Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?

BS 7671 permits an unfused spur (or cable branch) from a 30 A or 32 A ring final circuit supplying accessories conforming to the relevant part(s) of the BS 1363 series to be wired with thermoplastic (PVC) or thermosetting insulated copper line and neutral conductors of cross­sectional area as small as 2.5 mm² (433.1.204) where each accessory forming part of the ring final circuit itself is supplied via two such 2.5 mm² conductors. This article explains why this practice is acceptable.
 

Introduction

Designers and installers will recognise that the current-carrying capacity of 2.5 mm² thermoplastic (PVC) insulated conductors is generally less than the 30 A or 32 A rated current of the fuse or circuit­breaker at the origin of the ring circuit, particularly once the installation method and any relevant installation factors have been taken into account. This is also the case for cables employing thermosetting insulation where, additionally, they are selected on the basis of their conductor operating temperature not exceeding 70 °C as is generally necessary for compatibility with the accessories to which they are connected.

Consequently, the circuit protective device cannot protect the conductors of the unfused spur against overload current; if the spur conductors were to become overloaded, this may result in damage to the insulation, sheath, the immediate surroundings of the cable, or to connected accessories, due to the temperature of the conductors rising to above their rated value.
 

Why is it permitted to use 2.5 mm² conductors for an unfused spur?

Referring to regulation 433.1.204, the following conditions apply to the wiring of a ring final circuit:

• The current-carrying capacity of the cable, corrected for the particular installation conditions (Iz) must not be less than 20 A, and
• The circuit design, under intended conditions of use, should be such that the load current in any part of the circuit is unlikely to exceed the current­carrying capacity of the cable (Iz) for long periods of time (typically in excess of 1 hour). Where this is the case, it can be taken that the condition given in indent (ii) of regulation 433.3.1 for the omission of overload protection is met.

It should be noted, however, that a conductor size of greater than 2.5 mm² may be required in order to meet voltage drop limits, particularly for long cable runs (see Section 525).
 

What measures can be taken to make overload of the spur sufficiently unlikely for compliance with BS 7671?

In general, it can be taken that the live conductors of a spur are not likely to carry overload current for long periods of time where the spur feeds only:

• One single or one twin socket-outlet, or
• One or more items of fixed equipment supplied via a fused connection unit, or
• One or more socket-outlets supplied via a fused connection unit.

Additionally, sufficient numbers of socket­outlets should be provided, distributed throughout the installation, to provide reasonable sharing of load around the ring final circuit (553.1.7).

What about the possibility of short-circuit?

Referring again to indent (ii) of regulation 433.3.1, overload protection may be omitted on the grounds that conductors will not be subjected to sustained overload, subject to said conductors being protected against fault current in accordance with the requirements of Section 434.

Using the adiabatic equation given in regulation 434.5.2, it can be seen that any of the 30 A or 32 A fuses or circuit­breakers (including those incorporated in RCBOs) listed in regulation 433.1.204 for use as the protective device for a ring final circuit will protect the 2.5 mm² live thermoplastic or thermosetting insulated conductors of an unfused spur against short-circuit current.

And what about earth-fault?

In order to determine whether the 30 A or 32 A protective device will protect the, typically, 1.5 mm² circuit protective conductor (cpc) within a 2.5 mm² BS 6004 flat twin & earth cable the adiabatic equation given in regulation 543.1.3 should be used.

This will show that any of the 30 A or 32 A fuses or circuit-breakers (including those incorporated in RCBOs) listed in regulation 433.1.204 for use as the protective device for a ring final circuit will protect the 1. 5 mm² cpc, provided that the earth fault loop impedance (Zₛ) at the points supplied by the spur is within the maximum value required by BS 7671 to provide a 0.4s disconnection time.

Further guidance

Further guidance on the installation of ring and radial final circuits for household and similar premises in accordance with regulation group 433.1 is given in Appendix 15 of BS 7671.