Cable Calculation Question

[luke12345]

Hi there,

Just after some advice for a submain/DB I’m quoting. A client has asked for a 12way TP board to be installed in a warehouse and the length of run is 150 meters.

Section panel I’m coming from has a ZE of 0.05.
MCCB will be a Square D CDAE 63A so max zs 0.68 at 5 second.

Here’s my attempt at calculating it.

I’ve used the supply voltage of 230 even though it’s a 3 phase supply. I assume this is correct because I have single phase equipment fed off my new db. Just to confirm the max volt drop is from origin (meters) to final circuit and not just the submain?

I read somewhere to use 1.5% VD for sub mains, so 3.45V, which leaves some for the final circuits.

So firstly, do I calculate at 63amps as technically it can carry that or do I use amps calculated from the circuits I’m installing, which is about 20 amps per phase?

Calculated at 20Amps

TABLE 4E4B For 50 mm² : mV/A/m r = 0.99
Voltdrop = (mV/A/m x Length x Design current) / 1000
Voltdrop = ( 0.99 x 150 x 63 ) / 1000
Voltdrop = 2.97 Volts

Calculated at 63Amps

TABLE 4E4B For 150 mm² : mV/A/m r = 0.32
Voltdrop = (mV/A/m x Length x Design current) / 1000
Voltdrop = ( 0.32 x 150 x 63 ) / 1000
Voltdrop = 3.02 Volts

As you can see there’s an enormous difference. Which one do I use?

R1 for 50 mm² = 0.387 (milliohms/M)
R2 for 50 mm² = 0.387 (milliohms/M)
multiplier for 70 °C = 1.2

Zs = Ze + ((R1 + R2) / 1000 ) x length x multiplier
Zs = 0.04 + ((0.387 + 0.387 ) / 1000 ) x 150 x 1.2
Zs = 0.179


R1 for 150 mm² = 0.124 (milliohms/M)
R2 for 150 mm² = 0.124 (milliohms/M)
multiplier for 70 °C = 1.2
Zs = Ze + ((R1 + R2) / 1000 ) x length x multiplier
Zs = 0.04 + ((0.124 + 0.124 ) / 1000 ) x 150 x 1.2
Zs = 0.085 ohms


Both are well below 0.68 I need for the MCCB.

Firstly are these calculation correct? Another company has quoted 35mm for the same install but I can’t see how they’ve got to that.

 

[Pete999]

Hi there,

Just after some advice for a submain/DB I’m quoting. A client has asked for a 12way TP board to be installed in a warehouse and the length of run is 150 meters.

Section panel I’m coming from has a ZE of 0.05.
MCCB will be a Square D CDAE 63A so max zs 0.68 at 5 second.

Here’s my attempt at calculating it.

I’ve used the supply voltage of 230 even though it’s a 3 phase supply. I assume this is correct because I have single phase equipment fed off my new db. Just to confirm the max volt drop is from origin (meters) to final circuit and not just the submain?

I read somewhere to use 1.5% VD for sub mains, so 3.45V, which leaves some for the final circuits.

So firstly, do I calculate at 63amps as technically it can carry that or do I use amps calculated from the circuits I’m installing, which is about 20 amps per phase?

Calculated at 20Amps

TABLE 4E4B For 50 mm² : mV/A/m r = 0.99
Voltdrop = (mV/A/m x Length x Design current) / 1000
Voltdrop = ( 0.99 x 150 x 63 ) / 1000
Voltdrop = 2.97 Volts

Calculated at 63Amps

TABLE 4E4B For 150 mm² : mV/A/m r = 0.32
Voltdrop = (mV/A/m x Length x Design current) / 1000
Voltdrop = ( 0.32 x 150 x 63 ) / 1000
Voltdrop = 3.02 Volts

As you can see there’s an enormous difference. Which one do I use?

R1 for 50 mm² = 0.387 (milliohms/M)
R2 for 50 mm² = 0.387 (milliohms/M)
multiplier for 70 °C = 1.2

Zs = Ze + ((R1 + R2) / 1000 ) x length x multiplier
Zs = 0.04 + ((0.387 + 0.387 ) / 1000 ) x 150 x 1.2
Zs = 0.179


R1 for 150 mm² = 0.124 (milliohms/M)
R2 for 150 mm² = 0.124 (milliohms/M)
multiplier for 70 °C = 1.2
Zs = Ze + ((R1 + R2) / 1000 ) x length x multiplier
Zs = 0.04 + ((0.124 + 0.124 ) / 1000 ) x 150 x 1.2
Zs = 0.085 ohms


Both are well below 0.68 I need for the MCCB.

