Let's talk electrical codes, AWG, and breaker sizes

[Anthony Whitesell]

Hi Bob, not to dampen your day but you have a problem.

20 Amp breakers max AWG is 12 gauge

30 Amp breakers max AWG is 10 gauge

You can use 10 gauge on a 20 amp breaker, but not 12 gauge on a 30 amp breaker.

If you have a 12/2 wire fed by a 30 amp breaker you are outside/violating the NEC.

[Rob Russell]

What Anthony said.

The real issue is that - if the machinery really pulls close to 30A through the #12 conductors - you have the potential for the wires to overheat and that's a potential fire hazard.

[Bob Albrecht]

OK, then will the 12-2 carry the load of the saw and the 3HP/ 230V planer if I switch to a 20A breaker? Oh and they're not run at the same time either, one of the other as it's a single recepticle.

I really don't relish the idea of pulling wire thru the conduit for this line as let's say, it will be a tremendous pain in the you know what!!!!

[Rod Sheridan]

Hi Bob, there isn't much use in discussing the HP of the motor, because the full load current of motors can vary 50% depending upon speed, efficiency and power factor. You need to look at the nameplates on the motors and check the FLA (Full Load Amperes) to determine how much current your motor will draw at full load.

My saw is 3HP, 12.3 amperes full load, and will run on a 15 or 20 ampere circuit. (If it ran at 100% load constantly, it would need to use a 20 ampere circuit, however that's not likely to happen at home).

Since you have #12 conductors, you can use a 20 Ampere breaker and be code compliant.

The starting current of your saw will be much higher than the 2 X figure used earlier, it may range from 5 X to 10 X depending upon the motor and circuit impedance. Don't worry about that, the 20 ampere breaker will be able to handle it.

For a cord on your saw, I would reccommend a 12/3 heavy duty flexible cord, available at any supplier.

Note that flexible cord counts the ground wire so 12/3 will have 2 current carrying conductors, and a ground.

12/2NM (or Romex) will have 2 current carrying conductors, and a ground. Kind of confusing, however many trade designations are legacy issues.

regards, Rod.

[Bob Albrecht]

OK Rod, but this the planer is what concerns me more. Is that going to be an issue?

[Chuck Saunders]

Not wanting to start anything, just a question, are you allowed to pull nm (Romex) through conduit? I had it in my head that conduit was for single conductors if the conduit was connected box-to-box. Is any of that correct or did I dream it all up.

[Bob Albrecht]

Chuck,

Don't ask me about codes as I'm not an electricia. I know how to wire switches, recepticles and breakers, but that's it.

All I know is this line is in pvc conduit under concrete, it ain't a straight run at all and it will be a pain in the a** to pull a new line. That's why I asked.

My original issue is this, the present line is 12-2 on a 30A breaker (no idea why it's 30, been that way for years), it's only about 25'-30' from the main panel and it was rarely used.

I want to run a saw and a planer that has a 3HP/ 230V motor and wanted to know if that present line would handle it. It APPEARS it will, but it also APPEARS I should replace the 30A with a 20A breaker.

[Anthony Whitesell]

What does the nameplate on the machine or motor state for an amperage requirement?

Wire is based on amperage and not on horsepower. If you did a search, you'd find there is a large "discrepency" in the correlation between amperage and horsepower.

[Jim Mims]

Wow. Feeling like I may be stepping into it, but here goes.
Yes the code limits you to 20 amps on 12 gauge wire.
Yes the code limits you to using 80% of a circuit breaker's rating.
Yes the code limits you to 75 degree centigrade for almost all applications not utilizing specialized connectors.
In fact, I'd even limit it to a 60 degree centigrade rating because you're in the floor (no air flow) and you have multiple circuits in the same conduit.
However, the NEC also allows you to up the circuit breaker size to limit "nuisance tripping" of a dedicated motor circuit.
So, it is permissible, and well within the code, to use a 30 amp breaker if you experience occasional tripping.
This reasoning is also in the code because the motor is going to have internal current limiting devices, either thermocouples or thermal "heater elements" in the motor starter that will stop the motor before it burns up.
With all that said, here's the over-riding factor:
How many amps are you pulling?
Your installation, with 12 gauge wiring, on a 20 amp circuit breaker, pulling a nominal 10 amps will be fine.

