RCBO size calculation

heavyduty · 6 May 2011

what calculation or device is often used to calculate size of RCBO?

so in my main reception i have 2 chandelier lights with 12 x 60W bulbs = 3A
i want this to be on a single 1.5mm circuit.

so i need 6A RCBO.

am i missing something in this calculation like cable run length?

i want to understand this so i can compare my shopping list with my electrician's.

is this a good tool to use?

http://www.digitalrebellion.com/webapps/power_calc.html

for kitchen, i've added up all the appliances and not sure whether to use 40A or 45A...
thanks.

riveralt · 6 May 2011

heavyduty said:
what calculation or device is often used to calculate size of RCBO?

so in my main reception i have 2 chandelier lights with 12 x 60W bulbs = 3A
i want this to be on a single 1.5mm circuit.
so i need 6A RCBO.
am i missing something in this calculation like cable run length?

The design of a circuit and consequently the protective devices it needs takes a number of factors into account including some of the ones you have identified above.
There are eight basic steps - simplified here.
Determine the design current Ib
Select the rating of the protection In
Select the relevant correction factors (CF's)
Divide In by the relevant CFs to give a tabulated cable current carrying capacity It
Choose a cable size to suit It
Check the voltage drop
Check for shock risk elements
Chech for thermal complaints.

I want to understand this so i can compare my shopping list with my electrician's.

is this a good tool to use?

http://www.digitalrebellion.com/webapps/power_calc.html

for kitchen, i've added up all the appliances and not sure whether to use 40A or 45A...
thanks.

For appliances you should also add the diversity factor into the equation.

Your best bet is to buy the On Site Guide - that will give you a basic understanding of the calculations you will need to make to design circuits.
Alternatively leave it to and trust the electrician you are employing.

bernardgreen · 6 May 2011

heavyduty said:
for kitchen, i've added up all the appliances and not sure whether to use 40A or 45A...

Adding up the appliance loads with or without diversity is irrelevent to the choice of protective devices.

The maximum safe current carrying capacity of the cable used is the MAIN factor is deciding the amperage at which the protection device will operate.

Adding up the appliance loads with or without diversity will only tell you what size cable or cables are needed. Then you choose protection to suit those cables.

ricicle · 6 May 2011

bernardgreen said:
Adding up the appliance loads with or without diversity will only tell you what size cable or cables are needed. Then you choose protection to suit those cables.

Unless I am mis-reading you have it wrong Bernard. When designing a circuit you ascertain the design current of the circuit, then the protective device, then the cable to suit the protective device (along with the other deciding parameters)

JohnW2 · 6 May 2011

ricicle said:
Unless I am mis-reading you have it wrong Bernard. When designing a circuit you ascertain the design current of the circuit, then the protective device, then the cable to suit the protective device (along with the other deciding parameters)

Indeed. The limited range of MCB ratings available means that one sometimes has to use a larger cable than actually necessary for the circuit, simply because of what MCBs are available. For example, a dedicated supply to a load requiring, say, 24A could be run in 2.5mm² (clipped direct), were it not for the fact that the lowest rating usable MCB available would normally be 32A, requiring 4mm² cable.

Kind Regards, John.

ban-all-sheds · 6 May 2011

433.3

bernardgreen · 6 May 2011

ricicle said:
Unless I am mis-reading you have it wrong Bernard. When designing a circuit you ascertain the design current of the circuit, then the protective device, then the cable to suit the protective device (along with the other deciding parameters)

I read it that maybe the cable was already in place and the OP was possibly ignoring the need to limit current to the cable's safe carrying current.

I know that when designing find out the client's anticpated load, double it and go from there for how many cables are needed and what size they need to be.

ban-all-sheds · 6 May 2011

heavyduty said:
so in my main reception i have 2 chandelier lights with 12 x 60W bulbs = 3A

720W?

Do you hand out welding goggles to your guests?

i want to understand this so i can compare my shopping list with my electrician's.

You should shop for an electrician you trust.

is this a good tool to use?

http://www.digitalrebellion.com/webapps/power_calc.html[/QUOTE]
No.

//www.diynot.com/wiki/electrics:mcbtocable

http://www.tlc-direct.co.uk/Book/4.3.1.htm

http://www.tlc-direct.co.uk/Book/4.3.11.htm

http://www.tlc-direct.co.uk/Book/5.3.1.htm

http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf

http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

for kitchen, i've added up all the appliances and not sure whether to use 40A or 45A...

Click to expand...

Use an electrician.

JohnW2 · 6 May 2011

ban-all-sheds said:
433.3

If this is a response to my post (hard to tell

), what is meant to be its significance?

Kind Regards, John.

ban-all-sheds · 6 May 2011

JohnW2 said:
If this is a response to my post (hard to tell ),

It was immediately after your post, so yes.

what is meant to be its significance?

