A-B series 193 motor overloads come equipped with wire stubs to attach them to A-B motor starters. I measure these to be 12 ga. solid wire.

How can an overload with a range of 12 to 32A be fed with 12ga. wire?

(The trip point is 120% of dial setting, according to the label on the side of the OL case.)

Table 310.16 (2002NEC) shows 12AWG THHN rated at 90 deg C to have an ampacity of 30A*.

This table refers to 240.4(D) with the asterick.
Table 240.4(G) refers to Article 430, Parts III, IV, V, VI, & VII for motor and motor control circuit conductors.

How, when, the calculations for sizing branch circuit conductors for, let's say, a 20hp 460V 3ph AC Induction-Type Squirrel Cage motor with a FLC of 27A (from Table 430.150 2002 NEC), multiplying by 120% gives us a value of 32.4A. Refering to Table 310.16 shows 12AWG copper THHN rated 90deg C as 30A allowable ampacity.

It appears to me that 10AWG would be required for branch circuit conductors, so, how can the Overload device be used as supplied with 12AWG and a max setting of 32A. (And a trip setting of 120% of indicated dial setting.)

Am I wrong here?

You're talking apples and oranges here. The NEC applies only to the installation of appliances, not the internals of the appliances themselves.

While you've got your micrometer out, check out a replaceable element fuse. You will also find a "conductor" a lot smaller than what your understanding of code requires. Some things are supposed to get hot. That's how they work.

Even if you were to apply the code, there are at least ten exceptions recognised by the code. Several of them relate to motors.

I'll congratulate you for reading the code- few even try. I suggest that you try to look at the NEC as a whole, and not get too hung up on any one part. And- don't forget the introduction, which outlines what the code is intended to cover, and what limitations there are.

bp-redbear:

Also consider that the length of the 12ga wires on these overloads are less than 3" long. I'm positive that the voltage drop in such a short length of wire would be negligible by Ohm's law even at 30-plus amps of load. Thus no worries of overheating.

I've used several of these overloads for different applications (Two for a dual pipe organ blower starter I designed and several for other fan and machine apps.) They've proven their value with reliable performance and no false trips. The phase loss feature is the best!! And to boot, the cost is actually reasonable.

A-B also makes a DIN-rail terminal kit so they can be used with other brands of contactors. (Also very inexpensive!)

What is meant by the phase loss feature?

I have, also, had good results with many of these A-B products, most of the motors in the food processing plant where I work are protected by these.

Except for a few of them tripping continuously at lower settings than indicated on the dial, but, in reality these failures were most surely due to the water that our cleaning crew gets in our panels.

If you've ever tried to run a 3ph motor on two phases it becomes pretty obvious. All of the smoke comes out in a hurry. These cool little gizmatron's won't turn on, or will turn off if one of the three phases drops out. Cheap insurance for the motor.

TW

Actually the phase loss feature is pretty primitive when compared to phase loss relays. What these OLs have is really phase loss sensitivity. They have a common trip bar that moves at a constant "speed" when all three phases see the same current, if the currents are not balanced (i.e. single phasing) the trip bar twists resulting in a faster trip. I have seen many of these OLs used on single phase applications with absolutely no problems, nusiance tripping only occured when the motor was almost fully loaded.

Actually, even under the Code, 90 degree #12 wire has an ampacity of 30 amps. It's restricted to 20 amps, if I understand correctly, simply to provide an extra safety margin on circuits that are often played around with by unqualified personnel.

I was unaware that these A-B overload devices had the phase loss feature.

I am aware that if one phase drops out, a motor may continue to run, but will not re-start, and will be subject to damage if tried to be re-started. Or, if it is started with one phase lost, the motor may be damaged.

So, these Allen-Bradley series 193 overloads will only allow a motor to be started if all three phases are present.

Will they trip if a phase drops out while the motor is running, even if all three were present when it was started?

Thanks for the replies.

BP

yes, there are dimples that protrude from the mounting surface, but, not quite a quarter inch, more like an eighth or sixteenth.

Thanks, People, I feel safer after reading the replies.

BP

Woops, disregard last post by me, it was meant for the thread "24in. flourescent-too hot?"!!!

Anyone ever done that before?

Don't mean to hog this thread, (not talking to myself!!), but even the A-B OL's that are rated up to 45A have 12AWG.

So, apparently 12AWG can safely carry 45A?

Seems like the 12AWG would get hot, resulting in tripping the overload, but maybe that's how they are designed to work, as John Stienke has stated.

Seems wierd to me, what about the code article about industrial machinery (Art. 670 2002NEC), or NFPA 79 dealing with industrial machinery?

Yes many of us have posted to the wrong thread at least once, nothing to worry about.

I do not mean to sound harsh but you really just have to forget about 310.16 relating to the wire on these OLs.

310.16 has nothing to do with what manufacturers do. That is not bad, the manufacturers equipment is tested by UL or some other organization to make sure it will work without a danger.

Again forget about the NEC relating to what the manufacturers do. It does not apply to them and would really mess things up if it did.

That said 310.16s ratings are based on Insulated Conductors Rated 60°C Through 90°C Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)

The fact the conductors are insulated makes a huge difference. The fact they may be bundled in a cable makes a difference.

On the other hand a bare 12 AWG copper in 'free air' can handle, without melting out, much more than 45 amps.

310.16 ratings are based on keeping the conductor cool enough not to damage the insulation.

Back to the OLs, in order for the OLs to do the job they are supposed to, they must get hot. Many people refer to OLs as overload heaters for this very reason.

By selecting the correct 'heater' the bi metal strips in the OL contact block will be heated to the point the contacts open if the motor starts drawing more amps then the heater is designed to allow.

Bob

For what it is worth, I have read that 14 AWG copper will melt out at about 200 amps.

Of course long before the copper melts out the insulation would be smoke and ash.

Bob

[This message has been edited by iwire (edited 11-30-2004).]