MV/LV Distribution Design — Feeder Sizing, Fault Levels, and Selectivity

What Is a Distribution System?

An electrical distribution system is the network of conductors, switchgear, transformers, and protective devices that delivers power from the utility supply or generation source to individual loads. It spans two voltage tiers — medium voltage (MV), typically 1kV to 36kV, and low voltage (LV), typically below 1kV (most commonly 480V in North America, 415V or 400V in IEC jurisdictions).

Distribution design is one of the most consequential engineering disciplines in a project. The decisions made at this stage — how many transformers, what impedance, which feeder sizes, how protection is coordinated — directly affect capital cost, operational reliability, and safety for the life of the facility.

System Architecture

Most industrial and commercial distribution systems follow a recognizable hierarchy:

• Utility / generation source — incoming supply at transmission or sub-transmission voltage, typically metered at the point of common coupling (PCC)
• Main MV switchgear — receives the incoming supply, distributes to transformers via MV feeders; may include bus sections and interconnecting breakers for redundancy
• MV/LV transformers — step down to utilization voltage; number and sizing determined by load demand and redundancy requirements
• Main LV switchboard (MLVS) — receives transformer secondary, distributes to sub-boards, MCCs, and major loads; the heart of the LV system
• Sub-distribution boards / MCCs — serve area loads, motor feeders, and lighting panels; typically located close to load clusters
• Final circuits — individual feeders to motors, lighting, small power, and instruments

Redundancy is designed into the architecture at the transformer level (N+1 or 2N configurations) or at the bus level (bus section breakers with auto-changeover schemes). The appropriate level depends on the facility criticality and owner requirements.

Feeder Sizing — The Three Checks

Every feeder in the distribution system — MV or LV — must satisfy three independent sizing criteria. All three must pass; the largest resulting conductor size governs.

• Current-carrying capacity (ampacity) — the cable must carry the design load current continuously under its installation condition (conduit, tray, direct buried), with derating for ambient temperature and grouping applied
• Voltage drop — the accumulated voltage drop from the source to the load must stay within the permitted limit, typically 3% for branch circuits and 5% combined under NEC; 3–5% under IEC depending on project specification
• Short-circuit withstand — the conductor cross-section must survive the let-through energy of the maximum prospective fault current for the duration of the protective device clearing time. The governing formula is the adiabatic equation per IEC 60364-5-54 and IEEE Std 242 (Buff Book): A = I × √t / k, where A is conductor area (mm²), I is fault current (A), t is clearing time (s), and k is a material constant (115 for copper PVC, 143 for copper XLPE)

In practice, ampacity governs for most LV feeders. Short-circuit withstand governs when fault levels are very high (near the transformer secondary) or protection clearing times are long. Voltage drop governs for long cable runs — particularly motor feeders in large plant layouts.

Fault Level Calculations

The available fault current at any point in the distribution system must be calculated to:

• Confirm switchgear and cable short-circuit ratings are adequate
• Verify conductor withstand per the adiabatic equation
• Set overcurrent protection devices correctly
• Perform arc-flash hazard analysis (IEEE 1584 / IEC TR 61641)

The fault level decreases as you move downstream from the source — impedance of cables, transformers, and bus reduces the available current at each node. The highest fault level in a system is always at the transformer secondary terminals, where only the transformer impedance limits the fault current.

For a distribution transformer, the three-phase fault current at the LV terminals is approximately:

• I_sc = kVA / (√3 × V_LL × %Z/100)

A 1000kVA, 480V transformer with 5.75% impedance produces approximately 20.9kA of three-phase fault current at the secondary terminals. This is the starting point for all downstream fault level calculations.

Selectivity (Discrimination)

Selectivity — called discrimination in IEC terminology — is the ability of a protection system to isolate only the faulted section of the network while leaving the rest of the system energized. A properly selective system means a fault on a sub-board feeder trips only the sub-board incomer, not the main LV switchboard breaker.

There are three main selectivity methods:

• Current selectivity — upstream device has a higher pickup current than downstream. Simple, but limited to cases where fault currents at each node are significantly different. Not reliable for close-in faults.
• Time selectivity — upstream device has a longer intentional time delay than downstream. Reliable, but each level of delay adds fault duration, increasing arc-flash energy and equipment stress. Commonly used at MV level.
• Zone selective interlocking (ZSI) — devices communicate: when a downstream device detects a fault and starts timing, it signals the upstream device to delay. If the upstream device detects a fault without a downstream signal, it trips instantly. Combines speed with selectivity. Available on most modern electronic trip units (ETUs).

