Help with Volt Drop Calculations

[shelby3]

I have a 3 core 2.5mm SWA running from my house consumer unit to an external consumer unit in the garage. I've just upgraded this from a standard garage breaker to a RCD protected unit with 7 mcb's.

I've just been running through some figures in the TLC Volt Drop calculator to work out if I need to upgrade my current 2.5mm SWA.

These are my inputs

- SWA (3 Core) PVC
- Underground
- 2 of the circuits will be touching as they run through the same conduit (this is the 2.5mm supply cable to the consumer unit and the 2.5mm supply cable from the consumer unit to the shed)
- 3 KW of power
- Length of 10 metres

The calculator is saying my required cable size is 2.5mm and the MAX Cable Load is 16amp.

It doesn't look like I need to upgrade my supply cable from the house consumer unit, which is currently running off a 16amp breaker.

If I did however jump upto a 4mm supply cable, then running the figures through the calc and anticipate a higher usage of let's say 4kw then the current cable max load is 20amps. So therefore would a 4mm cable require a 32amp breaker?

Is there any harm in running a 2.5mm tail from the house consumer unit, to a 4mm SWA which terminates in the garage for a temporary period ?

 

[sparkybird]

beware the TLC calculator - it's a bit pessimistic!

Best to do the calcs yourself - it'll make quite a difference. You'll need to refer the to regs for the volt drop of your specific cable and then apply the forumla

SB

 

[ColJack]

to be fair though.. any calculator from a supplier is going to be on the side of the supplier..

they want you to buy the next size cable up as it's more profit for them..

I'm not saying that they fiddle the results massively, but if it's borderline 4mm but you can get away with it, then they're going to recomend 6mm aren't they..?

 

[Spark123]

You need to use the 2 core calculator as you are dealing with single phase, the 3 core option assumes 3 phase.
2.5mm seems very small for a supply to another CU imo.

 

[shelby3]

Well the supply from the house CU is running into an RCD (63amp) CU in the garage.

2.5mm seems quite small to me aswell. When running through the calc using single phase and taking into account the 2 touching circuits it say's I can run 2.5mm with upto 4kw of power going through it. More than I would ever need it for.

Is there any logic in upgrading my supply to let's say 6mm. For the sake of saving a few quid, I'd rather do the job now properly. The single phase 6mm will allow me to push through 7kw !!!

Now, my RCD incomer in the garage being 63amp, what's the max cable size you should be supplying to it ?

 

[Pensdown]

To be honest, IMO 6mm should be the minimum size for all sub-mains to out buildings.

 

[shelby3]

Advice taken !

The SWA terminates at a waterproof junction box inside the garage. I then run a 5 metre tail (6mm T&E) to the garage consumer unit in a 20mm conduit.

Now - the garage CU then runs a 2.5mm supply to the shed. Are there any issues with this running in the same 20mm conduit, and terminating in the same junction box, before being glanded and out to the Shed.

Basically, is there an issue with the 2 supplies using the same piece of conduit and sharing a junction box ?

 

[Spark123]

I think you'll be pushing it to physically fit both a 6mm and a 2.5mm in a 20mm conduit.
Re the supply, you appear to be trying to design to volt drop. You need to start by designing from the other way around, i.e. you need to start with the design current, i.e. what your loads are going to be. You then need to work out the size of cable required taking into account correction factors, then check it complies with the volt drop figures. The volt drop needs to be calculated to the point at which it is used, i.e. if you are allowed a 3% volt drop for lighting circuits then given for example a 1% VD for the final circuit inside the garage, you can only have 2% VD for the circuit feeding the fuse box.
Also, are there any metal piped services in the garage i.e. water? Are there any other metal parts in there in contact with earth?
I take it you are aware of the dreaded part p??

 

[ericmark]

If the calculator says that get rid of it and start again.
I use Excel one sheet (Data) has a list cable size to volt drop taken from the red book. And in main sheet
Voltage 230 Volts
Cable size 2.5 mm²
Cable length 10 Meters
Current Amps
Power 4000 Watts
Volt Drop 3.130434783
Volt Drop % 1.361058601
3%= 6.9
5%= 11.5
Formula =LOOKUP(B2,Data!C4:C15,Data!D415)*B3*IF(B4="",B5/230,B4)/1000
Where B2 = Cable size used Validation and enter cable sizes between comers so only valid sizes can be selected. Data 4 to 15 C & D is the list cable size to volt drop. B3 is length in meters. The If statement allows entry as Amp or Watts since table is mv/A/m on 230 supply watts to amps also on 230 supply and the 1000 changes it from millivolts to volts. Once made so easy to check any cable size with PDA on Phone also with excel so easy to make sure you have made no mistakes we know 80 meters limit on ring main so enter 2.5mm at 32 amp at 20 meters (80/4 as not ring main) and should read 11.5 volts and it does.
I can't see volt drop will be a problem on that distance you will be overloading cable well before the volt drop is exceeded.
I agree with Pensdown but I do think he should say why! To be it is in order to get the equivalent of 16mm copper on steel wire armor so any local earthing will not be likely to draw enough current to damage earth under fault conditions and normally you need to run 3 core in order for SWA to be thick enough not that you need 3rd core. But if there is no chance of any external earthing then not really a problem.

 

[shelby3]

Thank you very much for the detailed answers. That has certainly helped.

The incomer in the garage is a 63amp RCD, what's the maximum cable size you should be supplying to it ? and what's the ideal ?

 

[ericmark]

Oh dear you are quite a bit off track. The RCD has 4 ratings one it what leakage it will trip at i.e. 30ma the next is how long 40ms max is normal but for fire protection you have S type which includes a delay. Next is the maximum current that should normally be passed through the unit this is the 63A rating and finally how much current in can handle under fault conditions which is normally a number in a box non of these has any thing to do with supply cable size with maybe the exception of the prospective short circuit current i.e. the number found in the box.
It is the MCB or fuse supplying the garage in the house which will need to be smaller than what the cable can handle SWA cable will have a series of current rating according to where and how it is run Table 4D4A will most likely be the one you would use so in the ground ref method D for single phase 2.5mm² is rated 29 amp so would require a max of 25 MCB to protect it. With 4mm² is rated 37 amp so would require a max of 32 MCB to protect it. With 6mm² is rated 46 amp so would require a max of 40 MCB to protect it. With 10mm² is rated 60 amp so would require a max of 50 MCB to protect it. This is assuming in the ground if it was passed through an insulated wall this could be reduced. If for example it was connected to some 6mm² twin and earth for part of its run then it could be reduced to as low as 23.5 amp if going through insulation although I would not expect that to happen.
I would read http://www2.theiet.org/Publish/Wire...matters_electrical_installations_outdoors.pdf which covers most of your questions and remember you will need to consider earthing method which is why I said to use 6mm² SWA 3 core only to ensure the earth is up to scratch the forms http://www.theiet.org/publishing/wiring-regulations/forms/formspdf6.cfm?type=pdf will need completing and also it will come under Part P see http://www.planningportal.gov.uk/Pp...gportal.gov.uk/uploads/br/BR_PDF_ADP_2006.pdf sorry to say wiring a garage is not just sticking in a cable and you do need to consider all aspects and inspect and test once complete gone are the near enough days.
Eric

 

[dingbat]

Ericmark, you also appear to be a bit off track.

You might want to brush up on your understanding of circuit design procedures and earthing.

 

[shelby3]

thank you all for your inputs.