Replace Garage CU on existing supply cable - calcs check

[JB1970]

Hi

I've recently been carrying some work out on a friends house. I've rewired a radial circuit & lights in the garage back to the existing CU. This could do with replacing as it is an old unit with rewireable fuses, one of the carriers cannot be fastened back properly exposing the busbar if the fuse is pulled What I'm trying to confirm is whether the existing supply cable is up to the job. I'd rather not replace unless its not up to standard as it means lifting concrete. I've done the calc from the 17th but could do with the heads up whether I'm correct. Details as follows:

- House supply - TNC-S
- Protective device for the garage (in house CU) - Type 2 20A BS3871 MCB.
- Design current - Full 16A for the garage radial plus 4 x 58w fluorescent fittings (4 x 58 x 1.8 / 230 = 1.82A) 18 Amps.
- Cable - 2.5mm FT&E (1.5mm CPC) first 20 metres beneath thermal insulation not exceeding 100mm. Last 10 metres 1.5mm 2 core SWA 70 degree direct in ground.
- Current carrying capacity of existing 2.5 FT&E 21 Amps for method
- Current carrying capacity of existing 1.5mm SWA 21 Amps for method
- Volt drop 6.48v for 2.5 & 5.22v for SWA. Total 11.7V/230V x 100 = 5.08%. OK for 6% max (Table 4Ab Low voltage installation from private LV supply)
- Table B6 in the on site guide states I need to meet max measured Zs of 1.32 ohms for compliance on disconnection times.

Any help appreciated. I am 17th edition & have C&G 236 (from way back)

Many thanks

 

[EFLImpudence]

Don't know why you have not been answered.

I've recently been carrying some work out on a friends house.

You shouldn't really need to ask if working on someone else's property.

I've rewired a radial circuit & lights in the garage back to the existing CU. This could do with replacing as it is an old unit with rewireable fuses, one of the carriers cannot be fastened back properly exposing the busbar if the fuse is pulled What I'm trying to confirm is whether the existing supply cable is up to the job. I'd rather not replace unless its not up to standard as it means lifting concrete.

Have you tested it?

I've done the calc from the 17th but could do with the heads up whether I'm correct. Details as follows:

- House supply - TNC-S
- Protective device for the garage (in house CU) - Type 2 20A BS3871 MCB.
- Design current - Full 16A for the garage radial plus 4 x 58w fluorescent fittings (4 x 58 x 1.8 / 230 = 1.82A) 18 Amps.
- Cable - 2.5mm FT&E (1.5mm CPC) first 20 metres beneath thermal insulation not exceeding 100mm. Last 10 metres 1.5mm 2 core SWA 70 degree direct in ground.
- Current carrying capacity of existing 2.5 FT&E 21 Amps for method
- Current carrying capacity of existing 1.5mm SWA 21 Amps for method
- Volt drop 6.48v for 2.5 & 5.22v for SWA. Total 11.7V/230V x 100 = 5.08%. OK for 6% max

All seems correct.
Have you installed an RCD?

(Table 4Ab Low voltage installation from private LV supply)

Is it really a private supply?

Table B6 in the on site guide states I need to meet max measured Zs of 1.32 ohms for compliance on disconnection times.

Correct. It shouldn't be a problem.

 

[securespark]

What's your Zdb?

 

[JB1970]

Thanks for the responses.

Testing - The replacement radial & lights have been temporarily reconnected to the existing CU. As this was to be replaced immenently, currently i've only carried out the dead testing - continuity, polarity, insulation resistance. Polarity has been checked again after being energised. Haven't measured zs of the circuits yet, waiting to change the CU first.

RCD - Currently the circuit supplying the old CU is on the the RCD side of the split load CU in the house. My intention was to move this MCB to the non RCD side of that board & have 30mA RCD main switch on the new garage CU. That would be my preference but last couple of metres drop from loft to CU in house is within wall. Can I omit RCD protection on the feeder cable in this case or is it a no no.

Private supply - was unsure of this. My thinking was as a sub main from the house CU - yes, i use the 6 pc volt drop from table 4AB for the feeder cable only, then use the 3 & 5 pc volt drop for the two final circuits on the garage CU?