Full selectivity (total discrimination) means no upstream device trips for any downstream fault. Partial selectivity means selectivity is achieved up to a certain fault current level — above which both devices may trip simultaneously.

Cable Derating in Distribution Design

Distribution feeders are rarely installed in isolation. MV feeders share cable trenches; LV feeders share cable trays or conduit banks. Grouping reduces the ampacity of each cable, and this must be accounted for in feeder sizing.

Key derating factors to apply:

• Ambient temperature — correction factor applied when ground or air temperature differs from the reference (30°C for NEC; 40°C for IEC outdoor/ground)
• Grouping factor — applied when multiple loaded cables share a tray or duct; NEC Table 310.15(C)(1) and IEC 60364-5-52 Table B.52.17 both provide grouping factors
• Soil thermal resistivity — for buried cables and duct banks; higher resistivity means less heat dissipation and lower ampacity
• Installation depth — deeper burial increases soil thermal resistance; correction factors apply beyond standard depths

It is common in distribution design to size feeders at 80% of the derated ampacity (a 0.8 utilization factor) to allow for future load growth without immediate cable replacement.

Where Standards Diverge

• Voltage levels — North American LV distribution is predominantly 480V three-phase (with 208V and 120V derived from step-down transformers or panel secondaries). IEC jurisdictions use 415V or 400V three-phase (230V phase-to-neutral). This affects transformer ratings, breaker ratings, and cable sizing — NEC and IEC ampacity tables are not interchangeable.
• Fault level notation — ANSI/IEEE C37 standards express fault current as symmetrical RMS amps (kA_sym). IEC 60909-0 uses a voltage factor c (1.0 to 1.1) and calculates initial symmetrical short-circuit current I_k". Always confirm which convention applies before comparing equipment ratings.
• Neutral earthing — TN-S (separate neutral and protective earth), TN-C, TT, and IT earthing systems are common in IEC practice and affect earth fault current levels, protection design, and safety requirements. NEC practice is predominantly solidly earthed (TN equivalent) for LV systems, with specific rules for high-resistance grounding (HRG) in industrial applications.
• Switchgear standards — IEC 62271 governs MV switchgear; IEEE C37 series governs North American MV gear. LV circuit breakers: IEC 60947-2 vs UL 489. Ratings are defined differently — IEC uses Icu (ultimate breaking capacity) and Ics (service breaking capacity); UL uses interrupting capacity at specific voltage ratings. These are not directly comparable without reviewing test conditions.

Common Design Pitfalls

• Undersized neutral conductors — in systems with significant non-linear loads (VFDs, computers, UPS), triplen harmonics circulate in the neutral and can cause it to carry more current than the phase conductors; the neutral must be sized accordingly
• Ignoring motor contribution to fault current — running motors contribute to fault current for a few cycles during a fault; this increases the available fault current above the simple transformer-only calculation and must be included for accurate arc-flash analysis
• Missing voltage drop on motor starting — large motor DOL starts cause momentary voltage dips that can affect other loads on the same bus; check voltage drop under starting conditions, not just full-load
• Selectivity assumed but not verified — specifying that the system "shall be fully selective" without running a coordination study leaves this unverified; selectivity must be confirmed with time-current curves (TCCs) or manufacturer coordination tools
• Transformer impedance tolerance — transformer impedance carries a ±7.5% manufacturing tolerance per IEC 60076; always check fault levels at both minimum impedance (maximum fault current) and maximum impedance (minimum fault current for protection sensitivity)

Summary

• Distribution design spans MV and LV tiers — architecture decisions at this stage drive cost, reliability, and safety for the facility life
• Every feeder must pass three checks: ampacity, voltage drop, and short-circuit withstand — the largest resulting size governs
• Fault level is highest at the transformer secondary and decreases downstream as impedance accumulates
• Selectivity must be designed and verified — ZSI provides the best balance of speed and discrimination for LV systems. Short circuit calculations are the prerequisite for any coordination study
• NEC and IEC differ in voltage levels, fault current notation, neutral earthing systems, and switchgear rating conventions — confirm applicable standard before sizing