submain to new build flat

[Jimmyb]

installing a sub main to 5 x new flats but the service heads and meters will be about 50meters away

Ive worked out the design current to 74 amps due to it being electric heating and immersion ect (no gas)

The cable are to be brought in same duct and around 30c temp.

I worked out that 25mm will do the job but will leave with a voltdrop of 0.5volts for lighting so decided 35mm would be what was need.

I was going to install switch fuses after the meters and run swa into the flat but I was after some advice on what to do about connecting the swa into the consumer unit, I was thinking of a metal box in the service hatch which is running behide all the flats then mounting the consumer unit on the directly opposite side and bringing the cores straight into the cu to avoid any joints.

Will it be ok to use 35mm 2 core and get the earth from the swa the reason I wondering is I need to earth bond the water as it copper pipe and i thought it would be ok to use the swa for the main earth and bonding back to the MET but the guy i working with seems to think we cant use swa armouring if its for bonding and either we need 3 core or to run a separate 16mm earth for bonding

Any advise given i will be very greatful.

Many Thanks

Jimmyb

 

[Guest55]

having the dno supply 50mtrs away from the proposed flats is hardly ideal.
and to get all the armoureds running together you will need 6" ducting and a hefty grouping factor applied at the design stage.

personally i'd run a single cable into the building and have all the meters in the lobby mains cupboard.

 

[Jimmyb]

meters cant go anywhere else as the flats are being made inside of a massive building and are 50 meters from a outside wall and its the nearest place yedl will install them.

I have included the grouping factor which with a design current of 74amp and 35mm cable be used for voltdrop the grouping wasn't a problem, they have installed a service corridor which they are installing 12" duct for power cables and another 12" duct for water supplies.

 

[jackhammerJIM]

can you show your calculations for the volt drop it doesn't sound correct .

 

[Engineer54]

Sorry but 50 metres between the service heads/meters and the building with these 5 flats is a total nonsense, and one that i can't see the DNO accepting, especially using the installation method proposed eg a common duct/pipe!! And NO you can't use 2 core SWA cables for tails to supply each flat, the armouring will not have a sufficient CSA to fulfill that which is required for bonding purposes....

To be honest can't for the life of me see how your calculations are working either??

 

[Jimmyb]

I used software to work it out so ive copied and pasted it below

Supply voltage = 230 Volts
Ib - design current = 74 Amps
Protective device type = BS88
In - protective device rating = 80 Amps
Length of run of cable = 50 metres
Power factor = 1

Installation Method : Method E - (Cable Touching)

Ambient temp = 30 °C
Number of circuits including this one = 5
Length of cable in thermal insulation = none

Apply Correction factors:
From TABLE 4C1 : Cg = 0.8 (Grouping)

It = tabulated current carrying capacity
It = In / (Cg x Ci x Ca x Cc)
It = 80 / (0.8 x 1 x 1 x 1 )
It = 100.00 Amps
From TABLE 4D4A Cable selected = 25 mm²

TABLE 4D4B For 25 mm²; mV/A/m r = 1.75 and mV/A/m x = 0.17
mV/A/m Corrected for Power Factor = (cosØ mV/A/m r)+(sinØ mV/A/m x) = 1.75

Voltdrop = (mV/A/m x Length x Design current) / 1000
Voltdrop = ( 1.75 x 50 x 74 ) / 1000
Voltdrop = 6.48 Volts

If you saw the layout of the building you would understand that the meter cant get any closer and it is the only place that the DNO are willing to install them which I have confirmed with them today and also asked if they had any problems with the flat being 50 meters and they answered "why would we''

I thought 2 core 35mm swa worked out to be 19.3mm copper which is over half conductor size. The only problem I can see if it is PME and I needed to have a bonding conductor installed but since the incoming water is in plastic and there is no gas I don't need one and I understand that if a bonding conductor was needed it couldn't be part of inner cores or the armouring and had to be separate conductor due to the thermal effect of circulating currents.

 

[shanky887614]

I used software to work it out so ive copied and pasted it below

Supply voltage = 230 Volts
Ib - design current = 74 Amps
Protective device type = BS88
In - protective device rating = 80 Amps
Length of run of cable = 50 metres
Power factor = 1

Installation Method : Method E - (Cable Touching)

Ambient temp = 30 °C
Number of circuits including this one = 5
Length of cable in thermal insulation = none

Apply Correction factors:
From TABLE 4C1 : Cg = 0.8 (Grouping)

It = tabulated current carrying capacity
It = In / (Cg x Ci x Ca x Cc)
It = 80 / (0.8 x 1 x 1 x 1 )
It = 100.00 Amps
From TABLE 4D4A Cable selected = 25 mm²

TABLE 4D4B For 25 mm²; mV/A/m r = 1.75 and mV/A/m x = 0.17
mV/A/m Corrected for Power Factor = (cosØ mV/A/m r)+(sinØ mV/A/m x) = 1.75

Voltdrop = (mV/A/m x Length x Design current) / 1000
Voltdrop = ( 1.75 x 50 x 74 ) / 1000
Voltdrop = 6.48 Volts

If you saw the layout of the building you would understand that the meter cant get any closer and it is the only place that the DNO are willing to install them which I have confirmed with them today and also asked if they had any problems with the flat being 50 meters and they answered "why would we''

I thought 2 core 35mm swa worked out to be 19.3mm copper which is over half conductor size. The only problem I can see if it is PME and I needed to have a bonding conductor installed but since the incoming water is in plastic and there is no gas I don't need one and I understand that if a bonding conductor was needed it couldn't be part of inner cores or the armouring and had to be separate conductor due to the thermal effect of circulating currents.

let me ask you a question

if your getting 6.48vd at board how much at end of line lights?