Zdb - currently not tested but will do tomorrow. Assume I can do this as a direct measurement from garage or measure the R1 + R2 of the feeder & add to ze measured at the main CU.

Appreciate i've gone about this backwards! Power was needed in the garage for a new door & i found the existing feed to the spur was in 1mm from a socket, accessories were smashed & there were single insulated cables through ungrommeted cable entries (& screw holes) into the light fittings. For this reason i rewired the garage end first.

 

[JohnW2]

RCD - Currently the circuit supplying the old CU is on the the RCD side of the split load CU in the house. My intention was to move this MCB to the non RCD side of that board & have 30mA RCD main switch on the new garage CU. That would be my preference but last couple of metres drop from loft to CU in house is within wall. Can I omit RCD protection on the feeder cable in this case or is it a no no.

If any of the cable is buried <50mm deep in walls, then it needs RCD protection (unless it is adequately 'mechanically protected').

Private supply - was unsure of this.

It's not a private supply - you're stuck with the 5%/3% VD figures (although they are only 'guidances')

Zdb - currently not tested but will do tomorrow. Assume I can do this as a direct measurement from garage or measure the R1 + R2 of the feeder & add to ze measured at the main CU.

If I understand what you're saying, you've probably got that back-to-front. Zdb is the loop impedance as measured at the (house) CU. Add R1+R2 to that and you should get the Zs at the end of the circuit. Alternatively, you could estimate Zdb by subtracting R1+R2 from Zs measured ast the end of the circuit.

Kind Regards, John

 

[JB1970]

Thanks John

Had a look around regarding zdb & found a lot of discussion/argument/confusion on other forums as to what should be recorded & how the result should be obtained. As I understand (or possibly don't!):

For the distribution circuit in the house CU, either:
a - carry out a measurement of zs at the supply terminals of the remote db
b - calculate this zs by adding the r1 + r2 of the distribution circuit to ze
Record this on the schedule of results for the house CU

For the remote CU in the garage:
a - zdb is measured in the same manner as ze would be at the house. This is going to give a result the same as step a above, though there seems to be a lot of debate as to whether the incoming earth at the remote CU should be disconnected for the test (unlikely to be any parallel paths unless anything is bonded to the remote CU?)
Record this on the schedule of results for the garage CU, but be sure to indicate that it is zdb & not ze that is noted.

As for the RCD protection on the cable to the garage, possibly easier to re route the last couple of metres of the cable from the loft to the CU into a length of trunking in the cupboard, to avoid needing to deal with discrimination between the two RCD's

Kind regards

John

 

[ericmark]

I make it 11.65 volt drop taking account of correction factor so although over the limit for power not that much over where the problem lies is with lights.

4 x 58w fluorescent fittings will not like the volt drop and could fail.

Also there is the design current issue in the main we take the MCB or fuse size as being the design current so 6 + 16 = 22A not 18A assuming two MCB's in garage and since you can't get a 21A MCB it would need to be 20A in the house so design current 20A.

Even with 2.5mm² throughout still a 10.6 volt drop well over the 6.9 volt allowed. 4mm² gives 6.2 volt drop so design wise it would need to be 4mm².

Using HF lights likely you could get away with the volt drop without any real problems but the question is when doing it for some one else can you really not conform?

Even with a 13A supply still 7.9 volt drop even that need 2.5mm² all the way to comply. 13A at 30 meters with 2.5mm² I get to 6.44 volt drop.

My calculations will be a little out as working on installation method 100 but not really out enough to worry about.

If it was my own then I would use LED lights which will take a huge volt drop and not worry. But for some one else it's another story.

 

[JohnW2]

Thanks John ... Had a look around regarding zdb & found a lot of discussion/argument/confusion on other forums as to what should be recorded & how the result should be obtained. As I understand (or possibly don't!):For the distribution circuit in the house CU, either:
a - carry out a measurement of zs at the supply terminals of the remote db
b - calculate this zs by adding the r1 + r2 of the distribution circuit to ze
Record this on the schedule of results for the house CU.