Firstly are these calculation correct? Another company has quoted 35mm for the same install but I can’t see how they’ve got to that.

Perhaps they used the 3 phase voltage instead of SP 230Vfor their Calculations

 

[Richard Burns]

Calculating the volt drop just for the submain with 35mm² cable carrying 20A would give a 3.75V drop which could well be OK if the final circuits are not too large.
You would need to calculate the final circuit volt drop as well to find the total.
The volt drop is from origin to furthest point of final circuit.
The resistance of the cable would be 0.189 @ 70°C.
This would appear to be the best cable for the job.

You should be using the Z values for your VD calculations (not that it will make a lot of difference) and the design current that the sub main cable is intended to carry.
Ffor a submain it is often necessary to plan for future expansion and only you would know if this is required.
Using a 63A MCCB seems to be overkill for a design current of 20A.

 

[davesparks]

Is the 63A setting necessary for a particular reason or can it be wound down?

Your competitor may well have worked out their cable size by pure guesswork.

You've used the same value of resistance per metre for both R1 and R2, are you planning to run a CPC of the same size and material as the line conductor?

 

[luke12345]

Thanks for the replies. As the cable run is so far they're wanting to install a 63 amp supply which will leave some room for future installs.

So just for future reference when installing a sub main, do I calculate at the protective device size?

Im also looking to install a separate earth cable.

 

[Richard Burns]

When installing a submain where there is future planned expansion then you would need to gain an idea of the likely demand that may be applied in the future.
This is not really a well defined design process, so it may be best to design the submain with that capability to take the In current of the protective device, however this should be discussed and agreed with the client because they will be the ones paying out for currently unnecessary cable.
Why would you want to run an additional earth cable? what cable are you planning to run at the moment?

 

[Pete999]

Thanks for the replies. As the cable run is so far they're wanting to install a 63 amp supply which will leave some room for future installs.

So just for future reference when installing a sub main, do I calculate at the protective device size?

Im also looking to install a separate earth cable.

If it's a SP supply and you are running 2 core cable, up it to 3 core save running a separate earth and yo can use the sheath as the bonding conductor if the steel meets the required size limitations

 

[Ian1981]

Normally inductance and reactance need to be included in the overall volt drop of the calculations for cables above 25mm.

 

[luke12345]

Its a 4 core swa on an old concrete warehouse. So I just though it would be a good idea to throw a separate earth. Just oit of interest how do I calculate the swa armouring? Is there a table?

 

[Pete999]

Its a 4 core swa on an old concrete warehouse. So I just though it would be a good idea to throw a separate earth. Just oit of interest how do I calculate the swa armouring? Is there a table?

Yes in 7671 or the OSG can't recall which book

 

[Richard Burns]

Change the cable to a five core and you are good to go for bonding as well.
The armour can be readily used as the earth but check the table in appendix B of guidance note 8, I think it has been posted on here recently.
Though the latest edition states numbers in red are not suitable and failed to colour any numbers in red!!

 

[Ian1981]

Its a 4 core swa on an old concrete warehouse. So I just though it would be a good idea to throw a separate earth. Just oit of interest how do I calculate the swa armouring? Is there a table?

If you can get hold of the IET design guide there are tables for swa in the appendices for mv/a/m for armour.
GN8 has data for sizing of swa for compliance with table 54.7

 

[Ian1981]

Change the cable to a five core and you are good to go for bonding as well.
The armour can be readily used as the earth but check the table in appendix B of guidance note 8, I think it has been posted on here recently.
Though the latest edition states numbers in red are not suitable and failed to colour any numbers in red!!

I've posted it but dare not again as I've already been told off by westward haha

 

[Pete999]

If you can get hold of the IET design guide there are tables for swa in the appendices for mv/a/m for armour.
GN8 has data for sizing of swa for compliance with table 54.7

 

[davesparks]

Its a 4 core swa on an old concrete warehouse. So I just though it would be a good idea to throw a separate earth. Just oit of interest how do I calculate the swa armouring? Is there a table?

Yeah just throw in an extra Earth, why not? It's not your money you're wasting