ECN Forum Forums Technical Discussion Electrical Theory and Applications LOOPS IN BOX

>If you are counting on coiling the wires of the circuit together to cancel inductances, better include the ground.

Uh, oh! That had better not be.


Okay, Mr. Moderator, here's the scenario.
My cable make 237 full tight loops before it gets a motor. The motor shorts out to the housing which is hooked via GRC which thanks to my careful workmanship provides a low impedance path back to the panel comparable to the grounding wire.

Now is the fault current going to be stymied by those 237 loops?

Dspark,
Any way you look at it, your conductor routing has the EGC run with the ungrounded conductors and/or grounded conductor. In the tightly wrapped cable it is present inside the jacket. In the raceway it is either a conductor inside the raceway or is the raceway itself or is run on the outside of flex as allowed by NEC. If this is not so then the installation is not Code compliant. If there is a 3 phase motor being fed from cable on a wire reel and there is no EGC in the cable, there seems to be no problem as long as the inductances are being cancelled by all the current flowing in the ungrounded conductors in close proximity to each other. If there is a short to ground and the current finds a return outside of the cable the problem appears. With an EGC and other parallel paths available through conductive structures the EGC is still the lowest impedance path.

If there is a short to ground and the current finds a return outside of the cable the problem appears.
That is the scenario that I described.

With an EGC and other parallel paths available through conductive structures the EGC is still the lowest impedance path.

(That it is the lowest impedance path is untrue if you read the problem as I wrote it.)

Okay, I will explain the point for you and anyone else.

A lot of current will return on the conduit.
That means that the magnetic fields around the 237 loops will not be cancelled. They will act as chokes limiting the fault current.

Whether it is significant or not requires some calculation and kowledge of whether the imbalance will act as an inductor to reduce the impedance on the EGC. That's why I am hoping that Scott will answer.

If fault current return path is not in close proximity to the current supplying the fault, the installation is not in compliance with 300-3(b). That requires the EGC to be in the same raceway or cable as the other circuit conductors. If the raceway is suitable for an EGC that meets the requirement. If using cable there must be an ECG in the cable if not allowed outside by 300-3(b)(2). A cable with 237 loops probably would not fall in one of the allowed conditions.

The NEC cannot dictate the path that fault current shall follow. If it gets to the ventilation ducts, the plumbing, or the structural steel, the OCPD had better work.

If loops (chokes) in the ungrounded conductors could pose an actual problem in such a case, then loops should be avoided. That's a big "if". And I would like to see someone who actually knows weigh in on this.

I have no idea as to right or wrong here but if there is even a chance that it could be dangerous then i will make sure that i never loop again in a box ,but tell me if u want to leave extra wire in the box how would u do it ?
and remember guys try to keep theses conversations in a format that us un educated and less knowledgeable can follow

Loops are bad if there is another conductor going through at that might ever have a high enough voltage relative to the size of the loop.

So don't ever loop around another conductor or a metal pipe or rod.

But to make a single loop to align with a termination or such does not seem to me to be a bad thing.

As for leaving extra cable in a box, can you fold it up like ribbon candy?

Dspark,

Your 237 loops will be effected by ground fault current flowing through the conduit. Not that it will cancel out the total XL of the loops, but it will have some ability to reduce a field. This would be in the realm of how a Shaded Pole motor works. That consept is the reason that split phase motors can start on 1 phase AC - the "Auxillary Winding" reduces the stationary magnetic field produced by 1 phase AC on the "Run Winding", which allows the Rotor to have current Induced into it [and begin to rotate].

Back to the Q;

There's also factors on which direction the magnetic fields cross at - simply meaning is the coiled wire wound "In Step" [in 0/180 polarity], or "Out Of Step" [at 90° angles].
In step would exibit an influence on the inductor, whereas out of step will have very little effect between the two flows.
This is a topic of Breadboarding, when 2 or more Inductors are mounted on the same PCB and in close proximity. Placing them at right angles is done to reduce the amount of inductive coupling [that's why Toroids are SOOOOOO much nicer to use than normal stacked coils - the Toroids contain their fields almost entirely within themselves].