Indeed - but see comments below about parallel paths.

For the remote CU in the garage:
a - zdb is measured in the same manner as ze would be at the house. This is going to give a result the same as step a above, though there seems to be a lot of debate as to whether the incoming earth at the remote CU should be disconnected for the test (unlikely to be any parallel paths unless anything is bonded to the remote CU?)

I don't really understand what difference you are postulating between 'Measure Zs at the supply terminals of the remote DB" and "Measure Zdb (I presume you mean of the remote CU) in the same manner as Ze would be at the house" - aren't they essentially the same? In both cases, you would need to disconnect all possible 'parallel paths' if you wanted accurate answers. Zs of final circuits supplied by the remote CU would presumably, as usual, be measured with parallel paths in place.

As for the RCD protection on the cable to the garage, possibly easier to re route the last couple of metres of the cable from the loft to the CU into a length of trunking in the cupboard, to avoid needing to deal with discrimination between the two RCD's

I'm not sure what 'two RCDs' you're talking about - if the distribution cable to the remote CU was RCD protected, there would be no need (unless you are a 'belt and braces' person!) to have any further RCDs downstream.

Kind Regards, John

 

[JB1970]

Hi

Not sure the 11.65V should make the lights fail (in this instance). Perhaps if the supply voltage was at exactly 230V, but in this instance I believe (but will recheck) it's more like 254V measured.

With regard to design current I've taken the full 16A of the radial MCB but used actual load of the lights. As a 5m x 5m open space not likely to have any further lighting fitted. Initially I did the same allowing for 22A & 4mm

 

[EFLImpudence]

The earthing conductor is only disconnected for measuring Ze.
(It is removed so the measurement discounts all parallel paths)

Ze is the external EFLI.
There is only one Ze; it is at the origin of the installation.

All other EFLI measurements including remote CU/DBs are Zs which include parallel paths.
(otherwise you would have to disconnect ALL parallel paths at the house.)

 

[JohnW2]

I make it 11.65 volt drop ... 4 x 58w fluorescent fittings will not like the volt drop and could fail.

I do wonder about this, in practice.

As we know, the great majority of UK supplies are >230V, most probably being nearer to (including well above) 240V. That means that, for nearly all people, the supply voltage could actually be between 6% and &#8805;10.3% of what it currently is and yet still be within the permitted range (i.e. >216.2V). Are you suggesting that, say, "4 x 58W fluorescent fittings" might 'fail' (even with minimal VD in the cables supplying them) if the consumer was in the unlucky very small minority of consumers who have a supply voltage less than 223.V (230V - 3%)?

Kind Regards, John

 

[JohnW2]

The earthing conductor is only disconnected for measuring Ze. (It is removed so the measurement discounts all parallel paths). Ze is the external EFLI. There is only one Ze; it is at the origin of the installation.

Indeed.

All other EFLI measurements including remote CU/DBs are Zs which include parallel paths. (otherwise you would have to disconnect ALL parallel paths at the house.)

Perhaps not well enough expressed, but that was meant to be my point. IF one wanted to get the same answer (for the EFLI at the remote CU) using both methods mentioned by OP, one would have to "disconnect ALL parallel paths" when undertaking the EFLI ("Zs") measurement at the remote CU.

Kind Regards, John

 

[JB1970]

Regarding RCD's, yes the circuit feeding the garage is RCD protected in the house. However it's an old Wylex NN split load board so a fault in the garage will take out half of the house (don't suppose you can get an RCBO to fit this). I'd rather have it trip the garage only.

 

[JohnW2]

Regarding RCD's, yes the circuit feeding the garage is RCD protected in the house. However it's an old Wylex NN split load board so a fault in the garage will take out half of the house (don't suppose you can get an RCBO to fit this). I'd rather have it trip the garage only.

Fair enough - in fact I'd forgotten that you'd told us most of that before. In that case, as you suggested, if you want to be totally compliant without RCD protection at the house end, you'll have to dig the offending couple of metres of cable out of the wall.

Kind Regards, John

 

[EFLImpudence]

You could have a separate RCCB or RCBO in a small enclosure near the CU.