 

[shanky887614]

need to felete

 

[davesparks]

I thought 2 core 35mm swa worked out to be 19.3mm copper which is over half conductor size. The only problem I can see if it is PME and I needed to have a bonding conductor installed but since the incoming water is in plastic and there is no gas I don't need one and I understand that if a bonding conductor was needed it couldn't be part of inner cores or the armouring and had to be separate conductor due to the thermal effect of circulating currents.

Why can't a cable core be used as a bonding conductor?

 

[shanky887614]

Why can't a cable core be used as a bonding conductor?

if a core cant be used why have the dno and national grid wired most of the country in 3c+

 

[davesparks]

if a core cant be used why have the dno and national grid wired most of the country in 3c+

I think the national grid have done most of theirs in uninsulated singles, probably made up of an aluminium skin with a steel strength layer and an air core.

 

[shanky887614]

I think the national grid have done most of theirs in uninsulated singles, probably made up of an aluminium skin with a steel strength layer and an air core.

true lol, but they have no sleaving or coating to confuse there poor sparks lol
air gap up them towers

 

[Jimmyb]

Why can't a cable core be used as a bonding conductor?

Here's the regs for using SWA steel armouring as a cpc, main earth or main bond or any combination of these. Source of info BS7671 and NICEIC Technical Manual A57-1

1, As a cpc - Yes. It must comply with the thermal constraints of a fault current refered to in 543-01-01 which can be calculated with the adiabatic equation ( k values for armouring given in table 54D) or can be selected from table 54G(cross sectional areas of steel can be found in GN1 tables)

2, As a main bonding conductor -Yes - The compliance of reg 547-02-01 (except for PME) which is a copper equivalent of half the csa of the main earth and not less than 6mm and need not exceed 25mm. For PME, it is NOT acceptable for use as a main bonding conductor due to the thermal effect of circulating currents as this affects the current carrying capacity of the cable. It is also NOT acceptable under PME conditions to use a core of the armoured cable for the main bond due to the same reasons.

3, As a main earth - yes - in definitions, a main earth is a cpc, therfore it must comply with 543-01-01 as above, also for PME, this must act as a main bond and therefore is unsuitable due to heating effects, unless it can be shown by design that this will not affect the thermal constraints of fault currents and current carrying capacity of the cable, and show compliance with table 54H as required by 547-02-01.

4, Any combination of these - yes - If all of the requirements are met for the relevant uses, then there is no reason not to have more than one usefor the steel armouring, the same applies for a core of the cable.

from the above, it is clear that there are concerns in mixing the bonding and earthing for PME suppliies with the armouring or cores of an armoured cable, however, it is fine to use as a cpc (subject to calculation)

 

[davesparks]

Ok, so considering that 3 core SWA is designed to have 3 cores carrying full rated current during normal use (it is after all designed for 3 phases, hence the colours) then, how will these circulating currents heat the cable more than if it was being used this way?

 

[Jimmyb]

tbh I cant really get my head round it, this is what the guy im working said, I wanted to use the swa as earth and he stated you cant and sent me this on the night.

I have no problem running separate earth for bonding if needed but I wont need to bond anything so my understanding is that I can use the armoured for earth as it is only a cpc and when Ive checked the chart is shows the copper equivalent to be 19.2mm so is at least half of conductor size and is not less than 6mm.

Does anyone agree/disagree ?

also are my cable sizes wrong if so can I have some advice plz

 

[davesparks]

What chart are you using to get copper equivalent?

You should be calculating the size of the cpc using the adiabatic equation using the appropriate K factor.
Or using the appropriate calculation from table 54.7 depending on conductor size

 

[shanky887614]

What chart are you using to get copper equivalent?

You should be calculating the size of the cpc using the adiabatic equation using the appropriate K factor.
Or using the appropriate calculation from table 54.7 depending on conductor size

to be honest the mv/a/m value you should be using is the one in the correct table in bs7671

it varies depending on different factors just for the volt drop then there is temperature,grouping etc to consider in your next calc

i got the digital version for £60 odd, not worth getting now unless you need it.

the next amendment comes out in jan

 

[Jimmyb]

here is chart here

 

[Engineer54]

here is chart here

Jesus how many more times, .....throw that bloody old table in the bin where it belongs!!!

 

[shanky887614]

at least use upto date info, in your bgb that you should own if your designing circuits