15 amp power tool vs. 13 amp power tool on in-place 15 amp circuit

[iantwright]

I'm renting a house for the next 18 months, and am looking to buy a circular saw. I'm down to a 15 amp PC saw vs. a 13 amp Makita. I like the 15 amp PC a little more, and it is actually a little cheaper than the Makita, but I don't want to trip the breaker on my circuit when I use it heavily. I understand that at idling and easy cutting, saws won't draw close to max power, but when you increase the resistance and RPMs that you get closer to the 15 (or 13) amps. Is the 15 vs. 13 amps the maximum current that the saw will draw when fully loaded where if you were to increase the resistance, the saw wouldn't be able to cut further?

I'm just a DIY type, and so heavy cutting is cutting through a pressure treated post, or something along those lines. I've read several threads and they were all very informative, but none really answered my question. Several people mentioned that they used 15 amp tools in the field off standard 15 amp circuits with no problems, but that doesn't seem too scientific to me, although helpful. Most responders just recommended installing 20 amp circuits in the shop, and that isn't an option for me. I don't want to trip the breaker when using a 15 amp saw, but will get the 13 amp saw if that is what will happen. I suppose I could get the 15 amp saw and just not punish it to the point of drawing 15 amps (don't force it through tough wood, etc), but it'd be nice to not have to worry about it at all.

So basically two questions:

1) Is the 15 vs. 13 amps the maximum current that the saw will draw when fully loaded where if you were to increase the resistance, the saw wouldn't be able to cut further? 2) Will using the 15 amp saw trip the 15 amp breaker in my garage?

Thanks everyone.

[Leon]

In many cases the heaviest amp draw is during start up. With no load that drops down to a minimum and increases as you add more load. Chances are that 15 amp is not going to be any more troublesome than the 13 amp saw. IIRC the stated amperage is the most used under heavy cutting conditions unless you push the sat too hard. If you stall the motor you will likely draw more amps and trip the breaker if you dont' switch it off quickly. Either way I have never blown a 15 amp breaker with any 15 amp tool. IMHO you may be more likely to trip the breaker by running the 15 amp saw, and a shop vac at the same time. For instance on a 15 amp circuit I run a 15 amp router, a shop vac, fan, and lights at the same time. IF my 15 amp compressor comes on the at the same time the breaker trips after 2 or 3 seconds.

[Teamcasa]

I just tried a new Bosch saw, 1677MDT. It is a 15amp saw and man was I impressed! Since Bosch purchased Skil they have remade the tried and true MAG77 into this sleek new saw branded with the Bosch label.

We tested it on a Honda Generator in Mexico, the 15 amp circuit breaker never tripped once. But after a hard day of framing, the saw and generator worked great. I'd suggest this saw. Its the real deal.

Dave

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[bent]

sorry, I have not read any o/ posts.

the fuse blows to protect the wire only, its a pull thing, not a push. You do not overload the anything in the box. The reason for the fuse is to protect the wire only. Thicker wirer, same position, higher fuse. Wires get hot. Fuses are protection. Thinner wires are cheaper. period. You can get a time delay fuse that takes the fuse a little while to blow at the rating, a free overload time period. Not sure if in milliseconds, or seconds. Get em a at HD, etc., anywhere

[Toller]

15a is the maximum it will draw during normal operation; during the heaviest cut it can make. If you increase the difficulty of the the cut, it will stall.

If you have nothing else on the 15a circuit, it should not matter. If you have lights and a fan, the 15a is more likely to trip the breaker. One thing to consider is the length of the circuit. Even if it doesn't trip the breaker, if you are a long way from the breaker box there could be enough voltage drop to cause a problem. In that case you are better off with the 13a. Lights etc. will exacerbate the problem. Do a search on voltage drop if you don't know what I am referring to.

[barry]

Since most mfgs' ratings of current draw and power output are meaningless, as in: what happens milliseconds before armature is brought to a standstill, and essentially inflated, I'd not let current-draw-rating decide anything. Just so's the mfg is not on record stating that a 20a circuit is required.

Sustained 15a draw on a 15a circuit is a _bad_idea_ anyway.

Breakers are not precision devices, either.

You _do_ want to reduce voltage drop, as "arriving" at the saw. The longer the extension cord, the more you need "heavier" cable _and_ connectors.

J

[chuck]

Generally, a tool rated at 15 amps will work fine with a 15 amp circuit breaker. Whether you can have other loads on the circuit while using the tool depends largely on whether you are pushing the tool to its maximum power. A 15 amp breaker is supposed to be able to carry 15 amps continuously, which, I guarantee, your circular saw will NOT do! That is not to say that the saw will never trip the breaker. But if it happens, it should happen very rarely.

Remember that the 15 amp rating could be just advertising hype, intended to convey a sense of "raw, brutish power" to a prospective buyer. Wouldn't be the first time. I don't think there's anything to prevent a manufacturer from overstating maximum current draw. Understating it could be a serious problem, of course, so on which side do you suppose they err?

I would suggest that you 1) try to determine whether any other loads are on the circuit(s) you intend to use; and 2) buy the saw from a firm that will allow you to return/exchange it if it doesn't work out for whatever reason. Then go for the one you hope will last a lifetime.

