Consumer Unit Upgrade?

[mortman]

I am hoping to install an electric shower where one has never existed before. The consumer using is rated to 100A and by adding up the values of the existing MCBs I get a total of 96A. The shower I would like to install is rated at 10.5Kw and requires a 50A MCB and 30mA RCD and would be connected using 6mm cable (as per manufactures recommendations) I have a spare slot in the CU but by adding the 50A MCB I'd be way over the 100A rating. Would I need to upgrade the CU or the live feed to accommodate the additional load or is the CU rating not directly proportionate to the value of the MCBs located within it?

Any help greatly appreciated.

 

[user56565]

we have to first check for diversity. what exactly are the other ways on the CU?? ie what amp ratings are they and more importantly what do they supply??

also 6mm isn't going to be big enough. where is the cable going to be routed (ie through floorboards, insulation, etc...), and how long is the run going to be?? i would never put in less than 10mm, and you may need even bigger.

 

[mortman]

32A Cooker
32A Ring main
16A Immersion Heather
8A Lights Up
8A Lights Down

Cable length 10m or less .. 6mm was recommended by the manufacturer, details below

10.5kW
nominal current @ 240v = 45amp
isolating switch 45amp
mcb = 40 or 45amp
max 6mm cable run 28m
cable direct in plaster or clipped to vertical wall

think may of made typo above, mcb 45amp

++++++++++++++++++

Further reading ..

Power rating @ 230v = 9.6kW
9.6kW requires 40-45A MCB

___________________________
moderator

please note 10 a

 

[Steve]

10.5KW needs 10mm cable and 50A MCB. If you put that on a 6mm cable, you will find the smell of burning invading your bathroom very soon . . . . .

 

[Big_Spark]

Crafty is correct, tha cable should be 10mm, however a 45 RCBO would be sufficient for this as even at 240V the nominal current drawn is 43.75A. An RCBO for a shower is far better than a standard MCB.

 

[user56565]

here's a guide to diversity: http://www.tlc-direct.co.uk/Book/6.2.3.htm

even with it however, you'll be going over your consumer unit rating. you 'could' turn on your shower and a few other appliances, and then more than 100A could be flowing through your CU (which is only rated for 100A). in practice this is unlikely to occur and if it did, it would only occur for short periods (assuming a 5-10min shower) so is unlikely to cause much damage. so some may argue you could just add it in and it should be ok. however i would add in a switchfuse between the meter and consumer unit, and put in an 80A fuse. this would protect the consumer unit, so in the rare case you overload your supply, the fuse will operate.

you must make sure you have a 100A supply first obviously. call your supplier and be 100% sure. the reason i have said 80A in the switchfuse is because it will blow above 100A but before the supplier's fuse. does your CU have a built-in RCD?? if not you may have to get an expensive RCBO or change the CU. ignore the manufacturer's recommendation and put in 10mm. are you aware of part p: //www.diynot.com/forums/viewtopic.php?t=27936. its up to you whether you comply or not. are you 100% comfortable with doing everything above? ask more questions on what you're unsure with..

 

[andy]

here's a guide to diversity: http://www.tlc-direct.co.uk/Book/6.2.3.htm

even with it however, you'll be going over your consumer unit rating. you 'could' turn on your shower and a few other appliances, and then more than 100A could be flowing through your CU (which is only rated for 100A). in practice this is unlikely to occur and if it did, it would only occur for short periods (assuming a 5-10min shower) so is unlikely to cause much damage. so some may argue you could just add it in and it should be ok. however i would add in a switchfuse between the meter and consumer unit, and put in an 80A fuse. this would protect the consumer unit, so in the rare case you overload your supply, the fuse will operate.

you must make sure you have a 100A supply first obviously. call your supplier and be 100% sure. the reason i have said 80A in the switchfuse is because it will blow above 100A but before the supplier's fuse. does your CU have a built-in RCD?? if not you may have to get an expensive RCBO or change the CU. ignore the manufacturer's recommendation and put in 10mm. are you aware of part p: //www.diynot.com/forums/viewtopic.php?t=27936. its up to you whether you comply or not. are you 100% comfortable with doing everything above? ask more questions on what you're unsure with..

should be OK with whatever fuse is in. my house has a 40A service fuse and often have cooker and 9.5KW shower on at the same time. added to power used by lights/ringmain etc. so far it has never blew in over 6 years

 

[mapj1]

Dont get too hung up on the total current thing - adding up the breakers is very pessimistic, unless you have reason to think it really will be all maxxed out at once. Most houses average no more than a 2KW load, integrated over the day, and the electricity boards size their equipment based on this. A quick look at a few old bills will show your true demand.
Most houses subdivide the wiring over more breakers than the minimum it could be (2 rings in less than 100msq, 2 lighting circuits where 1 big one would just about do). This is for convenience if one pops off the other light stays on etc, not because the load is really that high.
The diversity calcs are a step in the right direction, but some additional sense needs to be applied.
Consider that a 100 A company fuse will carry 200A for some minutes, and 150A certainly for long enough to have a decent shower, and '100A consumer units' are generally sized to scale witha 100A fuse, in the same way - the rating is continuous - if you are not operating the switch with the load on, then the rating is thermal, (i.e. not trying to interrupt the full load current) so a brief overload that gives no time for things to heat up, while not ideal, is not going to cause anything more serious than a slight acceleration of the normal ageing process.

It is not unknown for a house to have a 200A breaker total, a diversity calculated load of 140 to 160 amps, running off a 60A company fuse, for years, without incident.
I'd say install it in the existing CU, make sure its all nice and tight, run it, and if you are concerned, see if it gets warm in use. Only then shell out for a second CU, if you really need it.
Remember to derate cables if they share trunking or capping with those from other circuits.