Outdoor Sockets - Wiring Question

chateauneufdupape

I have had two quite different opinions from two qualified electricians, so would appreciate some advice.

I have a hot tub outside which is wired up from the house via a 63A 30mA RCD and 6mm 3 core SWA. There is a rotary isolator close to the tub. Around the tub, I would like 2 outdoor double sockets. From one double socket, I want to run a stereo and some outdoor fairy lights which will be permanently plugged in. The second socket will run some garden lights and various garden tools such as hedge trimmers.

The 1st electrician has suggested that I could place the 2 double sockets in line between the rotary isolator and the tub. The 2nd electrician has suggested that I should take a fused spur off the 6mm SWA cable and then run 2.5mm SWA cable to the sockets. He suggests that I have an RCD on the fused spur or on the sockets.

Although the 2nd solution is more complex and more expensive, I am wondering if it is over the top. Any thoughts would be appreciated. N.B. I am not an electrician, so nothing to technical please.

Thanks in advance.

muckybutt

Right then for you to have any socket outside being used on that supply the they need to be down rated, you cannot have sockets wired on a 63A supply direct, best way would be new 20A radial circuit installed on a 20A RCBO.

Do you have another fuse board near the tub ?, only reason im asking is that 63A is a helluva load and the fact is in a overload condition it would not trip, as you say its a RCD for you to have overload protection you need an RCBO, im concerned that you may just have a 63A supply to your tub that isnt properly protected, was this signed off under building reg part p as it should be ?

chateauneufdupape

The RCD comes from the main consumer fuse box unit. There is no other fuse box. I believe that the electrician has installed in accordance with Part P, but can not confirm until the NICEIC paperwork arrives from his office.

When you say a new 20A radial supply, do you mean a completely independant cable from the consumer unit?

zax47

Muckybutt: Why do you consider the 63A rating for the RCD as the most relevant bit - it's providing fault protection so the 30mA is the relevant bit - if it were an MCB providing overload protection then that would be different.

Don't see any reason in my big red book why he can't have a fused spur off at the hot tub end and a couple of sockets on it. The FCU will give him overload protection - the supply RCD will give him fault protection. If he adds another 30mA RCD then he'll have to change the hot tub supply RCD to 100mA to provide some discrimination, can't have two 30mA RCDs on the same circuit. No word on what the existing overload protection (MCB?) is for that hot tub supply circuit but it must have something - the RCD DOES NOT provide any.

Who does it is relevant though - it's part P notifable work. May have been installed originally before Part P became relevant - we don't know.

EDIT: Was still typing when chateauneufdupape replied.

chateauneufdupape

I am getting a bit out of my depth here, but here goes!

On the consumer unit, there is another RCD that says 80A,240V, 30mA. The pull out fuse for the tub circuit says B40. It also has a trip switch on the fuse. Let me know if there is anything else I should look for.

The original tub was installed about 4 years ago. The current electrician has just rewired the tub in a new location. He saw no problem with the original installation (by a different electrician), but did replace the RCD as the original one was slow on response time during one of the many tests he carried out.

zax47

To go back to basics a little;

Fuses, or circuit breakers (or MCB, Miniature Circuit Breakers) give overload protection, they will fail or trip if the circuit tries to take too much current for the capacity of the circuit cable (usually caused by faults involving both live and neutral -like a "short circuit"). The current rating shows at what level it will trip - 40A in the case of the hot tub circuit "B40" breaker. (and the letter B denotes how much "leeway" there is on that figure).

RCDs (Residual Current Device) give fault protection - they will trip if they sense a difference in the current passing through live and neutral, usually caused by a fault of one of them to earth. They sense this difference (of 30mA in the case of your RCD) and trip before the current rises enough to kill you. They have to do this (in most cases) in under 0.4 sec. ("response time"). If it takes longer then it is faulty (or "sticky" - see later) and must be replaced - as your electrician found out. The current rating is only there on an RCD to show what the maximum current is that the device can switch - it doesn't "trip" at that figure, but must be capable of handling the current that may pass through it.

It is important to periodically use the "test" button found on the RCD(s) in your consumer unit (CU) - this keeps the mechanical parts of the RCD in good working order - so that it can meet the required disconnection time. There should be a sticker on your CU advising to do this every 6 months, but not enough people do this and the mechanisms get "sticky" over time leading to a slow response time and the need for replacement.

We have to have BOTH types of protection on pretty much all circuits now under the current regs. One RCD can cover several circuits, whereas each circuit will usually have it's own fuse or MCB.

Like your electrician, I see no problem with the original installation as you have described it. It has both RCD fault protection (which all outside circuits need) and a suitably rated MCB (or "trip switch") providing for overload protection. Your electrician can add a 13A Fused Connection Unit (FCU or "fused spur") after the isolator switch outside and add a couple of sockets (suitable for outside use), no problem.

The FCU adds local overload protection, by way of a fuse at the correct rating for the sockets and their cable (13A) and the supply RCD gives fault protection. The switch on the FCU meets the "safe isolation for maintenance" requirement, as does the isolator switch for the hot-tub.

There is little point adding another RCD in there - you can't have two of the same 30mA rating on the same circuit anyway - which would trip first? This means he would have to change the supply side RCD to a 100mA one and be sure it ONLY supplied that hot tub circuit - a waste of time, money and effort.

chateauneufdupape

zax47,

Thanks very much for such a detailed response in laymans terms. I greatly appreciate you taking the time to explain this to me. Not only have you helped me to understand what I need but also why. Additionally, you have saved me some money by not having to buy further RCD's.

zax47

No problem, at least my 3 years of training (plus regular updates) and 20+ years as an electrician aren't totally wasted.

muckybutt

zax47 wrote: »

Muckybutt: Why do you consider the 63A rating for the RCD as the most relevant bit - it's providing fault protection so the 30mA is the relevant bit - if it were an MCB providing overload protection then that would be different.

Don't see any reason in my big red book why he can't have a fused spur off at the hot tub end and a couple of sockets on it. The FCU will give him overload protection - the supply RCD will give him fault protection. If he adds another 30mA RCD then he'll have to change the hot tub supply RCD to 100mA to provide some discrimination, can't have two 30mA RCDs on the same circuit. No word on what the existing overload protection (MCB?) is for that hot tub supply circuit but it must have something - the RCD DOES NOT provide any.

Who does it is relevant though - it's part P notifable work. May have been installed originally before Part P became relevant - we don't know.

EDIT: Was still typing when chateauneufdupape replied.

In op 1st post there is no mention of mcb or fuse backing up the rcd that was what concerned me the most as there was no overload protection, now op has posted further all become clearer note 40A fuse still a little though for a 6mm. Now thats been cleared up then yes perfectly acceptable to spur off at spa end into an fcu, likewise 20+ years experience.

zax47

muckybutt wrote: »

In op 1st post there is no mention of mcb or fuse backing up the rcd that was what concerned me the most as there was no overload protection, now op has posted further all become clearer note 40A fuse still a little though for a 6mm. Now thats been cleared up then yes perfectly acceptable to spur off at spa end into an fcu, likewise 20+ years experience.

Understood mate. I assumed that there HAD to be some sort of overload protection back at the CU and the OP just hadn't mentioned it. 6mm SWA buried is good for ~44A by my maths (230V, 10kW max load, 10m run, 20 degree ambient, 1.3% V drop) so maybe 40A is on the cautious side, but next step is 50A.

mikey72

What's wrong with the other electrician's suggestion of putting the sockets inline to the tub?