Good luck

Chuck

[e-mail] wrote:

[CW]

[CW]

Yes, there is. Horsepower is a different thing, however.

[Chris Friesen]

Actually, I think you're not supposed to exceed 80% of the rated amperage for continuous use.

Chris

[chuck]

Hello Chris,

Well, the NEC is sometimes oversimplified so as to suggest the 80% rule you mention. Some points:

Breakers are designed and tested under UL procedures to carry 100% of their rated load continuously.

The NEC specifies the 125%/80% rule ONLY for continuous loads. These are defined as lasting for three hours without interruption. If the OP's saw draws 15 amps continuously for three hours (and I doubt that it is capable of that, but even if it is, the OP is probably not) then, and only then, will it violate the rating of the branch circuit under the NEC. In no case should it trip the breaker, provided the breaker is not defective.

I know it's a little confusing. There's a very good explanation of all this here:

a circuit breaker.

Chuck

Chris Friesen wrote:

[chuck]

Hello CW,

I think you're off a bit on both points.

Power tool horsepower ratings are fabrications and comparisons between one tool and the next are meaningless when based on published HP ratings.

In order for the UL rating on amps to make any sense, it must mean that the tool will not, under any circumstances defined by the manufacturer and UL, exceed the listed amp rating. So if it says 17 amps, it is entirely permissible for the tool to NEVER draw more than 16 amps. The user is appropriately notified by the label that his branch circuit must be able to handle a maximum of 17 amps. That's reasonable and safe, and is what I suggested in my initial post. It does not mean the tool will or could draw 17 amps.

You can find a more thorough explanation of UL standards for amp ratings on corded power tools here:

of the Trade: Playing the numbers: performance standards would benefit toolmakers as well as tool buyers. So why does the industry continue to shirk them?

Chuck

CW wrote:

[Chris Friesen]

Excellent information. Thanks for the clarification and the link.

Chris

[Mike Berger]

I have the Porter Cable and have never tripped a breaker. It's a well made saw, but it's very heavy.

[e-mail] wrote:

[Ian - DIY Extraordinaire]

Awesome info guys, way more than I needed. I've since bought Wiring Homes (For Pros by Pros) by Rex Cauldwell. What is everyone's opinion on that?

By the way, I went with the Porter Cable 324Mag 15 amp saw, reviews are great and it feels comfortable and balanced in my hand.

[CW]

The article is actually pretty accurate as to the U.L. amp rating. You seem to have misunderstood the testing procedure. A tools amp rating is the maximum current it can draw before the insulation melts, wiring burns, ect. The only way a manufacturer could limit amperage draw of a tool is with a fuse or circuit breaker. I can get a PC 690 to draw fifty amps or more easily, though it will only do so for fractions of a second before burning up. At fifty amps, I could rate that router as eight horsepower though that would be very misleading.

[chuck]

Hello CW,

I'm operating at a real disadvantage by not having access to the applicable (and expensive) UL standards. But according to the article, the amp rating is "the maximum allowable continuous current a motor can handle without exceeding temperature limits". The temperature limits are not given in the article and the actual load that produces the maximum allowable continuous current is not on the tool nameplate. The procedures probably do specify temperature rise limits.

So how could such a test be conducted?

Start with zero load and increase the load incrementally until the specified temperature rise is observed and stabilized. The current would be recorded and UL would require the manufacturer to post that number on the label. Unfortunately, the load that produced that current/temperature rise is not stated on the label. The testing doesn't appear to be destructive as you suggested.

This is all hypothetical, of course, but I hope reasonable.

I have some problems with the scenario I just described. First, it seems to reward tools with heavy, metal cases that can quickly dissipate heat generated by the motor, and it seems to penalize tools that incorporate high temperature insulation and other components along with lighter weight (plastic?) housings, etc. So a manufacturer makes a lightweight, double-insulated tool that can withstand higher temperatures but is nonetheless penalized for those higher temperatures. Make sense to you? Not to me.

This does not seem to be a game tool manufacturers should be anxious to play. Therefore, I suspect that there is more to it than has been revealed, and that manufacturers have an opportunity to "game" the system. The fact that virtually none of us consumers has seen UL 745-1 adds fuel to that notion. Also, the only way a user could take advantage of knowing the amp rating is to measure current while the tool was in use and try to translate the varying load/current pattern into a continuous load/current equivalent and then what? What the rating does tell us is that if you can figure out how to make the tool draw its rated current, it will operate at that load "continuously" without melting insulation? without shortening motor life? without exceeding some arbitrary rise in temperature?

I would be reluctant to conclude that the tool with the higher amp rating 1) will last longer; 2) will develop more peak power; or 3) will be more appropriate to some application than the tool with the lower amp rating.

I admit it is not easy to define a single, universal parameter or test procedure that captures what are important performance measures for a portable electric tool. I am not convinced that the amp rating does it or even that it is better than nothing.

So I guess my problem is failing to understand, rather than misunderstanding, the article. Maybe someone technically involved in the rationale behind the UL amp testing procedures can illuminate the area for us.

Chuck

CW wrote: