Anova replaced a non-compliant sub-main to a detached garage in South London, upgrading to a correctly specified SWA cable with full certification and Building Control notification. This case study highlights our commitment to compliance, transparency, and quality workmanship.
Introduction
When a South London homeowner contacted Anova to add more power to their detached garage, they assumed it would be a quick job. But our inspection revealed a deeper issue: the existing sub-main cable supplying the garage was undersized, lacked earthing, and suffered from excessive voltage drop — all of which made the installation unsafe and non-compliant.
This project shows how Anova goes beyond just “making it work.” We design installations with compliance, safety, and long-term performance in mind — even when the regs don’t force us to.
Project Scope: Detached Garage Rewire Triggered by Unsafe Supply
The property was a semi-detached home in South London with a detached garage at the rear of the plot. The client’s goal was simple: they needed reliable power for lighting and additional sockets in the garage. However, our inspection of the existing supply revealed:
- A cable with no earthing conductor
- Excessive voltage drop well beyond acceptable limits
- Undersized conductors for the protective device in use
Rather than patch or ignore the problem, we proposed a new sub-main installation — properly designed, safely isolated, and fully tested.
Our Solution: Safe, Smart, and Compliant
Anova supplied and installed a new 6mm² 3 core SWA (steel-wire armoured) cable, selected based on a full assessment of:
- Current-carrying capacity
- Installation method (clipped direct)
- Route length and environmental factors
- Voltage drop at expected load
We installed the following:
- Building Control notification in line with Part P
- A dedicated isolation point at the origin
- 6mm² SWA cable routed to the detached garage
- A new metal-clad garage consumer unit
- A full Electrical Installation Certificate (EIC)
Although we weren’t obligated to provide full design calculations for this job, we chose to ensuring the client wasn’t overcharged with an unnecessarily large cable, while still maintaining full compliance with BS 7671.
Cable Calculation Snapshot
Design current (Ib): 30A
Circuit length: 25 metres
Installation method: Clipped direct
Protective device: 32A Type B RCBO
Cable spec selected: 6mm² 3 core SWA
Reference method: C (clipped direct), adjusted for outdoor ambient conditions
Current-Carrying Capacity Check
From Table 4D4A BS 7671:
- 6mm² 3-core SWA clipped direct (ref method C) at 30°C = 38A
Now apply derating factor for higher ambient temp (e.g. 35°C):
- From Table 4C1 (ambient temp correction), factor at 35°C = 0.94
→ Adjusted capacity = 38A × 0.94 = 35.7A ✅
This still comfortably exceeds the design load of 30A.
So the cable is suitably rated for the application.
Voltage Drop Check
From Table 4D4B BS 7671:
- 6mm² 3-core SWA: 7.3 mV/A/m
→ VD = 30A × 25m × 0.0073 = 5.475V
→ Allowed limit (5% of 230V) = 11.5V
→ Voltage drop is acceptable
Summary
So with these adjustments:
- 6mm² SWA is fully compliant when derated for external ambient temperatures
- Voltage drop is within limits
- Cable was correctly selected based on performance and not unnecessarily oversized, which supports your cost-conscious, safety-first approach
Need Electrical Work?
Whether you’re adding power to a garden room, upgrading an old outbuilding supply, or starting fresh with a new garage fit-out — Anova is here to help.
We take care of everything from cable design and compliance to safe installation, certification, and Building Control notification.
- Honest guidance
- Transparent pricing
- Fully tested, fully compliant work
- Domestic & commercial projects welcome
Get in touch today to book a consultation or request a quote.
📞 Call: 02080516482
📩 Email: info@anovauk.com
💬 WhatsApp: Click the button below