Published: 7 February 2014
Category: Q&A
James asks this question, which is about an internal garage consumer unit supply cable and protection for it. It has been answered by the NICEIC (Theme – Distribution Equipment including Cable Management):
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Question: Is it acceptable to run the supply cable from a spare way on an existing 17th Edition consumer unit into an internal garage, terminating on a Starbreaker 63A 30mA DP RCD, two x MCBs (one x 6A and one x 32A) IP2X?
Note that the supply circuit breaker will be a 40A MCB on a 30mA RCD.
The main consumer unit is a Crabtree High Integrity 5+5+3 17th Edition board, and the cable will be ducted underground in three inch PVC ducting and will be non-armoured.
Answer by the NICEIC: The proposed method is acceptable with current standards and would be safe for its use.
In situations where an existing installation requires alteration, you must consider Regulation 132.16 to ensure that what is already in place is adequate in all aspects, including earthing and bonding requirements.
You should also consider Regulation 314.1 so that discrimination of protective devices is afforded for ADS. However in this instance, two RCDs would be in place at 30mA, so no discrimination will be afforded and it would be likely that if an earth fault develops in the garage, both the front end RCD and garage RCD may trip together or the front end first.
You may need to consider that if the original RCD is adequate and meets the requirements of the current BS 7671:2011, then a second RCD may not be formerly required with the garage location, as all of the final distribution circuit and final circuits are already RCD protected.
Answer by the ECA: Your proposal sound fine to me.
However, if this garage is an internal – as you say it is - I do not understand the need for the underground ducting. Why are you doing this?
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