9.5 kw shower on 6mm ?

[Guitarist]

9.5KW shower at 240V takes 39.6A

9.5KW shower takes 37.9A at 230V

40A Circuit Breaker will hold indefinitely for a 9.5KW shower.

Will not trip what ever you have to do in the shower.

The debate is that some people believe 6mm cable is adequate for a 40A MCB, not that a 40A MCB is ok for a 9.5 kw shower.

 

[Yorkshiresparky]

Personaly i would upgrade to a 10mm T+E, better to be safe than sorry. Also your giving the customer options in the future to go for a higher Kw shower

 

[Lester Lecky]

The debate is that some people believe 6mm cable is adequate for a 40A MCB, not that a 40A MCB is ok for a 9.5 kw shower.

Also 9.5kw at 230W is actually 41.3A!!!!!!!

 

[Kenny.]

Supply Voltage = 230 Volts

Power factor = 1

Ib - Design current = 41.3 Amps

Protective Device Type = MCB type B (BS EN 60898)

In - Protective Device Rating = 45 Amps

Cable Type : Twin & Earth

Length of run of cable = 10 metres

Installation Method : Single-core or multicore cable direct in masonry having a thermal resistivity

not greater than 2 K.m/W - without added mechanical protection.

(method C)

Ambient temp = 30 °C

Number of circuits including this one = 1

Length of cable in thermal insulation = none

Apply Correction factors:

From TABLE 4C1 : Cg = 1 (Grouping)

From TABLE 4B1 : Ca = 1 (Ambient temp)

From TABLE 52.2 : Ci = 1 (Insulation)

Protective device factor for Buried cables : Cc = 1

Cd = 1

Cs = 1

Protective device factor : Cf = 1

It = tabulated current carrying capacity

It = In / (Cg x Ci x Ca x Cf x Cc x Cs x Cd)

It = 45 / (1 x 1 x 1 x 1 x 1 x 1 x 1 )

It = 45.00 Amps

From TABLE 4D5 Cable selected = 6 mm²

Current capacity of cable selected = 47 Amps

TABLE 4D5 For 6 mm²: mV/A/m = 7.3

mV/A/m corrected for power factor = mV/A/m x Power Factor = 7.3 x 1 = 7.3

Voltdrop = (mV/A/m x Length x Design current) / 1000

Voltdrop = ( 7.3 x 10 x 41.3 ) / 1000

Voltdrop = 3.01 Volts

(Maximum permissible voltdrop (regulatuon - 525) = 11.5 Volts)

Calculated Cable size = 6 mm², Actual Earth conductor size = 2.5 mm²

Maximum Cable Length = 38.1 Metres

 

[Lester Lecky]

Supply Voltage = 230 Volts

Power factor = 1

Ib - Design current = 41.3 Amps

Protective Device Type = MCB type B (BS EN 60898)

In - Protective Device Rating = 45 Amps

Cable Type : Twin & Earth

Length of run of cable = 10 metres

Installation Method : Single-core or multicore cable direct in masonry having a thermal resistivity

not greater than 2 K.m/W - without added mechanical protection.

(method C)

Ambient temp = 30 °C

Number of circuits including this one = 1

Length of cable in thermal insulation = none

Apply Correction factors:

From TABLE 4C1 : Cg = 1 (Grouping)

From TABLE 4B1 : Ca = 1 (Ambient temp)

From TABLE 52.2 : Ci = 1 (Insulation)

Protective device factor for Buried cables : Cc = 1

Cd = 1

Cs = 1

Protective device factor : Cf = 1

It = tabulated current carrying capacity

It = In / (Cg x Ci x Ca x Cf x Cc x Cs x Cd)

It = 45 / (1 x 1 x 1 x 1 x 1 x 1 x 1 )

It = 45.00 Amps

From TABLE 4D5 Cable selected = 6 mm²

Current capacity of cable selected = 47 Amps

TABLE 4D5 For 6 mm²: mV/A/m = 7.3

mV/A/m corrected for power factor = mV/A/m x Power Factor = 7.3 x 1 = 7.3

Voltdrop = (mV/A/m x Length x Design current) / 1000

Voltdrop = ( 7.3 x 10 x 41.3 ) / 1000

Voltdrop = 3.01 Volts

(Maximum permissible voltdrop (regulatuon - 525) = 11.5 Volts)

Calculated Cable size = 6 mm², Actual Earth conductor size = 2.5 mm²

Maximum Cable Length = 38.1 Metres

Haha!! I wondered if someone couldn't resist posting the calculation !!

