Cables in Trefoil
Cables in Trefoil
(OP)
Hello All,
I have a situation that i'm seeking some advice on. Basically we have a ~1500kVA transformer supplying power to our Motor Control Centre. The run is only short, around 20-30 meteres. However total current is rather high as each phase is made up of 3 400mm2 single cores unarmoured run in non trefoil. (i.e each phase is bunched like so Red-Red-Red, Blue-Blue-Blu, White-White-White). Having looked around the build i could not find anything ferrous enclosing the cables like tray etc. but the guys are still concerned about heat issues.
I tend to think that because these cables are bunched and run next to each other, could i not model them as single conductors of some arbitraty large size run flat next to each other with no enclosed ferrous materials around? Will it still cause all kinds of issues?
I have a situation that i'm seeking some advice on. Basically we have a ~1500kVA transformer supplying power to our Motor Control Centre. The run is only short, around 20-30 meteres. However total current is rather high as each phase is made up of 3 400mm2 single cores unarmoured run in non trefoil. (i.e each phase is bunched like so Red-Red-Red, Blue-Blue-Blu, White-White-White). Having looked around the build i could not find anything ferrous enclosing the cables like tray etc. but the guys are still concerned about heat issues.
I tend to think that because these cables are bunched and run next to each other, could i not model them as single conductors of some arbitraty large size run flat next to each other with no enclosed ferrous materials around? Will it still cause all kinds of issues?






RE: Cables in Trefoil
How are the cables in the short run laid, are they inside the duct bank? In my opinion heat dissipation is also affected by laying of cables & what is the Insulation type is it XLPE cables.
RE: Cables in Trefoil
RE: Cables in Trefoil
RE: Cables in Trefoil
The three phases bunched together will react with one another like heat sources.
One phases emitted heat will be transfered to the other phases. You should rate each cable assuming the other two are fully loaded and add them as a heat source to your thermal circuit.
Combining the phases into one big conductor would give you a higher result for ampacity than considering each cable independantly. You may be able to derive an overall rating for all three cables combined but individual ratings using this method would be questionable in my opinion.
Thanks,
Andrew
RE: Cables in Trefoil
Would i be right to assume that induced currents would be negligible in objects that do not contain a fully enclosed path (i.e cable tray without a lid)
RE: Cables in Trefoil
That said, the statements regarding mutual heating and so on are correct, but I'd be surprised if there isn't already a standard relevant for the installation location that already deals with this.
RE: Cables in Trefoil
Most national codes in IEC land will reference this document, for instance AS-NZS 3008 Electrical installations—
Selection of cables is based on this document with some changes made to suit local conditions.
RE: Cables in Trefoil
If you opt for 3 cables per phase you have to remember that each conductor has its own insulation that thermal conductivity is somewhat less than perfect. As such you cannot consider a triple bunch of cables as a single conductor. IEC60287 must be employed with all consequences.
Chris
RE: Cables in Trefoil
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If we learn from our mistakes I'm getting a great education!
RE: Cables in Trefoil
RE: Cables in Trefoil
"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)
RE: Cables in Trefoil
RE: Cables in Trefoil
Regarding ampacity - I would say in reverse: that the ampacity of a circuit does not double when we use two cables per phase.
RE: Cables in Trefoil
In your actual configuration buried in 1 K.m/w soil thermal resistance, depth of 0.7-1.2 m and soil temperature 20 oC no more than 540 A per one cable will be permitted.
As jghrist said if you change the configuration to flat the current will be unbalanced up to 20% so the cable close to the center will be more heated.
I would suggest first of all to change to Red-Blue-White configuration for each group and to keep at least 150 mm clearance between groups. In this case a 650 A will be permitted per cable. In order to increase the load up to 722 A per cable you have to keep at least 40 mm clearance between the cables in the same group.
In a flat configuration the cable order could be R-B-W W-B-R R-B-W so all the cable will get only 722 A [and also keep 150mm distance between groups].
RE: Cables in Trefoil
Maybe we are talking about two different things. Here's my point:
(In NEC land)
One 500 kcmil copper conductor has a 75 deg C ampacity of 380 A in conduit.
TWO 250 kcmil copper conductors have the same total cross-sectional conductor area but each has an ampacity of 255 A, for a total of 510 A.
So the ampacity more than doubles.
"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)
RE: Cables in Trefoil
Less but not necessarily negligible.
1 - Let's say you have a plate next to a cable so they are both in the same plane. Draw a loop around the outside of the plate. Flux is enclosed and eddy current can flow.
2 - Hysteresis losses can still occur in steel even without loop current flow.
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RE: Cables in Trefoil
Is this what you meant?:
510A > 2 x 380A
or I misread your post?
RE: Cables in Trefoil
RE: Cables in Trefoil
Anyway, thanks for all the advice guys.
Cheers,
Jereb
RE: Cables in Trefoil
If the six cables are in the same conduit I agree. I was simply trying, obviously not successfully, to explain use of very large conductors is not usually cost effective.
I'm going to bow out since we already have at least two separate discussions going on at the same time.
"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)
RE: Cables in Trefoil
"Ampacities for Cables in Randomly Filled Trays" IEEE Transactions on Power Apparatus and Systems, Volume: PAS-90, Issue: 3/1971.
If the cables are not exposed to direct sun-shine[a raised hood -6 inches above the cables will be a suitable protection] the ambient air 40oC, the maximum permitted load will be 540 A per one cable. In order to keep balanced load in a bunch have to be included one cable of each phase and in the order as shown:
RE: Cables in Trefoil
Nice picture there. But I am scratching my head why the cables should be arranged like that! Gimme some time to digest.
RE: Cables in Trefoil
This is the phase position in order to avoid the currents' unbalance. If you change this order you may get up to 30% percent unbalance and this may heat one cable more than other.