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Free Online HV/MV Cable Sizing Calculator (11kV / 33kV)

Size 11kV and 33kV medium/high-voltage cables by current rating, voltage drop and short-circuit withstand. NEC basis. Free online tool, no sign-up.

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HV/MV Cable Sizing (11kV/33kV) Calculator
Reference: NEC
🔌 ELECTRICAL
Size 11kV and 33kV medium/high-voltage cables by current rating, voltage drop and short-circuit withstand. NEC basis. Free online tool, no sign-up.
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ℹ About This Calculator

Medium and high-voltage cables for 11 kV and 33 kV distribution are sized on three checks: continuous current rating after derating, voltage drop over the run, and short-circuit withstand for the fault duration. This calculator applies them for XLPE-insulated MV cables per NEC.

MV cables are XLPE-insulated with metallic screens; sizing derates the tabulated rating for soil or air temperature, laying depth, grouping and soil thermal resistivity for buried runs. The short-circuit withstand often governs the screen and sometimes the conductor, because MV fault levels are high and clearing times set the I²t the cable must survive.

HV/MV Cable Sizing Basis

NEC

1. I_rated × derating ≥ design current
2. Voltage drop ≤ limit over the run
3. Short-circuit: A ≥ √(I²t) / k (adiabatic) derating = temperature × grouping × depth × soil
I = fault current, t = clearing time
k = material/insulation constant (XLPE copper ~143)

Worked Example

Example: An 11 kV feeder carrying 250 A, buried, with derating factors totalling 0.75, needs a cable rated for 250 / 0.75 ≈ 333 A in standard conditions - a 3-core 120 mm² XLPE cable. The screen and conductor are then checked against the fault current and clearing time so the cable survives a through-fault.

MV Cable Ratings & Derating (NEC)

Indicative current ratings for 11 kV XLPE copper cables (3-core, ground/air, base conditions):

Size (mm²)Buried (A)In air (A)
50~185~200
95~275~305
120~315~350
185~400~455
300~520~605

Derating & Fault Withstand

Ratings fall for high ambient/soil temperature, close grouping, deeper burial and high soil thermal resistivity - combined factors of 0.6-0.8 are common. Beyond continuous current, the conductor and metallic screen must carry the fault current for the protection's clearing time without overheating (adiabatic check). At MV, this short-circuit withstand frequently sizes the screen.

Indicative values - confirm against NEC-2 and the manufacturer's tables.

Frequently Asked Questions

How do I size an 11kV cable? +
Check three things: the continuous current rating after derating must exceed the design current, the voltage drop must stay within limits over the run, and the conductor and screen must survive the fault current for the protection clearing time. The largest of the three governs.
What insulation is used for MV cables? +
Modern 11 kV and 33 kV cables use XLPE (cross-linked polyethylene) insulation with a metallic screen, having largely replaced older paper-insulated lead-covered (PILC) cables. XLPE tolerates higher operating temperatures and is easier to joint.
Why does short-circuit withstand matter for MV cables? +
MV fault levels are high, so during a through-fault the conductor and especially the metallic screen carry large currents until the protection clears. The cross-section must survive that energy (I²t) without damage - this adiabatic check often sizes the screen.
How does burial affect cable rating? +
Buried cables are derated for soil temperature, laying depth, grouping with other circuits and the soil thermal resistivity. Poorly conducting (dry, sandy) soil traps heat and reduces the rating significantly, so a thermal backfill is sometimes specified.
What is the difference between HV and MV cables? +
Medium voltage (MV) generally covers about 1-36 kV (including 11 kV and 33 kV distribution); high voltage (HV) is above that. The sizing principles are the same, but higher voltages need thicker insulation and more attention to screening and terminations.
How much voltage drop is allowed on an 11kV feeder? +
Utility and design practice typically limit MV feeder voltage drop to a few percent so downstream transformers stay within their tap range. The exact limit depends on the network, but excessive drop on long feeders can force a larger conductor than the current rating alone.
Do MV cables need a screen? +
Yes - MV cables have a semiconducting layer and a metallic screen over the insulation to control the electric field and carry charging and fault currents to earth. The screen must be sized and earthed to handle the fault current.
What standard covers 11kV cable sizing? +
NEC (Part 2 for MV) and NEC-2 cover XLPE-insulated cables for 3.3-33 kV, giving construction, ratings and test requirements used with derating and fault-withstand calculations for sizing.

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