Just out of interest, I would have thought there was a good chance of some cable Grouping (Cg) at some point on the run but i know not always - the original post didn't specify either way. If in doubt i always assume there must be a few cables running together! Just one cable along side would take the requirement above 50A - just a thought!
Also, are there many MCB manufacturers who make a 45A Type B MCB now? If only 50A MCB available then 6mm again, no good.
Thought most modern boards (17th edition etc) only give the options of 40A & 50A. Only seen 45A on old Wylex breakers. You know anyone who supplies 45A MCB's with new RCD boards - be interesting to know for future reference!!??

 

[Guest55]

^^^ that is truely an excellent effort , doff my cap to Kenny.
So , even by the book , 6mm ok.

 

[Moses]

Supply Voltage = 230 Volts

Power factor = 1

Ib - Design current = 41.3 Amps

Protective Device Type = MCB type B (BS EN 60898)

In - Protective Device Rating = 45 Amps

Cable Type : Twin & Earth

Length of run of cable = 10 metres

Installation Method : Single-core or multicore cable direct in masonry having a thermal resistivity

not greater than 2 K.m/W - without added mechanical protection.

(method C)

Ambient temp = 30 °C

Number of circuits including this one = 1

Length of cable in thermal insulation = none

Apply Correction factors:

From TABLE 4C1 : Cg = 1 (Grouping)

From TABLE 4B1 : Ca = 1 (Ambient temp)

From TABLE 52.2 : Ci = 1 (Insulation)

Protective device factor for Buried cables : Cc = 1

Cd = 1

Cs = 1

Protective device factor : Cf = 1

It = tabulated current carrying capacity

It = In / (Cg x Ci x Ca x Cf x Cc x Cs x Cd)

It = 45 / (1 x 1 x 1 x 1 x 1 x 1 x 1 )

It = 45.00 Amps

From TABLE 4D5 Cable selected = 6 mm²

Current capacity of cable selected = 47 Amps

TABLE 4D5 For 6 mm²: mV/A/m = 7.3

mV/A/m corrected for power factor = mV/A/m x Power Factor = 7.3 x 1 = 7.3

Voltdrop = (mV/A/m x Length x Design current) / 1000

Voltdrop = ( 7.3 x 10 x 41.3 ) / 1000

Voltdrop = 3.01 Volts

(Maximum permissible voltdrop (regulatuon - 525) = 11.5 Volts)

Calculated Cable size = 6 mm², Actual Earth conductor size = 2.5 mm²

Maximum Cable Length = 38.1 Metres

Kenny my friend, you will find that the manufacturer states 9.5KW at 240V and this equates to 8.7KW at 230v.

Therefore the Ib you refer to is 8725/230 = 37.93Amp

 

[Jabbajaws]

Hi Guys,

ive come across this same problem at least 3 times. All caused by 9.5kW showers on 6mm sq pvc/pvc.

The first two occasions, the problem lay in the 45Amp shower isolators, where the neutral conductor at the supply side, had melted the insulation completely, yet the conductor itself, remained intact.
Now the estimated lengths of these circuits(all separate addresses):

one 15m run, one 20m run (wired to 3036 30a fuse).

Now the third problem is the shower which is not yet turned on, as the plasterer has to finish, prior to commissioning of the circuit. The run is less than 5m and ive got the same problem, all for the sake of 1A.

Im going to comission the circuit, which is protected by a 40A breaker, housed with 30mA RCD (shower mcb unit) and ill monitor how it goes by checking the isolator, every so often. An l would suggest everyone else with this issue does the same.

