Cable Size For Shower and Correction Factors For MCB's

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dcool

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Trying to work out cable size for a new shower and not sure what correction factors to apply to things these days. It's been a little while since I've done this.

Here's what I have done so far.

10.5kW @ 230 = 45.65A = Ib

Am I correct in saying that I can't apply diversity to this? The shower is thermostatically controlled, 10 settings.

Selected a 50A MCB (I know a lot of people prefer RCD these days but let's stick with MCB for now please).

Now I am really confused at this bit, do I need to 1.45 x 50A and divide by the correction factors to get Iz. It's the 1.45 x 50A that's stumped me, I'll still apply the other correction factors for grouping, ambient temp etc.

I find some stuff mentioning 1.45 and others not. So it is a bit unclear or have I completely got it wrong.

Any help appreciated.
 
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dcool said:
Selected a 50A MCB (I know a lot of people prefer RCD these days but let's stick with MCB for now please).
Click to expand...

These two devices are entirely different things and it is not a matter of preference.
Circuit breakers provide overcurrent protection; in this case you are concerned specifically with overload.
RCDs do not offer such protection but you will need a 30mA RCD for a circuit supplying equipment in a location containing a bath or shower.

Now I am really confused at this bit, do I need to 1.45 x 50A and divide by the correction factors to get Iz. I find some stuff mentioning 1.45 and others not. So it is a bit unclear or have I completely got it wrong.
Click to expand...

You do seem to have got it completely wrong. You are referring to reg 433.1.1, item iii). For circuit breakers this is irrelevant.

And forget the rating factors - in a domestic installation the only relevant one is Ci, for thermal insulation and if you apply this one you will not find a cable that can realistically be terminated in a shower, or any local switch, for that matter.

Select your cable from table 4D5 on p.282, depending on the installation method - you should avoid any contact with thermal insulation, otherwise you're back with the size problem.

But I have serious doubts as to your ability to reliably judge whether or not your installation will be compliant.
If you are confused by cable selection, what else do you not fully understand? And how will you carry out the inspection, testing and certification required by BS 7671?
 
I think you are over complicating things.
At 50A you likely will need 16mm² cable which method 100# is rated at 57A and with a B50 MCB the line / neutral loop impedance will need to be better than 0.92 ohms to trip with short circuit. The earth loop impedance would be controlled by RCD size which has to be max 30ma so really either 30ma or 10ma so would be Table 41.5 note 2 max 200 ohms.

The only consideration is if you used a B45 MCB then you could reduce cable size to 10mm² (Method 100#) and since that is so close to the 45.65217A it is unlikely to cause a problem.

I will point out you need both over current (MCB) and earth leakage (RCD) protection these can be combined with one unit to do all three functions (RCBO) I say three because a MCB has two overload devices in one so a B45 MCB will have a thermal trip which will after quite some time trip at 45A and a magnet trip which will trip near instant at 5 x 45 = 225A and this is the figure used to ensure it will trip with short so 230/225 = 1.0222 ohm minus the impedance of supply leaves you 0.67 ohms for cable.

It does seem a little open as to the line / neutral impedance as the regulations does say it needs to trip in 5s for TN (411.3.2.3) or 1s TT (411.3.2.4) comes under "411.3.2 Automatic disconnection in case of a fault" and this does not say "earth fault" it just says "fault" and it is open to interpretation. Many consider this only applies to an earth fault. Since all bathroom items must have RCD protection the RCD will provide the earth fault protection.

However I would consider it prudent to keep within the 0.67 ohms anyway.

If the installation method is not 100# then of course cable sizes will change it is all dependent on the thermal insulation properties of walls and celling.
 
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ban-all-sheds said:
PS - Eric - an RCD is NOT a work-round for a too-high Zs in a TN installation....
Click to expand...

Yes it is. Not a good one, admittedly, and certainly not good practice, but it is exactly a way of overcoming high Zs.

If it wasn't acceptable then there wouldn't be any TT installations.
 
LoveRocket said:
ban-all-sheds said:
PS - Eric - an RCD is NOT a work-round for a too-high Zs in a TN installation....
Click to expand...

Yes it is. Not a good one, admittedly, and certainly not good practice, but it is exactly a way of overcoming high Zs.

If it wasn't acceptable then there wouldn't be any TT installations.
Click to expand...

IMO best practice design is to select a cable that can satisfy max. Zs taking into consideration Ze. It would be lazy to just throw in an RCD to achieve disconnection, unless of course there is no way around it ie TT systems.
 
Which is what I meant - apologies for the unclear wording.

Would this make everyone happier?

PS - Eric - an RCD should NOT be used as a work-round for a too-high Zs in a TN installation....
 
