Updating cable calc spreadsheet

[MartinPaul]

Hi All,

Just looking for some advice: I'm in the process of updating an old cable calc spreadsheet that helps to spit out Zs and Voltage Drop figures but I'm just looking for some clarity on figures.

For 3-core 16mmsq SWA, single phase, I'm looking at table 4D4B in BS7671 and it states:

• Two-core cable, single-phase AC - 2.8 mV/A/m
• Three or four-core cable, three-phase AC - 2.4 mW/A/m

What would the correct value be in the context of the above cable?

Also for Zs and calculating R1+R2 for same, the on-site guide says in table I1 that for 16mmsq, copper conductor R/m is 2.3 Ohms. For ducted cable, should this be multiplied by 1.2 (Table I3), to give value of 2.76 Ohms?

Any guidance is much appreciated.

 

[buzzlightyear]

Three or four-core cable, three-phase AC - 2.4 mW/A/m

CORRECT

Also for Zs and calculating R1+R2 for same, the on-site guide says in table I1 that for 16mmsq, copper conductor R/m is 2.3 Ohms. For ducted cable, should this be multiplied by 1.2 (Table I3), to give value of 2.76 Ohms

yes 1.20
look in g/note 3 .

 

[oscar21]

Three or four-core cable, three-phase AC - 2.4 mW/A/m

Why would you choose the 3 core table if its single phase, surely you dont count the earth or spare core when doing the calculations.

CORRECT

yes 1.20
look in g/note 3 .

How can a 16mm SWA have a resistance of 2.3 ohms/M. You might as well use some string.

 

[buzzlightyear]

Why would you choose the 3 core table if its single phase, surely you dont count the earth or spare core when doing the calculations.

Cpc or did your work out that.is this a collage jobby.

 

[pc1966]

• Two-core cable, single-phase AC - 2.8 mV/A/m
• Three or four-core cable, three-phase AC - 2.4 mW/A/m

What would the correct value be in the context of the above cable?

If it is single phase then 2.8 mV/A/m applied to your nominal 230V L-N for both 2C and 3C cable (when 3rd core is used as CPC).

If it is three-phase then use 2.4 mV/A/m applied to your nominal 400V L-L for both 3C (3L such as delta motor load), and also 4C (3L+N) provided you have reasonable balance between phases (so your N drop is small).

Also for Zs and calculating R1+R2 for same, the on-site guide says in table I1 that for 16mmsq, copper conductor R/m is 2.3 Ohms. For ducted cable, should this be multiplied by 1.2 (Table I3), to give value of 2.76 Ohms?

Table I1 is for the cable when cold = 20C, other values are for cable hot which is usually assumed to be 70C for PVC insulation.

For cables running 90C with XLPE the drop is more as hotter copper means higher R, but few accessories are happy at tat sort of rating so you need to think twice before adopting that CCC limit.

 

[pc1966]

Why would you choose the 3 core table if its single phase, surely you dont count the earth or spare core when doing the calculations.

You might need a 10mm Cu CPC for PME bonding, or to meet disconnection times if armour R2 too high.

How can a 16mm SWA have a resistance of 2.3 ohms/M. You might as well use some string.

They probably meant Ohm/km but I would put mOhm/m and use the omega letter if I could find a simple way to type its code in...

 

[MartinPaul]

Thanks all, for the assistance. It's much appreciated.

The third core is CPC but looking at previous versions of the .xls shows some of the values are taken from the 3/4 core 3-phase table, so it's been throwing me a bit. Seems to be some difference on opinion on th correct Vd table.

With regards to 90⁰/XLPE, there seems to be some clarifications required on device compatibility in the regs for this so will stick to 70⁰ as a fail safe.

Thanks again.

 

[pc1966]

With regards to 90⁰/XLPE, there seems to be some clarifications required on device compatibility in the regs for this so will stick to 70⁰ as a fail safe.

70C is a safe assumption.

Where 90C helps are cases of a cable going through a hot area (e.g. near hot water pipes), or a region of high thermal insulation, so the ends are OK temperature-wise (70C or below) but the middle needs the extra capacity.

 

[MartinPaul]

70C is a safe assumption.

Where 90C helps are cases of a cable going through a hot area (e.g. near hot water pipes), or a region of high thermal insulation, so the ends are OK temperature-wise (70C or below) but the middle needs the extra capacity.

Thanks mate. The spreadsheet is typically used for street lighting design.