Wye Delta Wire Sizing

Somebody please check me on this. Im using the following formula to figure the Ampacity of the wiring between the starter and the motor, (6 leads):

(FLA/SQRT(3))*1.25 = Amps per wire.

I've been using this im my spread sheet for years. But now I'm getting some argument from other people about this and I'm having a hard time finding documentaion to verify my equation.

Thanks, Mike.

First off, NEC has nominal FLA and minimum wire sizing tables. You should be using those.

Second, the formula for three phase curent is:

A = (hp x 746) / (V x eff. x PF x 1.73)

1.73 is the square root of three

I have no idea where your formula came from. If you don't know it's derivation, you shouldn't trust it. It may be appliable to a special situation or certain conditions that may or may not be correct for your existing design. Always know the assumptions underlying any formula you use!

Tom,

I think you misunderstood my question, or I misstated it. The formula you posted is correct for the amps on a three wire system such as the feeder wires coming into the starter panel, or accross the line wiring, (3 wires to the motor).

What I'm working on is the 6 wires between the starter and the motor (open delta). The current on a single pair feeding one winding would be less than the current three wires feeding all windings.

Thanks, Mike.

I found a document on this website that says each pjase sees a voltage reduction of 1.73. The line sees an impedance increase of a factor of 3.
However, when you are in the wye connection you aren't anywhere near the operating point of the motor. The current will be significantly higher than what you see during normal run even though it is connected in a wye configuration. But your wiring is not typically sized for starting anyway. So I don't think starting in and of itself is something to be concerned with either way.
Is your point that, since you are running two wires per phase instead of one when you are connected in delta, the ampacity of each conductor doesn't need to match the nameplate current? I could see that point of view, but I don't think the square root of 3 has anything to do with it. The two conductors will share the current as equally as their relative resistances will allow (I think).

Keith

Mike

I would agree with your formula. Referring to the drawing below, when the motor is running in delta, Line 1 will have 2 wires going to the motor connecting to T1 and T6. The same is true for the other phases. Since each winding in a delta connected load will draw 1/SQRT 3 (.58) of the line current, each of the 6 motor leads must be sized for .58 x FLA. The Electrical code requires motor conductor ampacity to be 1.25 x the FLA in that conductor.

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kamenges

The point you are missing is that the current in each of the conductors per line is not in phase with each other. Hence the 1/SQRT 3 factor.

OK lets talk code, NEC Article 430.22C states:
For a Wye-Start, delta run connected motor, the selection of branch circuit conductors on the line side of the controller shall be based on the motor full load current. The selection of conductors between the controller and the motor shall be based on 58 percent of the motor full load current.

Hope that helps. I see Vic kinda beat me to it.

One more point about Wye-Delta conductor current.

Most Wye-Delta starters only have the overload in one conductor per phase. Therefore, the overload MUST be sized for .58 x FLA. If it is sized for FLA, the motor will have no overload protection. Some of the IEC overloads have 2 current scales on them for this purpose.

Thanks for the info and the cite ont the code. One of my suppliers also verified the calculation. Whew.... I was starting to sweat it out a bit!

Years ago I filled out this spreadsheet, and never looked back. It's been so long I've forgotten much of the stuff in it. I think my computer is makeing me stupid or someting like that. Sometimes it's diffucult to trust someone 10 years your junior, even if it's yourself.

Thank you all!

Mike.

P.S. Vic, thanks, the overloads are ok too. Thanks again.

Now I'm getting confused. That's not saying much, since I'm just a wide-eyed country boy playing in the big city. But Vic and Ron's posts don't come out to the same numbers.
The NEC reference Ron cites says the conductor should be sized for 58% of FLA. The formula Mike lists and that Vic breaks down says thee conductors should be sized at 72% of FLA:
(FLA/SQRT 3) * 1.25 = 1.25/1.73 = 0.72

Also, Vic says the the currents in each of the lines is not in phase with each other. Vic, do you mean the currents in the conductors going to the same connection point in the delta configuration? These currents need to be in phase or very dramatic things will happen, like excessive heating and arcing.

The 58% rule from the NEC seems to match up with the 125% single conductor rule used for normal motor wiring with a slight concession for the additional cooling capacity provided by two separate conductors as well as the lower net starting current per conductor. I don't see anything that would match up to SQRT 3.

Keith

rsdoran said:

The selection of conductors between the controller and the motor shall be based on 58 percent of the motor full load current.

Click to expand...

I presumed that rsdoran ment was 58% + the 125% required by NEC.

1/Sqrt(3)= .577, for round numbers 58%,x 125% of full load amps = required ampacity of the wire.

So FLA x.58 x 1.25 = wire ampacity required by NEC.

Or FLA x .725 = wire ampacity required by NEC.

I think what Vic ment to say is that the setup is spliting the phases among the windings.

It all works out.

Mike.

Keith

When wiring a motor to a Wye-Delta starter, 6 wires are run to the motor and connected to the motor terminals T1-T6. Each end of each winding has its own wire connected to it. See the drawing in my previous post. The wires are not tied together at either end.

By energizing the appropriate contactors in the starter panel, the motor can be connected in Wye or Delta. When starting in Wye, Line 1, 2, 3 are connected to T1, T2, T3 and T4, T5, T6 are shorted together. When the motor is up to speed, the contactors connect L1 to T1 and T6, L2 to T2 and T4, and L3 to T3 and T5.

The NEC states that motor conductors must be sizes for 1.25 times the motor FLA. The rule quoted by Ron is used to determine what the FLA is for each of the wires when the motor is wired to a Wye-Delta starter.

I hope this clears up your confusion.

I didnt elaborate, just stated what the code is. You are correct tho, NEC Article 430.22 requires conductors to be rated at 125% of the motors full load current rating as determined by 430.6 A(1)

I am so glad I bought the NEC Handbook, its much easier to use and offers in depth explanations even I can understand.

I would say you are correct using the 125% and I now understand your formula....may see it posted again somewhere sometime.

I just found the reference in the Handbook that Ron referenced. If I had to size this and I was using just the NEC code book, I would have got that wrong. I would have assumed that each subsection (A - F) of 430.22 was free-standing. The Handbook example clearly shows that the 125% (or the subsection A rule) applies to all the other subsections (B - F). I don't know that I agree with the rationale but the listed equation is correct based on the NEC.

Keith

Somehow I was sure I knew what I was doing..

But then somebody made me doubt myself.

I'm going to print out this thread and "post" it on the wall in somebodys office. moon2

This is a great forum!