"> Don't have my code book handy to verify; however, the use of a #10 conductor
The plate on my 5 HP motor indicates that it draws 20 Amps at full load. If it was a motor that operated for extended periods at full load, or on a long wire, I'd agree with your thinking on the I squared R loss. But my table saw will rarely load the 5 HP motor, and the run of #10 wire is less than 20 feet. Since I have a pretty good feel for the tradeoffs involved, I elected to use the smaller wire, not because of cost, but because it's easier to pull. Others may certainly choose another path.
DonkeyHody Even an old blind hog finds an acorn every now and then.
well, i cant vouch for that particular motor, but imho a 40 amp slow blow fuse on a 5hp table saw motor is kind of like saying, ok, once the thing catches on fire and explodes and the blade has spun out of control and the power wires are actually touching and welding together, we better stop supplying current to it
i would bet that saw has internal protection that blows long before that fuse does.
Well shucks. Y'all made me get out of my chair and go look it up. Check out Section 430, Part D - Motor Banch-Circuit Short-Circuit and Ground-Fault Protection. Paragraph 430-52 (b) All Motors. "The motor branch-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor." (c) Rating or Setting. (1) "A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table
430-152 shall be used." EXCEPTION NO. 1 (Paraphrased) Exception 1 allows you to go to the next size up if you are between sizes. EXCEPTION NO. 2 (Quote) "Where the rating specified in Table 430-152 as modified by Exception
1, is not sufficient for the starting current of the motor:" "(a) The rating of a nontime-delay fuse or a time-delay Class CC fuse shall be permitted to be increased, but shall in no case exceed 400% of the full-load current. "(b) The rating of a time-delay (dual element)fuse shall be permitted to be increased but shall in no case exceed 225% of the full-load current. "(c) The rating of an inverse-time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes."
The commentary in the handbook states: (Quote) "It should be noted that it is not necessary to size the branch-circuit conductors to the percentages (150 to 300) permitted for the branch-circuit short circuit and ground-fault protective devices." . . . "During a short-circuit or phase-to-ground condition, the extremely high current causes the protective fuses or circuit breakers to open the circuit. Excess current flow caused by an overload condition passes through the overload protective device at the motor controller, thereby causing the device to open . . ."
DonkeyHody Even a dumb old country boy gets it right once in a while.
Well, now you caught me without my code book but that seems to imply that it is a dedicated, hardwired motor, being supplied by a motor controller that incorporates both instantaneous (like an MCP) and overload protection AND that it pertains to the wiring between the motor controller and the motor.
I could be wrong, but I doubt you could convince a residential electrical inspector to pass an installation with #10 wire being protected by a 40A breaker.
Al, That section is specifically NOT about the factory installed wire between the factory installed motor controller and the motor. I went to the trouble to look up the reference for you and even took the time to quote chaper and verse. I'm not going to type the whole chapter for you. If you want to argue context, at least go to the trouble to read the chapter so you'll know what your're talking about.
Now I remember why I mostly just lurk here.
DonkeyHody We should be careful to get out of an experience only the wisdom that is in it - and stop there; lest we be like the cat that sits down on a hot stove-lid. She will never sit down on a hot stove-lid again---and that is well; but also she will never sit down on a cold one anymore. Mark Twain
Hey DonkeyHody, don't get your drawers in a knot!! I thought we were having a discussion. As anyone that has dealt with the NEC or other NFPA publications, interpretation is everything. And I have found that there is no universal interpretation of all aspects of the code.
I was merely expressing my doubts as to the wisdom of violating the 30A on a #10 wire rule in section 310. Most inspectors get hung up un that one and usually don't budge. Perhaps, my designs are too conservative, however, I have never had one rejected by a PE review before installation or by an inspector during/after installation.
Also, I started my comment by stating that I did not have my code book in front of me.
-- 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
dedicated, hardwired motor, being supplied by a
protection AND that it pertains to the wiring
inspector to pass an installation with #10 wire being
Given that a breaker exists only to protect the wire connected to it and that is fact:
1) a 30A breaker connected to and protecting #10 wire will give (by your NEC code) 30 x 80% = 24 amps of continues current
2) You must get the FLA off the nameplate of the motor. Motors are ALL built with different efficiciencies and housing structures - therefore the only accurate current draw is that which was tested at the factory on their design model. Motors, at startup typically drew about 6 x FLA at startup, but today motors can easily hit 12 x FLA at startup because of lighter housing, thinner internal wiring etc - in otherwords the motors have been cheapened up.
3) If you motor nameplate, and it read 22 amps FLA - then you'd have 2 amps left over and you are good to go. If the motor nameplate on the other hand said 26 amps FLA you need to revisit the breaker/wire combination. It will still work but likely it will trip, maybe not for hours (only the breaker trip curve could answer that question) - the code can't stop you from attaching a 35 amp load to a 30 amp breaker combo - it just won't stay live very long.
The code derates by 20% to give you the easily calculated values that allow you to enjoy continuous power on a circuit. The derating comes from the fact that circuit breakers are thermal devices (BTW so are fuses) in that they have as part of their protection system, a bimetallic element that works on heat principles. Once you put a breaker in an enclosure, heat can build up and trip the breaker. Enclosure are build with minimum cubic inches of "cooling space" and in conjunction with the 20% derating rule offer you a product that can supply current on a continuous basis. Put that product in the boiler room of a ship, where the ambient temp is high and the breaker will trip sooner. Same applies to a "hot" woodworking shop - this is why the
20% derating. If you want to spend the money, you can buy 100% rated breakers which are certified as such and you can draw 100% of the current. These breakers start at 400Amp and go to 6000Amp.
Anyway, I digress - don't try to re-engineer some that has been designed for you and enforced by the NEC:
Once you pick the right size "service" - make sure you have the proper device to protect your motor - the upstream breaker has nothing to do with that - but that a lot more typing and I'm not good at that.
I have the exact same Unisaw. I connect my saw to my 30 amp dryer connector via a 30 amp twist lock converter and I have never experienced any issue. I also have a 24" 5hp delta x5 planner that I also connect through my dryer outlet and also have never had an issue.
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