Which current is used for rating AC induction motor leads 2

[Clyde38]

As an example, let's say that the motor is a 50 hp, 460 V, 3∅, 60 Hz and the current ratings are as follows:

Full Load Amps: 59.2 A
No load Current: 25.0 A
Locked rotor current (II/In): 6.2 (does this mean that the locked rotor current is 6.2 times the no load current? I'm not familiar with the II/In value. Is it the KVA/hp rating?)
Nameplate code: G

Calculating the locked-rotor current from the KVA code letter G (KVA per hp of 5.6 to 6.2) would be between 351 A to 389 A.

I can't see rating the conductors for the motor leads for locked-rotor current. It seems that full load amps times some safety margin is more appropriate.
I realize that a motor of this size would most likely have a terminal box, but the conductors between the coil wire and the terminal box have to be designed for a value. I guess I'm just a little confused by the nameplate ratings.

 

[dpc]

If you are in the US:

Motor circuit conductors (from motor to the starter) are sized at 125% of the motor full load amps listed in NEC Article 430 (or your actual motor FLA IF GREATER). For a 50 hp, 460 V motor, the FLA are 65 A (per NEC) and 125% of that is 81.25A.

Minimum conductor size is #3 Copper.

If you are talking about the motor leads from the motor terminal box to the motor windings inside the motor, that is a completely different story. This may be covered in NEMA MG-1 or it may be left to the manufacturer to size based on a performance specifications and dependent on the motor insulation level.

 

[electricpete]

The motor T-lead size is not specified in NEMA MG-1....left to the OEM or rewind shop.

=====================================
(2B)+(2B)' ?

 

[dArsonval]

“What current is used for rating AC induction motor leads?”

The answer is… The Operating Current of the Apparatus.

In using fingers (wet or dry) touching the conductors to feel…
~WHATS THERE!!!~~ in the way of… [Difference of Potential and Current].
The insulation system surrounding the conductor dictates what it will handle.
Say What? : )

Numerous Charts have been published for decades based on the popular insulation systems in the market at the time.

Today, the disclaimer is something along this line… “It is best to use the lead wire size originally specified by the motor manufacturer.”

If you’re seeking a [boiler plate] answer to your question… it will only apply to the insulation system surrounding the conductor.

Is it Air, Neoprene, PVC, Teflon, Hypalon, Silicon, Rubber, Glass, Ceramic, Wood? And so on.

electricpete calls it. “It’s left to the OEM or rewind shop.”

John

 

[Clyde38]

Thanks to all . . .

dpc has given me most of what I'm looking for. I know how to determine what size wire, insulation, etc. What I was having trouble with is some of the various terms used in name-plating and the specifications associated with these types of machines. For instance I'm not sure what "Locked rotor current (II/In): 6.2" actually means. I don't see any correlation to locked rotor current in any of the specifications.

 

[jraef]

So where are you Clyde38? In the US? If so, what dpc said is actually the only thing you can legally use for determining the motor branch circuit conductor sizing.

Unfortunately using the term "leads" is unnecessarily confusing in that depending on what part of the industry you are in, it can have different meanings. If you are in the motor or motor repair business, "lead" refers to the wire coming off of the rather brittle magnet wire in the winding to the connection box, in order to facilitate connection to a power source. Thats why you see motors described as "6 lead", or "9 lead", they are referring to those flexible wire stubs in the box.

But a lot of electricians use that same term (incorrectly IMHO) to refer to the conductors going TO the connection box from the last power switching device, ie starter or disconnect. Technically, those are called the "motor branch circuit conductors" but that is too long, so electricians have over the years shortened it to "leads".


"Will work for (the memory of) salami"

 

[dpc]

Pete,

Thanks for confirming my very vague recollection on that point. They always seem small compared to our NEC-sized motor feeder.

Dave

 

[KllrWolf]

I agree with Jraef on the definition of motor leads. in submersible motors, the leads are the amount of wire coming from the windings, out of the motor, and what the cable is spliced to. The leads are sized based on FLA and always being submerged in the fluid. this can lead to funny splice situations such at 1/0 being spliced to 500 MCM cable when the well is really deep.

