[tfo]

Customer requested we change the bottom plate from the manufacturers aluminum bottom plate, to a fiberboard bottom plate. Said it had to be done to meet code. I havent been able to find any code on it though.
Do they have reason?

If you did't remember the title of the thread its for Mono-conductor teck cables.
Thanks all.

 

[Pete m.]

if it's a non-ferrous metal I don't see the concern.

Pete

 

[Vintage Sounds]

They might be worried about complying with this rule:

4-010 Induced voltages and currents in metal armour or sheaths of single-conductor
cables (see Appendix B)
(1) Where sheath currents in single-conductor cables having continuous sheaths of lead, aluminum, stainless
steel, or copper are likely to cause the insulation of the conductors to be subjected to temperatures in
excess of the insulation ratings, the cables shall be
(a) derated to 70% of the current-carrying rating that would otherwise apply;
(b) derated in accordance with the manufacturer’s recommendations and in compliance with
Rule 2-030; or
(c) installed in a manner that prevents the flow of sheath currents.
(2) Circulating currents in single-conductor armoured cable shall be treated in the same manner as sheath
currents in Subrule (1).
(3) Single conductor cables carrying more than 200 A shall not enter ferrous metal boxes through individual
openings.
(4) Where single conductor cables carrying more than 200 A enter ferrous metal boxes, precautions shall be
taken to prevent overheating of the wall of the box by induction.
(5) Precautions to be taken to prevent overheating of the metal shall include the use of non-ferrous or
non-metallic box connectors or cable glands, locknuts, bushings, and ground bushings.
(6) All cables making up a circuit shall enter the box through one common non-ferrous or insulating plate
having a minimum thickness of 6.0 mm unless a deviation is allowed in accordance with Rule 2-030.
(7) Where single-conductor mineral-insulated cables are used, all current-carrying conductors shall be grouped
together to minimize induced voltage on the sheath.

The aluminum plate is legal but since it's conductive it could still allow sheath currents to flow whereas a non-metallic plate wouldn't, as long as you didn't use a bonding locknut(and bond it) on the load side. That would mean having to derate the conductors.

 

[farlsincharge]

Last time I did one it was aluminum on the entrance cabinet and fiberboard on the gear. That was the code compliant way.

 

[Nick0danger]

aluminum is compliant, fiber board is what is commonly used.

 

[tfo]

They might be worried about complying with this rule:



The aluminum plate is legal but since it's conductive it could still allow sheath currents to flow whereas a non-metallic plate wouldn't, as long as you didn't use a bonding locknut(and bond it) on the load side. That would mean having to derate the conductors.

Perfect thank you. Im still an apprentice so still trying to learn all the rules for specific situations.

The ground was in fact left "floating" on the load side which is a 4160V/600v XFO. I'm just not really clear on why.

Why is this not a concern with a normal 3 phase teck cable? Is it because the 3 phases under the same sheath cancel each others electro-magenetic field?

If we had left the aluminum bottom plate would we have had to derate the cable?

 

[Vintage Sounds]

Well the thing about sheath currents is that for the same installation they increase as you space the conductors further apart. Single conductors have to be given a minimum 100% spacing from each other to avoid derating. You could space them further than 100% but the potential for sheath current increases. Those sheath currents amount to wasted energy, and they also heat the conductor inside up.

Let's say you're using the 75 degree ampacity and the installation as it is has the wire at 62 degrees under normal operation. If sheath currents are flowing it's possible they might heat the wire up beyond 75 degrees which would mean you have to derate the conductor, forcing you to either upsize the conductor or add another parallel single conductor. On the other hand if you just eliminate those sheath currents you don't have to do anything other than maintain 100% spacing!

A multiconductor cable, let's say a 3C 750 copper, already has all 3 phases together right next to each other and so it can't induce a sheath current in itself since the fields created by each phase conductor will cancel each other out. As I understand the twist in the conductors further reduces the magnetic field's intensity. The conductors also are already derated compared to single conductor 750 because under normal operation they'll heat each other. Again going by 75 degree column in Table 1 the ampacity for single 750 is 785A whereas three 750s in a cable or raceway would be only 475A according to Table 2.

Sheath currents are a different problem from induction heating. Induction heating is when the magnetic fields between single conductors heat up ferrous metal in between the different conductors like sections of enclosures, screws, connectors etc. Aluminum isn't ferrous, so it's a good candidate to replace the part of the box or whatever that the cables enter through but it doesn't eliminate sheath currents. Fiberboard fixes both problems. The single conductor installations I've seen have been aluminum on the line side(to bond all the sheaths together on one end) and fiber on the load side.

 

[tfo]

Thanks you Vintage, that clears it up quite a bit for me. So you are saying that our first install with aluminum bottom plate, 100% spacing and bonded at the source end only was ok?
It sounds like it was to me and the fiberboard wasn't needed but was something of a wishlist item.

I'm guessing that the cause of increased sheath current the farther apart the conductors are is because there's less "cancelling each other out"?

 

[KDC]

It all depends on the amperage when it comes to metal sheathed single conductors.

Up to 200 A, it's a pretty simple install. No plates needed, no worries about sheath currents.

Above 200A to 425A - Use non Ferrous plates on both ends.

Over 425A non ferrous plate (at source) and fiber plate at load. (Save for hazardous locations)

4-010 is a pain of a rule, in that most of the information is in appendix B. I know that in Manitoba the instructors at RRC like going over it every level because of that.