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
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?
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
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.
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
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
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.
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_
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
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.
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:
Tools 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?
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.
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
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
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
I know it's a little confusing. There's a very good explanation of all
Sizing a circuit breaker.
Chris Friesen wrote:
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
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.
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