JohnW2 said:
For example, a dedicated supply to a load requiring, say, 24A could be run in 2.5mm² (clipped direct), were it not for the fact that the lowest rating usable MCB available would normally be 32A, requiring 4mm² cable.

Read 433.3 et seq about only having fault protection.

pjcomp · 6 May 2011

Just to clarify, BAS means regulation 433.3 in Wiring Regulations, aka BS7671:2008, aka the Big Red Book ... or is the Big Green One out yet with the amendments?

PJ

JohnW2 · 6 May 2011

ban-all-sheds said:
JohnW2 said:

If this is a response to my post (hard to tell ),

Click to expand...

It was immediately after your post, so yes.

Thanks for confirming; 'immediately after' in itself obviously means little; responses very often appear several posts after the one they relate to.

ban-all-sheds said:
what is meant to be its significance?

Click to expand...

JohnW2 said:

For example, a dedicated supply to a load requiring, say, 24A could be run in 2.5mm² (clipped direct), were it not for the fact that the lowest rating usable MCB available would normally be 32A, requiring 4mm² cable.

Click to expand...

Read 433.3 et seq about only having fault protection.

That's what I suspected you meant, but (on the assumption that 433.3.3 did not apply) that provision for only having fault protection (i.e. no overload protection) would presumably only apply if 'the characteristics of the load were such that the cable was unlikely to carry an overload current' (433.3.1(ii)). I suppose you are suggesting that this would be satisfied if it were a corretly designed dedicated circuit, but is that necessarily the case? If, for example, one correctly designed a cooker circuit on the basis of the normal diversity figures, would not the 'overcurrent' due to switching every part of the cooker on simultaneously (maybe with the oven door open!) qualify as 'overload current' (certainly would not qualify as a 'fault current')?

Kind Regards, John

JohnW2 · 6 May 2011

pjcomp said:
Just to clarify, BAS means regulation 433.3 in Wiring Regulations, aka BS7671:2008, aka the Big Red Book ... or is the Big Green One out yet with the amendments?

Thanks - but as BAS knows only too well, that's one thing I do know well

I was asking him what he felt was the significance of 433.3 of the regs to the point under discussion.

Kind Regards, John

ban-all-sheds · 6 May 2011

JohnW2 said:
Thanks for confirming; 'immediately after' in itself obviously means little; responses very often appear several posts after the one they relate to.

Not from me they don't, and I'll have a go at people who indulge in such careless and confusing actions.

That's what I suspected you meant, but (on the assumption that 433.3.3 did not apply) that provision for only having fault protection (i.e. no overload protection) would presumably only apply if 'the characteristics of the load were such that the cable was unlikely to carry an overload current' (433.3.1(ii)). I suppose you are suggesting that this would be satisfied if it were a corretly designed dedicated circuit,

You'd designed it - apologies for not thinking that you'd not done it correctly.....

but is that necessarily the case? If, for example, one correctly designed a cooker circuit on the basis of the normal diversity figures, would not the 'overcurrent' due to switching every part of the cooker on simultaneously (maybe with the oven door open!) qualify as 'overload current' (certainly would not qualify as a 'fault current')?

In that case you may not be able to omit overload protection. You can't always.

But equally you don't always need 4mm² cable for a 24A load because the OPD is 32A.

Advance warning: This topic has been known to trigger pathological behaviour in certain posters here.

JohnW2 · 6 May 2011

ban-all-sheds said:
....provision for only having fault protection (i.e. no overload protection) would presumably only apply if 'the characteristics of the load were such that the cable was unlikely to carry an overload current' (433.3.1(ii)). I suppose you are suggesting that this would be satisfied if it were a corretly designed dedicated circuit, but is that necessarily the case? If, for example, one correctly designed a cooker circuit on the basis of the normal diversity figures, would not the 'overcurrent' due to switching every part of the cooker on simultaneously (maybe with the oven door open!) qualify as 'overload current' (certainly would not qualify as a 'fault current')?

Click to expand...

In that case you may not be able to omit overload protection. You can't always.

Quite so; that was the point I was making.

ban-all-sheds said:
But equally you don't always need 4mm² cable for a 24A load because the OPD is 32A.

Agreed, provided one can convince oneself (and others) that it is unlikely that the cable will ever carry an overload current - which one may well be able to do with some dedicated loads.

Given how over-cautious (sometimes to the point at which some people talk of 'silliness') the regs can get, this degree of common sense in the regulations comes as somewhat of a breath of fresh air - even if it's not all that common a situation we're talking about.

ban-all-sheds said:
[Advance warning: This topic has been known to trigger pathological behaviour in certain posters here.

I know, but don't worry about me - I've had a jab of the vaccine

Kind Regards, John.

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RCBO size calculation

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