What cable size do I need for a ring final circuit?

A standard ring final circuit serving socket outlets uses 2.5 mm² twin and earth cable protected by a 32A Type B MCB (or 30A for older installations). The circuit must form a complete ring from the consumer unit, through the socket outlets, and back to the consumer unit. Each ring should serve a maximum floor area of 100 m². The cable must be clipped direct or installed in plaster for the full 32A rating.

What is the maximum number of sockets on a ring final circuit?

BS 7671 does not specify a maximum number of socket outlets on a ring final circuit. The limit is the floor area served, which should not exceed 100 m². In practice, a typical domestic ring circuit may have 10-20 socket outlets. However, the total load should be considered: a 32A ring circuit can deliver approximately 7.36 kW, though diversity factors mean the actual demand is usually much lower.

Do all circuits need RCD protection?

Since the 18th Edition (BS 7671:2018 Amendment 2), additional protection by a 30 mA RCD is required for: all socket outlets rated up to 32A, all circuits in bathrooms, mobile equipment rated up to 32A used outdoors, cables installed in walls at a depth of less than 50 mm, and cables without earthed metallic covering. In practice, this means virtually all domestic circuits now require RCD protection, typically provided by RCBO (combined RCD/MCB) devices.

What cable size do I need for an electric shower?

Electric shower circuits depend on the shower rating: up to 8.5 kW requires 6 mm² cable with a 40A MCB, 8.5-9.5 kW requires 10 mm² with a 40A or 45A MCB, and 10-10.8 kW requires 10 mm² with a 45A MCB. These are for PVC cable clipped direct. If the cable is enclosed in insulation or thermal insulation is present, derating factors apply and a larger cable may be needed. The circuit must include 30 mA RCD protection.

What is the difference between Type B and Type C MCBs?

Type B MCBs trip at 3-5 times their rated current and are used for most domestic circuits (lighting, sockets, showers). Type C MCBs trip at 5-10 times their rated current and are used where there is a moderate inrush current, such as motors, fluorescent lighting banks, or air conditioning units. Type D MCBs (10-20 times) are used for heavy inductive loads like transformers. Most domestic installations use Type B throughout.

BS 7671 Circuits Reference — Free Trade Calculator

BS 7671 Domestic Circuit Specs — Cable Sizes and MCB Ratings

Standard domestic circuit specifications per BS 7671 18th Edition: cable sizes, MCB ratings, maximum lengths, and protective device requirements.

Standard Domestic Circuits

Circuit	Cable Size	MCB Rating	MCB Type	RCD Required
Lighting (6A)	1.0 mm² or 1.5 mm²	6A	B	Yes (30 mA)
Ring final (sockets)	2.5 mm²	32A	B	Yes (30 mA)
Radial (sockets, ≤50 m²)	2.5 mm²	20A	B	Yes (30 mA)
Radial (sockets, ≤75 m²)	4.0 mm²	32A	B	Yes (30 mA)
Immersion heater	2.5 mm²	16A	B	Yes (30 mA)
Cooker (≤13.5 kW)	6.0 mm²	32A	B	Yes (30 mA)
Cooker (>13.5 kW)	10.0 mm²	45A	B	Yes (30 mA)
Shower (up to 9 kW)	6.0 mm²	40A	B	Yes (30 mA)
Shower (9-10.8 kW)	10.0 mm²	45A	B	Yes (30 mA)
Smoke/heat alarms	1.0 mm² or 1.5 mm²	6A	B	No (dedicated circuit)
Outdoor socket	2.5 mm²	16A or 20A	B	Yes (30 mA)
EV charger (7 kW)	6.0 mm² or 10.0 mm²	32A	B or C	Yes (Type A or DC)

Cable Current Ratings (Clipped Direct, PVC T&E)

Cable Size (mm²)	Clipped Direct (A)	In Insulation (A)	In Conduit/Trunking (A)
1.0	15.5	11.5	13.5
1.5	20	14.5	17.5
2.5	27	20	24
4.0	37	27	32
6.0	47	34	41
10.0	65	46	57
16.0	87	62	76

Values from BS 7671 Table 4D5, Installation Method C (clipped direct). Derating factors apply for grouping, ambient temperature, and thermal insulation.

Related Calculators

Use the Cable Sizing Calculator for volt drop and derating, or the Lighting Circuit Calculator for domestic lighting design. See also the BS 7671 glossary entry and the Part P glossary entry.

How We Calculate This

Frequently Asked Questions

Last updated: March 2026

All calculations are estimates. Verify with your supplier.

BS 7671 Domestic Circuit Specs — Cable Sizes and MCB Ratings

Standard Domestic Circuits

Cable Current Ratings (Clipped Direct, PVC T&E)

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How We Calculate This

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