drawing anywhere close to Full rated amps 95% of the time. This is true because your hobby guy is not going to be resawing continuous harwood 4x4s for more than a few minutes at a time. Certainly not for 3 hours. My table saw is rated for 15A but idles at about 6 Amps and doesn't go above about 8 for cutting 1x material.

[LarryG]

your reference is what a electrical contractor told you.

No, it's not. It's what a licensed electrical engineer, who designs the electrical systems for the commercial buildings that we (and others) design, told me.

He didn't say that the breaker will immediately trip if the breaker passes more than 80% of its rated capacity. He said that the manufacturers will only guarantee that the breaker won't trip for the specified period (one hour or three; again, can't recall, but I can find out). After that, it is subject to tripping.