Thanks for your input.

dingbat said:
dcool said:
Selected a 50A MCB (I know a lot of people prefer RCD these days but let's stick with MCB for now please).
Click to expand...

dingbat said:
These two devices are entirely different things and it is not a matter of preference.
Circuit breakers provide overcurrent protection; in this case you are concerned specifically with overload.
RCDs do not offer such protection but you will need a 30mA RCD for a circuit supplying equipment in a location containing a bath or shower.
Click to expand...

Yes. Part of why I am saying stick with MCB is because I am 99% sure the existing shower is supplied from a split board with RCD.

dcool said:
Now I am really confused at this bit, do I need to 1.45 x 50A and divide by the correction factors to get Iz. I find some stuff mentioning 1.45 and others not. So it is a bit unclear or have I completely got it wrong.
Click to expand...

dingbat said:
You do seem to have got it completely wrong. You are referring to reg 433.1.1, item iii). For circuit breakers this is irrelevant.

And forget the rating factors - in a domestic installation the only relevant one is Ci, for thermal insulation and if you apply this one you will not find a cable that can realistically be terminated in a shower, or any local switch, for that matter.

Select your cable from table 4D5 on p.282, depending on the installation method - you should avoid any contact with thermal insulation, otherwise you're back with the size problem.
Click to expand...

I thought I had the 1.45 wrong. Never heard that about the correction factors before.

dingbat said:
But I have serious doubts as to your ability to reliably judge whether or not your installation will be compliant.
If you are confused by cable selection, what else do you not fully understand? And how will you carry out the inspection, testing and certification required by BS 7671?
Click to expand...

I won't be doing the the full installation and testing. A qualified electrician will. I'm just trying to get a feel for it to get stuff prepped for it.
Click to expand...
 
ban-all-sheds said:
Which is what I meant - apologies for the unclear wording.

Would this make everyone happier?

PS - Eric - an RCD should NOT be used as a work-round for a too-high Zs in a TN installation....
Click to expand...

I fully understand what you're saying, and am fully aware that you're refering to TN installations only. (Although my mentioning of TT installations might have caused confusion)

However it is still acceptable to use an RCD as a work-around of a too high Zs. Not great, but it IS acceptable.

If relying on an RCD to ensure disconnection wasn't deemed safe then there'd be no TT installations.
 
ericmark said:
I think you are over complicating things.
At 50A you likely will need 16mm² cable which method 100# is rated at 57A and with a B50 MCB the line / neutral loop impedance will need to be better than 0.92 ohms to trip with short circuit. The earth loop impedance would be controlled by RCD size which has to be max 30ma so really either 30ma or 10ma so would be Table 41.5 note 2 max 200 ohms.

The only consideration is if you used a B45 MCB then you could reduce cable size to 10mm² (Method 100#) and since that is so close to the 45.65217A it is unlikely to cause a problem.

I will point out you need both over current (MCB) and earth leakage (RCD) protection these can be combined with one unit to do all three functions (RCBO) I say three because a MCB has two overload devices in one so a B45 MCB will have a thermal trip which will after quite some time trip at 45A and a magnet trip which will trip near instant at 5 x 45 = 225A and this is the figure used to ensure it will trip with short so 230/225 = 1.0222 ohm minus the impedance of supply leaves you 0.67 ohms for cable.

It does seem a little open as to the line / neutral impedance as the regulations does say it needs to trip in 5s for TN (411.3.2.3) or 1s TT (411.3.2.4) comes under "411.3.2 Automatic disconnection in case of a fault" and this does not say "earth fault" it just says "fault" and it is open to interpretation. Many consider this only applies to an earth fault. Since all bathroom items must have RCD protection the RCD will provide the earth fault protection.

However I would consider it prudent to keep within the 0.67 ohms anyway.

If the installation method is not 100# then of course cable sizes will change it is all dependent on the thermal insulation properties of walls and celling.
Click to expand...

You are probably right about over complicating. The installation method is actually C and I'm pretty certain it's fed from an RCD.

Looks like I need to see if I can get a 45A MCB to make things a bit simpler based on ban-all-sheds reply too.

Thanks for your input.
 
dcool - I think you're confusing a bog standard 'RCD' (or RCCB to give the specific term) with an RCBO.

Anyway - 10mm cable on a B45MCB on the RCD side of your CU, keep the cable away from any insulation, job done.
 
dcool said:
Am I correct in saying that I can't apply diversity to this? The shower is thermostatically controlled, 10 settings.
Click to expand...

most showers have 2 heat settings, the other 10 "heat settings" are controlled by the flow of water through the heater.

the heater is either on high or low, the slower flow of water allows it to heat up more.
 
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