 

[elinBG]

Do you mean the 6 inside leads? From the winding to the terminal board of the motor?

http://www.eldvigateli.com

 

[Clyde38]

I am in the US. My questions apply to both NEMA and IEC motors. I know that I have caused some confusion with the term "leads". I apologize. I realize that the type of motors I'm referring to normally have a terminal box. This is a special case where the leads from the magnet wire (coil phases) need to go to a connector (like an Amphenol Radsok) instead of a terminal box. I need to size the connector. So elinBG to answer you question, yes. From the winding to the terminal box.

 

[electricpete]

It's been well covered by every single responder here, but I'll repeat again the approach depends on whether you're talking about "T-leads" (leads from motor winding to terminal box) or the "branch circuit conductors" (from terminal box to protective device at MCC / Load Center etc) to use jraef / code terminology.

On the surface it sounds like your op talked about T-leads (you said "the conductors between the coil wire and the terminal box") and you're using a code approach for branch circuit conductors.

Locked rotor current (II/In): 6.2 (does this mean that the locked rotor current is 6.2 times the no load current? I'm not familiar with the II/In value. Is it the KVA/hp rating?)

I haven't heard that terminology, but it seems like maybe someone was trying to express a ratio of currents. Maybe symbols IL / IN = Ilockedrotor/Inameplate where Inameplate = FLA. That would be about 6.2


=====================================
(2B)+(2B)' ?

 

[elinBG]

The old winders in our repair shop have a "rule" which is the Nominal current divided by 8 (In/8) and then they choose the next larger size of cable (in sq.mm)...I found that this works for delta motors, for wye-connected, multiply by sqrt(3)

)

http://www.eldvigateli.com

 

[jraef]

Discussed in the past by the way.

thread237-317969

"Will work for (the memory of) salami"

 

[rcw retired EE]

"Locked rotor current (II/In): 6.2" In = nominal rated current = full load amps from nameplate. = 59.2A
II = Inrush or intital starting Current. (I'm making an assumption).
II/In = 6.2 means starting current = 6.2 x 59.2A = 367A. That is in the range you have for Code "G" motor.

 

[dArsonval]

Clyde38 20 Feb 14 7:30

Clyde Writes: “…This is a special case where the leads from the magnet wire (coil phases) need to go to a connector (like an Amphenol Radsok) instead of a terminal box. I need to size the connector.”

Now I get the picture.

Size your connector pin current capacity to what you’re connecting it to.

Or said another way, match an extension cord size with another extension cord of the same size or slightly larger.

Measure the total cross-sectional circular mil area of the magnet wire(s) you are going to connect.

(You’ll need a detailed Magnet Wire Chart showing Bare Wire Diameter, Wire Gauge, and Cross-Sectional Area of the wire as measured in Circular mils for reference.)

You may also need a chart denoting the Circular Mil area of the stranded lead wire you intend to connect to the magnet wire coil ends of the motor.

Add up the total circular mil area of the motor coil end.

Size your American Phenolic Company [plug/jack] connector pin current capacity to match or slightly exceed the total circular mils measured from the motor coil end(s).

Coil ends in a motor stator can have one or multiple number(s) of conductors in parallel.
The enamel insulation needs to be removed from each magnet wire -conductor end- for accurate measurement.
(Use a micrometer, or set of calipers to make these measurements.)

Let’s use a Number 10 magnet wire for sake of Example:

A single motor lead end is determined to be a Number 10 American Wire Gauge copper wire.
(It’s measured at around .102 inches in diameter.)

Referring to a Magnet Wire Chart, a No. 10 wire has a cross-sectional area of 10,380 circular mils.

Two number 10 wires in parallel at the coil end would total 20,760 circular mils.

Size the Amphenol Connector pin to match or slightly exceed the total number of circular mils the motor coil end represents.

Always enjoy this forum,

John

 

[dArsonval]

Disclaimer:

Opps!

"Size your American Phenolic Company [plug/jack] connector pin current capacity to match or slightly exceed the total circular mils measured from the motor coil end(s)."

And, "Size the Amphenol Connector pin to match or slightly exceed the total number of circular mils the motor coil end represents."

Both sentences above have the potential to confuse the reader. (sorry

One has to know the current capacity of all conductors incorporated into the entire conductor arrangement. And that goes back into the circle of what
the insulation system consists of. This has the potential to lead the whole topic off into another subject.

Ideally one would match the measured motor coil end size to an appropriately sized connector pin size.

"But", what is the circular mil size of the particular connector pin?

I understand now why this is a somewhat problematic question, and how the approach in attempting to give a suitable answer has its downfalls.

Staying tuned,

John