BS 1361 Time Current Characteristics

[Colonel Hathi]

Hi Chaps
I am about to install an 8.5 Kw electric shower and a 2Kw water heater in a small flat that has a 60A 1361 main suppliers fuse.

The flat has 1*32A ring with 3 sockets, 1*20A radial with 5 sockets and 1*6A lighting circuit with 7 points.
If I allow 2amps for the lighting (at 66%), 40amps for the sockets (100% for the 32A ring and 8amps for the radial at 40%), then 37A for the 8.5Kw shower and 3.2amps for the 2Kw demand triggered water heater (at 40% of 8amps),
I get a total demand of 82 amps.

Am I reading the Time Current Chart correctly?
It seems to flatten out at about 3700 seconds at about 110 amps for a 60amp 1361.
Does this actually mean that it can carry 110 amps for 58 minutes?
Does it mean that it can carry my 82 amp estimate indefinitely?

Is my 82 amp estimate actually reasonable in the real world?
I cannot imagine that 8 sockets and 7 lightbulbs would really pull 42 amps?

Any advice is much appreciated.

CH

 

[Engineer54]

Quit worrying, you'll not see anything like your calculated loadings. Those diversity formulas (excluding cooker formula) are so way over the top, it's almost a joke!! You'll be very lucky to see 40A for the whole flat!! And that's being generous too!! ...lol!!

 

[i=p/u]

awnsered your own question, turn everything on and clamp meter the live , youl see not up there... some say just take highest breaker and multibly the rest by 0.4

so.. 32+20+6+8 x 0.4 =26A add 37 for shower=63A your shower wont use full current unless youve skin like an elephant... i just know this as a rule of thumb from other posts on thbis forum

 

[johnboy6083]

ther is no hard and fast rule for caculating diversity, and often your own knowledge of the installation in question will give more accurate figures than any calculation. Ive never seen a ring final in a house pull 32A. the 8.5kw shower will only pull 37A a) if your voltage is at exactly 230, and b) if it is working at full power. In reality,you wont have it anywhere near full power, and you wont have it on for long, so its very unlikely to cause an overcurrent device to pop anyway

 

[i=p/u]

my 8.5kw shower is on 30 A fuse, and never tripped ever in two years

 

[SirKit Breaker]

my 8.5kw shower is on 30 A fuse, and never tripped ever in two years

mine runs off my combi boiler, and that has never tripped a circuit breaker or a RCD:jester:

 

[topquark]

mine runs off my combi boiler, and that has never tripped a circuit breaker or a RCD:jester:

Glad to see they haven't stripped the sense of humour Howard!

 

[Engineer54]

mine runs off my combi boiler, and that has never tripped a circuit breaker or a RCD:jester:

And very sensible system it is to have too...lol!!

 

[telectrix]

my 8.5kw shower is on 30 A fuse, and never tripped ever in two years

so, 2 years between showers? worse than me. i have one every xmas, whether i need one or not.

 

[Engineer54]

so, 2 years between showers? worse than me. i have one every xmas, whether i need one or not.

Bet you two smell sweet, ....especially you Tel, on Chistmas day!!!

 

[resettleddave]

awnsered your own question, turn everything on and clamp meter the live , youl see not up there... some say just take highest breaker and multibly the rest by 0.4

so.. 32+20+6+8 x 0.4 =26A add 37 for shower=63A your shower wont use full current unless youve skin like an elephant... i just know this as a rule of thumb from other posts on thbis forum

Just spotted this thread, because it is similar to a question I have come up against, practicallities v regulations, what figures should Colonel Haiti use for design purposes and the regs?

Furthermore, and similar to my problem, if you take a similar DB with the new shower as defined by Colonel Hathi and put it 40m away from the incoming supply, and you have to join them with 3 core SWA cable buried in the ground, what size SWA should you use? Calculating the Max Demand using OSG gives the figure Colonel Haiti came up with, so do you use a SWA big enough for the calculated demand or one big enough from experience and which is the correct way by the regs?