High Voltage Transformer Feeder

[steve66]

OK, I'm specifying a 12KV feeder for a 2500KVA transformer. I was wondering if this sounds like the right way to go:

2500KVA/12KV/sqrt(3) *1.25 = 150 amps primary OCP. So I would use #2 copper XLP based on table 310.77. (I never know if this table applies to 3 conductors in a single burried conduit).

From the notes on table 310.64, I would assume it would be safe to use 133% although I'm not sure if the loop I am taking power from is grounded or not.

From Southwire, I got a cut sheet on their XLP cable:

http://appprod.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet105

It looks like #2 has about a 1" diameter. Thats 0.79 square inches. If I put 3 of these into a single conduit, I need 2.4% of conduit area at 40% fill. So it looks like a 3" conduit will work (see edit below).

Or would it be better to install one cable in each of 3 conduits? Can I do that with metal conduit, or would that have to be PVC?

A couple of more questions: Whats the difference between XLP and EPR? Would one be better for this than the other? How about the difference between "Tape" and "Wire" sheilds?

Steve

Edit:

I already see that jambing might be a problem, so it looks like it is 4" conduit.

 

[jtester]

It looks like #2 has about a 1" diameter. Thats 0.79 square inches. If I put 3 of these into a single conduit, I need 2.4% of conduit area at 40% fill. So it looks like a 3" conduit will work (see edit below).

Or would it be better to install one cable in each of 3 conduits? Can I do that with metal conduit, or would that have to be PVC?

A couple of more questions: Whats the difference between XLP and EPR? Would one be better for this than the other? How about the difference between "Tape" and "Wire" sheilds?

Steve

Edit:

I already see that jambing might be a problem, so it looks like it is 4" conduit.

Steve I think you'd be happy with 4", but you are performing the right calculations. I prefer one conduit for all cables, including neutral/ground, but a local utility in our area uses 3 conduits with a hot wire and neutral/ground in each. I specify wire shield over tape shield, but don't have a problem with either.

XLP insluation, cross linked polyethylene, is a bit older and often times considered somewhat less reliable than EPR. EPR, ethylene propylene rubber, is only slightly newer, and is considered by many to be more reilable. That will often incite an argument but I agree with it.

Be sure to determine grounding, and the need for a neutral. Often times padmounted transformers are wye-wye vs delta-wye and in the first type you must have a neutral.

Jim T

 

[bob]

Utilities here will install all cables in one conduit and a spare conduit.

 

[steve66]

All cables in one conduit with a spare sounds like a plan

I see there is also TRXLP:

http://appprod.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet52

And I have also seen XLPE cable. Does anyone know where I can find out what the differences are?

Steve

 

[jtester]

XLP and XLPE are thse same, cross linked polyethylene. The TRXLPE is tree retardant cross linked poly. A common mode of failure in XLPE cables in slow degredation of the insulation. It actually grows little tree like structures from the conductor, thru the insulation towards the ground. After a while the insulation has broken down enough to fault. When you disect the cable you can see little dendritic "trees" under a micorscope.

The TRXLPE is treated to inhibit the growth of these trees.

There should be a lot of info available thru a google search.

Jim T

 

[C_CLEV]

hope this helps

hope this helps

This is a web site I used to figure out the diffrence in cable insulation just download the pdf file and look towads the last few pages it will show what the insulation is sutiable for.

http://web.anixter.com/Anixter/anixter.nsf/WCTechnicalHandbook/InsulationandJacketMaterials

 

[kingpb]

Medium voltage devices and equipment are all 100% rated. No need to increase the cable by the 125% like is done with lower voltage systems (480V power circuit breakers are also 100% rated) Therefore, the current is 121 A. This does not necessarily affect the cable size, because for 15kV voltage class, #2AWG is typically the smallest cable manufactured. As you probably saw from the Southwire catalog.

Keep in mind, that Southwire, like others, do not manufacture a MV-90 cable anymore. They have standardized on MV-105. So the current ratings you see in their catalog are for 105 deg C, not 90 deg C.

As far as 310.77, unless your going to put concrete around the conduit, then technically, it does not apply. This is because it is usual and customary to put concrete around MV Conduit.

EPR is more flexible, however not quite as durable as XLPE. The PVC jacket will help protect the insulation during pulling.

 

[steve66]

I didn't know what they meant by tree retardant. I wonder if that is a problem even if the cable is never overdriven, or completely heated up.

Thanks for the link C_CLEV. That does give the differences between XLP and EPR. Right now I'm thinking the TRXLP will work.

Kingpb:

Southwires cut sheet specifically says 90 deg. under normal conditions, although their ampacities do seem to be higher than in the NEC tables. I still want the primary OCP on the transformer to be 125% the full load current, so thats why I applied 125%.

You say it is usual and customary to put concrete around MV Conduit. Does that mean it would be unusual to not concrete encase it??? Some of this run may be horiz. bored.

Steve

 

[jtester]

Steve

The trees are believed to originate from overvoltages not overloads. Lightning arresters, etc., can limit the energy into an underground system, but a voltage wave will still be past thru the riser and into the ug system. In my experience the tree retardant XLP is supposed to limit the insulation degredation from this overvoltage. I believe it is as much hype as it is effective, but probably helps a bit.

90C is as standard rating because the connectors, elbows, etc., are generally rated 90C.

Jim T

 

[bob]

As far as 310.77, unless your going to put concrete around the conduit, then technically, it does not apply. This is because it is usual and customary to put concrete around MV Conduit.

I am assuming that the this is a riser that that taps the utility 12 kv feeder
and cable runs from the pole to the transformer. I don't think its necessary to
install the conduit in concrete if it is installed in accordance with table
300.50. I would use PVC sch 80. I would use 36" as the min depth just for added safety.

OK, I'm specifying a 12KV feeder for a 2500KVA transformer. From the notes on table 310.64, I would assume it would be safe to use 133% although I'm not sure if the loop I am taking power from is grounded or not.

The notation 12kv is likely a 12.46 kv wye system with the neutral grounded.

 

[kingpb]

I understand that not all installations will require MV cable runs to be concrete encased. That's fine, but the point I was making is that 310.77 refers to exactly that, and therefore the current values provided do not directly apply. Look at 310.60 which is referenced. All the pictures are of concrete encased ducts. The portion of the riser would fall under conduit in free air, 310.73 which has lower current value. For conservative-ness that is what I would use.

 

[bob]

I understand that not all installations will require MV cable runs to be concrete encased. That's fine, but the point I was making is that 310.77 refers to exactly that, and therefore the current values provided do not directly apply. Look at 310.60 which is referenced. All the pictures are of concrete encased ducts. The portion of the riser would fall under conduit in free air, 310.73 which has lower current value. For conservative-ness that is what I would use.

I would agree. In table 310.73 the riser cable ambient should be hotter and have a reduced ampacity for the cable. If you look at the tables the ampacity is the same for both installations #2 155/165 amps and for most of the other cable sizes, the ampacity is higher in table 310.73.

 

[steve66]

But the figures in 310.70 has a legend that says "backfill - earth or concrete"?

Steve

 

[bob]

But the figures in 310.70 has a legend that says "backfill - earth or concrete"?Steve

Steve
Do you mean fig 310.60? You are correct. The ratings for #2 is the same for
both tables.