5

I'm getting ready to install a new, dedicated 240 V circuit for a 4 hp table saw that I just bought1. The saw hasn't actually arrived yet, so I'm working off information in the user manual I found on the product's website.

The wiring diagram in the manual shows a 6-20 plug (snippet below). The power requirements elsewhere in the same manual shows that a 30 A breaker is required (snippet also below).

But 210.21(B)(1) of NEC states

Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.

Exception No. 1: A receptacle installed in accordance with 430.81(B).

Exception No. 2: A receptacle installed exclusively for the use of a cord-and-plug connected arc welder shall be permitted to have an ampere rating not less than the minimum branch-circuit conductor ampacity determined by 630.11(A) for arc welders.

430.81(B) states

Portable Motor of 1⁄3 Horsepower or Less. For a portable motor rated at 1⁄3 hp or less, the controller shall be permitted to be an attachment plug and receptacle or cord connector.


After several hours of extensive Googling, I've discovered a few things:

  1. 20 A < 30 A
  2. 4 hp > 1/3 hp
  3. Table saw ≠ arc welder

Question

How do I satisfy the power requirements for this machine and stay code compliant? The 30 A breaker (with 10 AWG wire) and 6-20 plug seem to violate code. Is there something I'm missing? Are there exceptions for situations like this?


From the user manual, page 6:

power requirements from user manual

and page 17:

clip of wiring diagram from user manual


1 Actually, I'm getting ready to install a sub-panel for the whole workshop. This circuit will be in that sub-panel.

6
  • Will let the experts answer, but usually the installation instructions are what you must use, if the device(saw) is listed by one of the testing labs(UL, CSA,ETL). If you are using NEC then the CE label is not recognized.
    – crip659
    Jul 6, 2023 at 15:04
  • Motors have strange rules, so probably fine as is, though I would then label the rceptacle "cabinet saw only". But seriously: this thing weighs 600 lbs. You are not moving it anywhere. Hard wire it. That solves the "receptacle != breaker" problem because no receptacle. Jul 6, 2023 at 16:02
  • @keshlam I was using the designation "240V" pretty loosely. diy.stackexchange.com/a/167438/116928 Jul 6, 2023 at 17:28
  • @jay613 I considered that. Without knowing for sure, I'm assuming the 30 A breaker is specified just due to inrush current. Note in the table that the nominal current is 16 A, so I'm not super worried about the plug being sufficient. Although swapping out GFCI breakers until I find one that works is a little pricey of an experiment. Jul 6, 2023 at 17:30
  • 1
    Typo corrected: Mine is in that weight range and is on a mobile platform. But mine is also currently configured as 120V. I do expect to put in a 240 outlet for it but am in no rush –
    – keshlam
    Jul 6, 2023 at 17:36

1 Answer 1

3

After several hours of extensive Googling, I've discovered a few things:

Long enough to read most of a book. Like I always say, googling doesn't work: Google only answers questions, it doesn't tell you which questions to ask. For that, you need a well-rounded primer on the subject. A book.

Heck... NEC 90.1(A) even says it's a Code violation to use NEC as an instruction manual for untrained persons :)

What's got you confused is the statement "ampere rating not less than the branch circuit". You're going "well that must be the breaker then", and it's not. It's the wire. See Article 100.

So, 210.21(B)(1), Receptacle is 20A, branch circuit (wire) is 20A, we're all good. (B)(2), (B)(3) and 210.23 do not apply since it's a 1-socket circuit labeled "4 HP saw only".

Now how big can the breaker be? Off we go to 240.3, which refers you to the relevant code for a long list of specialized equipment. Motors, motor circuits and controllers refers you to Article 430. This gets restated again in 240.4(G) pointing you to Article 430 parts II-VII again.

And 430 says consult the equipment nameplate because UL will have specified maximum breaker when certifying the motor (based on performance of its onboard overcurrent protection).

The minimum breaker is also on the nameplate, and/or called out in NEC 210.23(A)(2) since remember, nothing prevents you from plugging that saw into a multi-outlet 240V/20A branch circuit. 16A ampacity is 80% of 20A, so it's welcome on a 240V/20A multi-outlet branch circuit which is breakered at 20A per Table 210.21(B)(3).

Yes, there is a range of allowable breakers. Why is that? Because with larger motors, overload protection tends to be on the motor itself, where it has able to be savvy to the motor's temperature. When that is so, the breaker no longer needs to provide overcurrent protection, and only needs to protect against hot-neutral shorts or hot-ground shorts. They allow larger breakers to reduce nuisance trips from (redundant) overcurrent protection. If a 20A breaker holds to your satisfaction, you're perfectly welcome to install it on a 20A circuit including a multi-outlet circuit, and then the rest of 210.21 and 210.23 apply.

For a cord-and-plug connected appliance, The only thing the "dedicated circuit" really buys you is the right to over-breaker to reduce nuisance trips.

7
  • Thanks for the detail and cross references. So if I understand this (and 430.52) correctly, a 6-20 plug, 12 AWG wire and a 30 A breaker will satisfy code based on the information on the nameplate? Jul 6, 2023 at 21:18
  • Also, "The only thing the 'dedicated circuit' really buys you is the right to over-breaker to reduce nuisance trips." That's not nothing. Jul 6, 2023 at 21:18
  • @Gern that's what I mean, yes. Now is your saw actually tripping 20A breaker under normal operating conditions? (you can't very well count a stall). Back in the day I woulda said "try it", 20A breakers are $10 and you can just test with a couple of 20A singles you already have (240V-only loads only require a handle tie). But now with NEC 2020 requiring GFCI breakers in many jurisdictions who did not delete that requirement, "try it" no longer makes sense. Jul 6, 2023 at 21:43
  • 1
    @Gern well since you're wiring the shop anyway, here's food for thought. Ever hear of a Multi-Wire Branch Circuit or MWBC? It's 2 hots sharing 1 neutral. Normally they're used to cheap out on wire by getting dual 120V circuits out of a single /3 cable (or 3 wires +ground in conduit). But they have a superpower: they can also power 240V loads. So you might consider using 20A MWBCs around the shop and fitting both NEMA 5-15 or 5-20 and also 6-20 outlets on it. It would let you try the saw on your regular outlet circuits and spare installing the dedicated saw circuit if it works out. Jul 6, 2023 at 22:11
  • 1
    Passed inspection today. Aug 7, 2023 at 13:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.