 

[telectrix]

i'd have liked kenny's post if i could reads it, but i've lost my magnifying glass. LOL.

 

[Guitarist]

That's fine if it is a true method C. Not very often that the cable doesn't pass through some insulation or conduit along its run tho.

 

[bigdaddy]

Ive seen a 9kw shower ran on a 32 amp MCB

My shower has been like this for 10 years now with no problem 32A mcb wired in 10mm T+E, i put on a 32A because thats all i had at the time and said i will change it to a 40A but have never had to:martini_shaken:

oh and its 9KW BTW

 

[Kenny.]

^^^ that is truely an excellent effort , doff my cap to Kenny.
So , even by the book , 6mm ok.

Thanks but it was cable calc software then copy and paste :cool4:

 

[bigdaddy]

i did a more or less the same calc yesterday for a job on sunday and came out with the same result, 6mm should be fine but i am going to put in 10mm for piece of mind.

 

[ARCElectrical]

Recommend upgrading to 10mm for sure, who knows what the cable is running through or under. In my opinion, if its borderline then it's wrong. No room for maybe's in this game..

 

[Geoffsd]

Recommend upgrading to 10mm for sure, who knows what the cable is running through or under.

When you replace it unnecessarily you will.

Customers like paying for unnecessary work. They call it being ripped off

In my opinion, if its borderline then it's wrong. No room for maybe's in this game..

No such thing as borderline. It complies or it doesn't.

 

[PC Electrics]

I'm putting a new shower circuit in today. Here's how I ensure method C, by maintaining an air gap around the cable where it passes through the insulation layer.
This isn't finished yet. There's a Crabtree shower pull below that joist and there's another cable to put in yet.
I've left slack in the cable above the isolator just in case the cables should ever need reterminating. To reduce the chances of that happening the ends are treated with cord end ferrules which stop the strands spreading and improve the quality of the connection.

 

[Moses]

I'm putting a new shower circuit in today. Here's how I ensure method C, by maintaining an air gap around the cable where it passes through the insulation layer.
This isn't finished yet. There's a Crabtree shower pull below that joist and there's another cable to put in yet.
I've left slack in the cable above the isolator just in case the cables should ever need reterminating. To reduce the chances of that happening the ends are treated with cord end ferrules which stop the strands spreading and improve the quality of the connection.

Nice fabrication!
Well done!

 

[PC Electrics]

Kenny my friend, you will find that the manufacturer states 9.5KW at 240V and this equates to 8.7KW at 230v.

Therefore the Ib you refer to is 8725/230 = 37.93Amp

At last! Someone else who undestands the shower ratings and the effects of Ohms Law when the voltage is reduced!

Except that I think you made an arithmetic error:

(9500 x 230) / 240 = 9100kW

Ib = 9500/240 = 39.6A
or
Ib = 9100/230 = 39.6A

So, fine on a 40A for non-continuous use (or leave a space on at least one side of the MCB)

 

[PC Electrics]

Nice fabrication!
Well done!

Thanks. It takes less than 10 mins to cut that out of soil pipe and just a couple of minutes to install. There's a similar arrangement at the other two points the cables pass through the insulation. And the battons are simply screwed to the roof frame uprights.
This method also has the advantage of minimising the disturbance of the insulation and creating less 'fibre-gas'.
And, yes, it's a 9.5kW shower on a 40A MCB.

 

[Moses]

I understood that we had professional Electricians on this forum, who could at least use the Ohms Law!!!!

You have proved me wrong.

Here is my proof :-

Power=VxV/R

Rated power for the shower @ 240V is 9500W

R is the shower heater resistance

@ 240V, R = 240x240/9500W = 6.063 Ohms

Shower Ohms are constant at all volts, but the supply volts change to 230V

Power = VxV/R @ 230V =230x230/6.063 = 8725 Watts

Load Current @ 230V = Power/Volts = 8725/230 = 37.93Amps