I don't have my NEC code book with me right now, but tonight I can look up and site the code section dealing with motors.

- Jim

[Lewis Cobb]

Chuck,

Don't ask me about codes as I'm not an electricia. I know how to wire switches, recepticles and breakers, but that's it.

All I know is this line is in pvc conduit under concrete, it ain't a straight run at all and it will be a pain in the a** to pull a new line. That's why I asked.

My original issue is this, the present line is 12-2 on a 30A breaker (no idea why it's 30, been that way for years), it's only about 25'-30' from the main panel and it was rarely used.

I want to run a saw and a planer that has a 3HP/ 230V motor and wanted to know if that present line would handle it. It APPEARS it will, but it also APPEARS I should replace the 30A with a 20A breaker.

Bob - seems like your situation is exactly the same as mine except you have a 30A breaker and I have a 20A one. From my calculations and listening to the folks here that know more about the codes etc. than me, I would say we're going to be fine - but you should go to a 20A breaker to keep the circuit within code.

Cheers,
Lewis

PS - I am an electronics engineer so I can run calculations to check things and they look fine to me - however, the temperature rise / code issues are not something I am familiar with which is why I posed the question. Even with my knowledge of electricity, I seldom shoot from the hip with household wiring without asking an electrician if it's something out of the ordinary.

[Lewis Cobb]

What does the nameplate on the machine or motor state for an amperage requirement?

Wire is based on amperage and not on horsepower. If you did a search, you'd find there is a large "discrepency" in the correlation between amperage and horsepower.

You're absolutely correct but I don't have the tools yet and the manual does not have this info - perhaps I should pose another question in case some of you have these machines -

What's the name plate amperage data for a Powermatic PM2000 3HP saw and a Powermatic 15HH 3HP planer ?


Lewis

[Jim Mims]

OK, putting the codes aside, lest me ask this.
I bought a new saw and planer. The old saw & planer are going to be used for rips and rough planing.
Where I intend on putting these old machines is fed by a 12-2 line on a 30A breaker.
The saw won't be an issue, but the planer is a 3 HP, 230V unit. Can this line still handle this load?
And please don't use all that electricaleze language, it hurts my head.

Yes, you'll be fine if you leave things as they are. Once again, I'll post a citation from the NEC code tonight.
But, I have to question whether you really need the 30 amp breaker?
Why not try a 20 amp breaker and see if you get any nuisance tripping?
When all's said and done, it's about keeping the wire insulation from melting.
That's why we have different ratings for different types of insulation.
12 gauge wire, run for a sufficient amount of time in excess of 20 amp, does present a danger of melting the insulation off. If your 3 hp motor is pulling around 10 amps, or for that matter, any value up to 16 amps, which is well within the rating of the wire, it'll be fine.
- Jim

[Jon Crowley]

Not wanting to start anything, just a question, are you allowed to pull nm (Romex) through conduit? I had it in my head that conduit was for single conductors if the conduit was connected box-to-box. Is any of that correct or did I dream it all up.

I am not positive that it is a code requirement, but is definitely good practice to NOT use romex within conduit. There are many debates about heat issues from having both the romex outer covering and the conduit. Is it a real issue? WHo knows... but it is MUCH easier to pull individual stranded wires, so you'll want to go that route anyways.

It is OK to put romex within conduit for physical protection over short distances. Picture an unfinished basement, and conduit runs vertically only a few feet from the ceiling joists down a concrete wall to an outlet.

[Jim Mims]

Ahh, here's a good treatise on the subject of motor circuits:
http://www.iaei.org/subscriber/magaz...c/johnston.htm

- Jim

[Chris Friesen]

Firstly, I assume you mean 12/2 with ground, which I would call 12/3.

The NEC says that the ampacity of up to three wires in cable for 12 AWG is either 25 A or 30 A, depending on the insulation type. For example, THWN is 25 A and THHN (the most common modern wire) is 30 A.

12/2 means 2 insulated conductors, plus ground.

Your ampacity numbers are correct, in that you can start with that value when derating for temperature and multiple conductors in a conduit. However, there's another table in the NEC that limits #12 to 20A for regular branch circuits.

However, there's yet another rule that allows you to upsize the breaker for motor circuits. Whether or not an outlet for a cord-and-plug connected 240V woodworking tool counts as a "motor circuit" for the purposes of code compliance is going to be up to the local inspector.