help calculate cable size for 130kw

[cuezed]

i am struggling to work out the cable size required for the following air con unit. the run of the 4 core swa cable will be about 90 metres, where 85% of it will be underground.

 

[Darkwood]

Welcome to the forum....... we are not here to give a free cable calcs service we need to see your understanding of what your doing so show us your calcs in accordance with the 17th edition and if we see errors we will be happy to guide you.
You haven't even provided us with protection device type, how the rest of the cable is run, ambient temp, etc etc but a clue for this would be to run parallel cabling system.

So give us a hint you are clued up with cable calcs and help with be plenty.

 

[ackbarthestar]

It sounds as though you are finding it difficult to follow through you calculations using BS7671 ?

 

[cuezed]

ok. been busy lately. the protection device type will be bs88. there will be 2 of these units therefore 2 runs of 4 core swa cable (3 phase and neutral). the complete length of the cables run will be approx 70 meters. 10 meters will be clipped against a wall in open air. the rest 60 meters will be buried 0.7m below ground, in soil.

i have done the following calculations. As Ib is 195A. In would be 200A.

Cg from app 4. table 4c2. 2 circuits and cables touching.
Ca at 20 deg C will be 1.0
Ci = 1.0

Iz = In/Cg
Iz = 200/0.75
Iz = 266.67A

from table 4e4a ref method D. cable size will be 185mm

please correct me where i have gone wrong.

 

[Darkwood]

Hope you didnt take my post wrong but il check later as im on my way out but at first viewing check your connecting to equipment rated at 90degrees if your using those tabled values or better still refer to 70 degree SWA (4D4A) as this is the standard rating of most equipment, if you calc for the cable is correct the addition of both cables needs to be over Iz not each cable .... you have just run 2 x 185mm cables giving you a total value of 486amps Table 4D4A

Table 4D4A .... 70mm = 143amp ref method D so 2 x 143 will meet your Iz all you need to do then is check your volts drop complies.

Il re-iterate i haven't confirmed your calcs so dont take my suggestion as the size you require but hope ive headed you down the correct path.

 

[cuezed]

Hope you didnt take my post wrong but il check later as im on my way out but at first viewing check your connecting to equipment rated at 90degrees if your using those tabled values or better still refer to 70 degree SWA (4D4A) and if you calc for a cable is correct the addition of both cables needs to be over Iz not each cable .... you have just run 2 x 185mm cables giving you a total value of 486amps Table 4D4A

Table 4D4A .... 70mm = 143amp ref method D so 2 x 143 will meet your Iz all you need to do then is check your volts drop complies.

no problem. thanks for the reply. the data sheet in the photo above is for one air con unit. there will be two units fitted. therefore there will be two 4 core swa cables.

 

[johnboy6083]

Would this be a good candidate for a submain to a panelboard or other switchgear nearer the load. You could then apply diversity to the submain and could end up with a more versatile installation.

 

[Darkwood]

What are you air conditioning ????? ..... London!!

 

[Darkwood]

no problem. thanks for the reply. the data sheet in the photo above is for one air con unit. there will be two units fitted. therefore there will be two 4 core swa cables.

If you can run 2 seperate parallel cable run 1m apart then you can benefit from large savings on cable sizing.... so as above then another run the same 1m apart, or as johnboy said account for both to a submains and split at load end, although the diversity may not be a candidate to reduce size as in hot weather both units could be expected to be at full load for long durations so wouldn't in this case go down that route.

 

[cuezed]

What are you air conditioning ????? ..... London!!

lol

If you can run 2 seperate parallel cable run 1m apart then you can benefit from large savings on cable sizing.... so as above then another run the same 1m apart, or as johnboy said account for both to a submains and split at load end, although the diversity may not be a candidate to reduce size as in hot weather both units could be expected to be at full load for long durations so wouldn't in this case go down that route.

i wont be able to run them 1 meter apart. maximum will be 0.5m apart. so i'm looking at 120mm