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

[Tom Veatch]

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

- Jim

But that begs the question of "What is a 'motor circuit'"?

Is any circuit that has a device containing an electric motor plugged into it a "motor circuit"?

If it is, then what about that same circuit when there is no device containing a motor plugged in? Is it still a "motor circuit"?

If not, how can the same physical circuit be both a motor circuit and not a motor circuit?

[Jim Mims]

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

From the NEC
Article 430.6 "Ampacity and Motor Rating Determination"
(A) "General Motor Applications"
(1) "Table Values"
... Table ... 430.248 ... shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating on the motor nameplate...

Table 430.248 lists a 3 hp, 230 volt motor as being rated at 17 amps.

430.22 (A) Tells us the branch circuit wiring must be sized for 125% of this rating, or 21.25 amps.
310.16 lists #12 AWG 60 degree centigrade wiring as having an ampacity of 25 amps, so it satisfies the code.

Note that 240.4 (D) states "Unless specifically permitted in 240.4 (E) or 240.4 (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG and 30 amperes for 10 AWG copper..."

So, Table 240.4(G) "Specific Conductor Applications" lists NEC Article 430 for Motor conductors.

430.52 covers sizing the motor branch-circuit short-circuit protective device. Using a circuit breaker, it may be sized UP TO 800% of the motor current rating for single phase motors. In this case 17 amps times 800% = 136 amps!
(For a time delay fuse it would be 175%, which is the only kind of fuse I'd use on a motor circuit).

Now before you say that's crazy, I need to cite article 430.32(A). It requires all motors over 1 hp to be protected by one of the following:
(1) Separate Overload Device
(2) Thermal Protector
(3) A protective device integral with the motor.

So, to review, because when starting the lock-rotor current of a motor can be up to 6 times the full-load current rating, we need a way of letting the motor start without tripping the breaker, but at the same time, protecting the motor, and it's wiring, from overload, and that is provided by the motor overload device. For most of us, this will be the thermal protector integral to the motor. Larger motors, say 5 hp and up, will normally use a contactor which will have thermal elements integrated into it's manufacture.

- Jim

[Charlie Knauer]

Lewis, if this concrete slab is a ground slab the romex is an issue. The NEC considers this a wet location.
Wet location. Installations underground or in concrete slabs or masonary in direct contact with the earth, and locations subject to saturation with water or other liquids, such as vehicle washing areas, and locations exposed to weather and unprotected.
336-4 (a). Type nm cable shall be permitted for both exposed and concealed work in normally dry locations. It shall be permissable to install or fish type nm cable in air voids in masonary block or tile walls where such walls are not exposed or subject to excessive moisture or dampness.

An easy fix for your situation would be to remove the romex and pull type UF cable, or pull a suitable wire in such as type THWN.
I know the experts are going to say that the PVC conduit is dry, however it is not treated that way in the code and in reality it is not due to condensation.

Charlie

[Jim Mims]

But that begs the question of "What is a 'motor circuit'"?
Is any circuit that has a device containing an electric motor plugged into it a "motor circuit"?
If it is, then what about that same circuit when there is no device containing a motor plugged in? Is it still a "motor circuit"?
If not, how can the same physical circuit be both a motor circuit and not a motor circuit?

Excellent question!
Article 430 is really used for one motor and a dedicated circuit. As such, it's acceptable to "protect" 12 gauge wire with a 30 amp circuit breaker.
I like to use common sense.
You really don't want to create a situation where you have 12 gauge wire pulling more than 20 amps on a 30 amp circuit breaker.

In this thread, it's a little easier, we're dealing with 230v circuits and 110v branch circuits haven't been discussed. And as such, we would not have receptacles or lighting on this circuit.

But, let's assume we did want to discuss 110v branch circuits that may or may not include a motor. Well guess what! That's article 210 which covers branch circuits.

Article 210.1 Scope
"This article covers branch circuits except for branch circuits that supply only motor loads, which are covered in Article 430. Provisions of this article and Article 430 apply to branch circuits with combination loads."

When we think of the separation between 230v and 110v it becomes a little easier to figure out which is which. By code, branch circuits are not allowed to supply more than 110v to ground (article 210.6) and 230v circuits are rarely dedicated to more than one piece of equipment. Think dryers, water heaters and 3 hp table saws.

- Jim

[Mike Cutler]

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

No. It's not going to be an issue. I've had a Jet 3HP planer on a 12/2 circuit protected by a 20 amp breaker for years, and have never had a problem, and short of a motor fault, I don't expect to have one.

[Lewis Cobb]

Lewis, if this concrete slab is a ground slab the romex is an issue. The NEC considers this a wet location.
Wet location. Installations underground or in concrete slabs or masonary in direct contact with the earth, and locations subject to saturation with water or other liquids, such as vehicle washing areas, and locations exposed to weather and unprotected.
336-4 (a). Type nm cable shall be permitted for both exposed and concealed work in normally dry locations. It shall be permissable to install or fish type nm cable in air voids in masonary block or tile walls where such walls are not exposed or subject to excessive moisture or dampness.

An easy fix for your situation would be to remove the romex and pull type UF cable, or pull a suitable wire in such as type THWN.
I know the experts are going to say that the PVC conduit is dry, however it is not treated that way in the code and in reality it is not due to condensation.

Charlie

Charlie - interesting - especially seeing as I had a licenced electrician install this for me. Maybe the code is slightly different in this repspect in this part of Canada but in general we mimick the NEC to the letter.

I will ask him about this but your point on the single wires being installed is a good one. There's plenty of room if I pull the 2 Romex cables out, to place 3 x 10AWGs for the 240V and 3 x 12AWGS for the 120V. In fact, I had this idea earlier when someone else mentioned single wires.

I consulted an electrican earlier this year when I was installing some surface conduit on the walls for 10AWG wires for a future cyclone and the jointer etc. So I know where to go to get some of the correct wires in less than 1000' rolls (that the local fools wanted to sell me ).

Thanks very much for this information, I appreciate it.

Lewis

[Jim Mims]

Lewis, if this concrete slab is a ground slab the romex is an issue. The NEC considers this a wet location.
<snip>
An easy fix for your situation would be to remove the romex and pull type UF cable, or pull a suitable wire in such as type THWN.
I know the experts are going to say that the PVC conduit is dry, however it is not treated that way in the code and in reality it is not due to condensation.
Charlie

Charlie, I was wondering about that too.
Lewis says "Jason - it's the normal Romex that's used for baseboard 240V heaters - red outer sheath, 12 AWG wires inside - one red, one black, and a bare copper ground wire."
The only "Romex" suitable for use only in dry locations I'm familiar with is white. I wonder just what it is he does have and if it does have a wet location rating?
One telling feature, and part of why it's rating is for use only in a dry location, is the use of brown paper around the conductors inside the sheathing.
- Jim

[Dick Strauss]

Jim,
Can't any dry application wire (including NM) be placed in the PVC if the PVC is glued/sealed? Doesn't the sealed PVC conduit become a dry environment assuming it doesn't violate other filling rules (for heat dissipation purposes), etc?

I'm not an expert but just asking a question?


Thanks in advance,
Dick

[Charlie Knauer]

Dick, This is addressed in the NEC.
310-8 (c) Wet locations Insulated conductors and cables used in wet locations shall be: 1. Moisture impervious metal sheathed. 2. Types MTW, RHW, RHW-2, TW, THW, THW-2, THHW, THHW-2, THWN, THWN-2, XHHW, XHHW-2,ZW or a type listed for use in a wet location.

Hope this helps.

Charlie

[Charlie Knauer]

PVC conduit is not 100% waterproof due to condensation.

Charlie