Challenge: Fit 50 EV Chargers into a 400A Apartment Supply

The Problem

A 50-unit apartment building in Sydney. The body corporate has approved EV charger installations. Each resident wants a 7 kW (32A at 230V single-phase) Mode 3 charger.

Total connected load: 50 × 32A = 1,600A at 230V = 800A per phase on 3-phase supply.

The building's supply: 400A 3-phase (from Ausgrid), with current non-EV demand of 280A per phase.

Available capacity for EV: 400 − 280 = 120A per phase

That's enough for only 11 chargers at full power (120A / 32A × 3 phases). But 50 residents want chargers. What do you do?

The Challenge

Design a solution that allows all 50 residents to charge their EVs while staying within the 400A supply. What's the maximum charging rate each resident can expect?

The Solution

Option 1: Static Load Management (Diversity)

Not all EVs charge simultaneously. Industry data shows:

• Peak concurrent charging: 40–60% of installed chargers (8 PM – 12 AM)
• Typical overnight average: 25–40%
• Off-peak (2 AM – 6 AM): 15–25%

With a diversity factor of 0.40 at peak:

Effective demand = 50 × 32A × 0.40 = 640A ÷ 3 phases = 213A per phase

This exceeds the 120A available. Static diversity alone doesn't solve the problem.

Option 2: Dynamic Load Management (OCPP + Smart Charging)

A Dynamic Load Management System (DLMS) continuously monitors the building supply and adjusts individual charger output in real-time:

Available EV capacity = Supply limit − Building demand (measured)

With CT monitoring on the main incomer:

Average overnight (10 PM – 7 AM): approximately 20A per charger = 4.6 kW

Over 9 hours at average 4.6 kW: each EV receives approximately 41 kWh — enough for 200+ km of range per night.

The Design

1. Each charger: 32A circuit (per NEC 625 / AS/NZS 3000 — must be rated for maximum even if managed)
2. DLMS controller: OCPP 1.6 or 2.0 compliant, CT-monitored main incomer
3. Algorithm: Equal share with minimum floor (6A per IEC 61851 minimum)
4. Failsafe: If DLMS loses communication, default to 6A per charger (50 × 6A = 100A per phase)

The Branch Circuit Sizing

Even with DLMS, each branch circuit must be rated for the full 32A continuous per NEC 625.41:

• Circuit breaker: 40A (32A × 1.25)
• Cable: 6mm² minimum per AS/NZS 3008
• RCD: Type A with DC 6mA detection (or Type B per BS 7671 Section 722)

The DLMS manages the aggregate — individual circuit ratings are NOT reduced.

The Metering Solution

Sub-metering each charger allows:

• Individual billing per kWh consumed
• Verification of DLMS limits
• Detection of charger faults or bypass

The Answer

All 50 chargers can be installed with dynamic load management. Each resident gets 4–7 kW overnight (depending on concurrent demand), delivering 35–60 kWh per session — adequate for daily commuting. No supply upgrade required.

Total cost: ~$150,000 for 50 chargers + DLMS vs ~$500,000+ for a supply upgrade to 1,000A.

Design your system: Model dynamic load management with the EV Charging Calculator.

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Frequently Asked Questions

What standards govern cable sizing calculations?

The primary standards are AS/NZS 3008.1.1:2017 (Australia/NZ), BS 7671:2018 (UK), IEC 60364-5-52 (International), and NEC Article 310 (USA). Each has different assumptions for ambient temperature, installation methods, and derating factors.

Why do different standards give different cable ratings?

Standards differ in reference ambient temperature (AS/NZS uses 40°C, BS 7671 uses 30°C), test conditions, grouping factor calculations, and installation method classifications. A 50mm² XLPE cable can vary by 15% between standards.

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