A few last things would be the diameter of the coil, permeability of the core [it's magnetic reluctance], layer depth [distance between each layer and row of coiled wire], trueness, or consistity, of the windings [AKA Perfect Layer] and finally the integrety of the coil [limited "Bouncing" between layers when currents peak or cross zero].
These factors will be used to determine the Inductance of the coil [rated in mH - milli Henrys]. Apply mH values here to obtain the XL of this coil, then apply XL to E in a simple Ohms Law Calc to find I.
If the XL looks like it would limit the I to a certain level [figure the coil's XL as a stand alone element - without the motor being in the circuit], then it would allow a level of ground fault to flow which might not be viewed as overcurrent to the OCPD.
With the motor involved in the circuit, it will create a non-stable X [XL and XC], plus R in series with the coil during normal operation - unless it is idling, or at a constant load.
Also, the motor will be contributing I during a fault [level depending on the motor's load at the time of fault]. This will change everything on the Inductor [coil], which may go as far as removing the XL created in it from the original current flow [before fault].

Scott SET

BTW: Anyone come up with anything regarding the coiled wire around the Rigid Nipple, with the DC current flowing in another coil thingee??

>Your 237 loops will be affected by ground fault current flowing through the conduit. Not that it will cancel out the total XL of the loops, but it will have some ability to reduce a field.
Okay, so whether the EGC is fairly parallel to the other conductors is not terribly important, right?

And if the fault current returns via a totally different path, then the loops may act as chokes and you mentioned how it could be calculated. Would you care to throw in some hard numbers as examples, like how many 2" loops to limit 100 amps or something?

>Back to the Q:
There was a request to keep "these conversations in a format that us uneducated and less knowledgeable can follow"...

>There's also factors on which direction the magnetic fields cross
So let's assume that they don't cross during a fault and they all cross during during operation.


>A few last things would be the diameter of the coil,
1 or 2"

>permeability of the core
air

>layer depth
Assume outside of pratical influence.

>trueness
perfect

>integrity of the coil
maximum

>These factors will be used to determine the Inductance of the coil
Okay, thank you. We're waiting.


>the motor will be contributing I during a fault
The fault occurred when the motor was being energized for the first time. It was not rotating and did not budge.

>Anyone come up with anything regarding the coiled wire around the Rigid Nipple, with the DC current flowing in another coil thingee??
No, but since we're trying to have fun, I'll venture that you just had me build the primary winding (AC) for a transformer whose core is the DC electromagnet. When I get near to the magnetic field, I become the secondary coil.

Dspark,

Sorry I haven't responded to the last post - I've been so swamped with stuff to do, I haven't been in the forums since last Wednesday or something!!
Got to run through the forums today, and man are things really happening!!!

Anyhow, I printed the thread and will throw some numbers your way ASAP. Not sure when I will be back in the forums, because this upcoming week's workload is mind-numbing!!

I do want to mention that you have obvious tech skills in advanced theory [unless you are just pulling things out of books at random ...just a joke] from what I have seen here and in the Holt forum.
Have you wound your own coils [Inductors] before??
That's where I learned most of this thread's topic from - reading many-many-MANY books on the subject.

I am impressed by the many people in this forum that have such a great knowledge of the advanced areas of theory!!! You guys know who you are!! Chime in and let the people that just browse here see your names!!
If not, I will do it - and you know that will include a bunch of jokes and 100x the text needed, so it's going to be your fault!!

BTW: The coil with the DC thingee is a crude example of a Magnetic Amplifier [AKA Saturable Reactor].
It's been discussed in threads before, mostly in the Holt forum between Bennie Palmer and Myself.
That was one of the first "Solid State" types of motor control, termed "Static Control Systems".
They're interresting in operation, but Jurasic in technology! [plus wasteful].

Scott SET