Purpose

Plan a garage or outbuilding supply by assessing load, route, earthing interaction, voltage drop, fault protection, isolation, and environmental conditions.

Learning objectives

• Build a load schedule
• Compare supply arrangements
• Assess exported earth risk
• Plan isolation and verification

An outbuilding circuit crosses boundaries between environments and sometimes earthing zones. A cable size and breaker rating cannot be chosen safely before the full system is understood.

Core theory

Record present and credible future loads, route length/method, grouping, burial or overhead exposure, mechanical protection, voltage drop, fault current, RCD strategy, surge risk, and distribution needs.

For TN-C-S/PME supplies, simultaneous contact with true Earth and exported protective potential can be significant; extraneous parts and special equipment may change whether exported TN or a local TT solution is suitable.

The outbuilding needs suitable origin and local isolation, distribution/protection, earthing and bonding, environmental enclosures, inspection/testing, labels, and documentation.

Assumptions: No site survey yet.

1. Survey: Classify metalwork, services, ground contact, loads, route, and supply characteristics.
2. Earthing: Assess PEN/open-PEN and simultaneous-contact risks using current rules.
3. Design: Choose the earthing/protection architecture, then size and verify the submain.

A site-specific load, route, and earthing-risk assessment must come first.

Reasonableness check: Conductor capacity cannot resolve an unsafe earthing interface.

Knowledge check

Answer: The protective architecture changes conductor and disconnection requirements. Earthing is a system decision, not a finishing detail.

Practical exercise

Prepare two conceptual supply options for a fictional workshop and list the evidence needed to choose between them.

Summary

• Start with load and site survey
• Assess earthing interfaces
• Verify the complete downstream installation