Medium Voltage Underground Cable / Smallest Available Size
Medium Voltage Underground Cable / Smallest Available Size
(OP)
I have an application for a 15 kV, 133% EPR Insulation (220 Mil) underground cable that needs to fit in 2" Conduit (1.913" I.D.). I only need an ampacity of 70 Amperes, but have been told by my local Okonite rep. that they will not manf. a medium voltage cable smaller than #2 CU per NEC Table 310.5 (2002 Version). Note that this is an utility application and we are not under obligation to meet NEC requirements. I checked the 2007 NESC and was not able to locate a minimum UG medium voltage cable limit. Does anyone know of a UG cable manufacturer who manufacturers cables smaller than #2 CU?






RE: Medium Voltage Underground Cable / Smallest Available Size
You might want to ask your Okonite rep what the peak stress would be for #2 (which they will make) or #4 (which they won't). I'm sure you'll see a dramatic difference in peak stress and I believe that is why they won't make a smaller size than #2 at 15 kV.
RE: Medium Voltage Underground Cable / Smallest Available Size
So it's the field caused by the small diameter conductor that makes the insulation issues become acute?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Medium Voltage Underground Cable / Smallest Available Size
Like magoo2 said...
It's not an NEC issue. The cables are manufactured in accordance with AEIC and ICEA standards; max electrical stress determines the minimum conductor diameter for a voltage class (15kv - #2, 25kv - #1, 35kv - #1/0, eg).
RE: Medium Voltage Underground Cable / Smallest Available Size
S = 0.868 E/[d_c log10 (D/d_c)]
E = l-g voltage
D = diameter of insulation in mils (under insulation shield)
d_c = diameter of conductor in mils
d_c - supposed to be d with the subscript c
See Southwire Power Cable Manual.
RE: Medium Voltage Underground Cable / Smallest Available Size
Always nice when the physics match reality.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Medium Voltage Underground Cable / Smallest Available Size
For cables I have always seen that the maximum electric field occured at:
Emax= V/R2ln(R1/R2)
Where R2 is the conductor radius from center and R1 is the radius to the outside of the insulation. This equation holds true for just a conductor surrounded by insulation with no Intersheath or shield.
A cable havin an intersheath or shield has its maximum field represented by:
Emax= (V/R3ln_a)(1/1+a)
a = R1/R2 = R2/R3
Where R3 is the radius of the conductor and R2 is the radius to the shield or sheathe and R1 is the radius to the outside of the insulation.
I belive that the shield or sheathe is used to reduce the maximum electric field in cables and can be seen by examles with the above equations.
At least this is how I understood it. Please correct me if I am wrong.
RE: Medium Voltage Underground Cable / Smallest Available Size
log10 x = (ln x)/2.303 = 0.4343 ln x
mine used the diameter and yours used the radius.
With these 2 substitutions, they match.
I didn't check your other eqn.
RE: Medium Voltage Underground Cable / Smallest Available Size
I have seen these equations and understand them in theory but never really had an explanation for how they are used in cable applications.
I am curious now, is the reason that an MV cable has a shield for the purpose of reducing the maximum electric field in a conductor. From the second equation I posted you can do examples which show the electric field diminishing by having this inner sheathe. Is this the reason MV cable is shielded?
RE: Medium Voltage Underground Cable / Smallest Available Size
The shield is intended to make the dielectric field (i.e., the electric field that occurs across the insulation) more controlled and more symmetrical.
I think the wear on an automobile tire is a good analogy. If I were to make a tire in a square rather than a round configuration, you'd get nonuniform wear (and a pretty bumpy ride), and the tire wouldn't last as long.
Without a shield, the stress on the inside of the cable is pretty nonuniform and the wear would be much higher. Conductors that are insulated for operating voltages above 2 kV are shielded (5 kV cable and higher). Below that, there's more insulation added to the nonshielded design.
RE: Medium Voltage Underground Cable / Smallest Available Size
Regards,
Jim
RE: Medium Voltage Underground Cable / Smallest Available Size
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RE: Medium Voltage Underground Cable / Smallest Available Size
Here's another one, courtesy of EPRI. It shows the electric field in Fig. 4-3 and the stress distribution on Fig. 4-4.
RE: Medium Voltage Underground Cable / Smallest Available Size
RE: Medium Voltage Underground Cable / Smallest Available Size
RE: Medium Voltage Underground Cable / Smallest Available Size
RE: Medium Voltage Underground Cable / Smallest Available Size
RE: Medium Voltage Underground Cable / Smallest Available Size
An NEC committee wasting time? I'm shocked, shocked to hear this.
RE: Medium Voltage Underground Cable / Smallest Available Size
RE: Medium Voltage Underground Cable / Smallest Available Size
Benjamin Lanz
Past Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants
RE: Medium Voltage Underground Cable / Smallest Available Size
Are you sure you need 133% or would 175 mil cable work? If it's in conduit you could consider non-jacketed cable which would be smaller in diameter too.
Just some thoughts.