Determining cable size

D.24

Hi all a friend is looking to install 2 security cameras that involve a 320 meter swa cable run,cable buried in the ground that runs to a 5amp spur,the Load Max 4amps. He's trying to do this job the cheapest/safest way. Maybe a 6sq/10sq swa should do it but I can't remember how to confirm this by using the regs, anybody know how to achieved this? With regards to volt drop I'm don't know the operating input voltage of the cameras all I know is that they are low voltage. Any suggestions on how to calculate the formula of cable selection and the cheapest way. Thanks

2011

D.24 wrote: »

Hi all a friend is looking to install 2 security cameras that involve a 320 meter swa cable run,cable buried in the ground that runs to a 5amp spur,the Load Max 4amps. He's trying to do this job the cheapest/safest way. Maybe a 6sq/10sq swa should do it but I can't remember how to confirm this by using the regs, anybody know how to achieved this? With regards to volt drop I'm don't know the operating input voltage of the cameras all I know is that they are low voltage. Any suggestions on how to calculate the formula of cable selection and the cheapest way. Thanks

Obviously a larger current will mean a larger cable. As this is a particularly long cable run an oversized cable will increase the cost significantly. So my advice would be to start off by determining the design current. I would think that the load is significantly less than 4A and if it isn't I would suggest that you look at cameras with lower power consumption. I would expect that the cameras would work with a volt drop in excess of the permitted value (4% of nominal).

An estimation of volt drop can be calculated using the mV per A per m tables in the National Rules for Electrical Installations (ET101:2008). Remember your overall volt drop calculation will have to include the volt drop to the point where the cable is connected (this could be a significant value).

Randyleprechaun

You'll also need to ensure that the fault loop impedance of the circuit is within the parameters allowed for the associated protective device.

Resistance of phase + earth of various lengths of various types of cable can be found online. Add this value to the impedance at the point of supply of the circuit to find overall impedance.

D.24

2011 wrote: »

Obviously a larger current will mean a larger cable. As this is a particularly long cable run an oversized cable will increase the cost significantly. So my advice would be to start off by determining the design current. I would think that the load is significantly less than 4A and if it isn't I would suggest that you look at cameras with lower power consumption. I would expect that the cameras would work with a volt drop in excess of the permitted value (5% of nominal).

An estimation of volt drop can be calculated using the mV per A per m tables in the National Rules for Electrical Installations (ET101:2008). Remember your overall volt drop calculation will have to include the volt drop to the point where the cable is connected (this could be a significant value).

Thanks for your reply where is the table located in Annex 52e the reference method H doesn't seem to be covered in this section tho I'm sure I've overlooked it;)

Steve

I can't see any camera drawing 4A @240V, you need to check that. Also, most will be happy with min 100V so bear that in mind. .75 swa would probably do .

D.24

Steve wrote: »

I can't see any camera drawing 4A @240V, you need to check that. Also, most will be happy with min 100V so bear that in mind. .75 swa would probably do .

Yea as far as I'm aware it's going to a small hub with 2 cameras thats why he needs the 230v supply

2011

Steve wrote: »

I can't see any camera drawing 4A @240V, you need to check that. Also, most will be happy with min 100V so bear that in mind. .75 swa would probably do .

A 320m cable run using 0.75 cable would but the earth fault loop impedance through the roof. Without doing a calculation I would guess this would even be too high for a 1A fuse.

Randyleprechaun

Ya. The impedance would be way too high for 0.75.

The rating of the MCB would help to determine the cable size

Or is it a completely new circuit

2011

Randyleprechaun wrote: »

Ya. The impedance would be way too high for 0.75.

The rating of the MCB would help to determine the cable size

...and that depends on the design current.
As per post #2 my suspicion is that 4A is too high.
This would result in too large a cable being selected.

Randyleprechaun

I'd be going for 2.5mm on a 6A MCB.

D.24

2001 any notes or examples of cable selection exercises you could PM me I need to brush up on my theory... It's been a while. Cheers

2011

D.24 wrote: »

2001 any notes or examples of cable selection exercises you could PM me I need to brush up on my theory... It's been a while. Cheers

I haven't used "traditional" techniques to size cables in a long time.
There is a graduate at work that is trained to use Amtech (cabe sizing software).
When I need a cable sized (not very often) I provide all of the data and he sizes it for me. One of the few advantages of getting old

Here are a few pointers, grab a copy of the National Rules for Electrical Installations (ET101:2008):

1) Calculate deign current (maximum current drawn by the load). This could mean looking at data sheet(s) / nameplate(s) /a bit of addition.

2) Select a suitably rated protective device for the load. Also consider characteristic (fuse curve) of the protective device. For example a B type MCB is generally used for a domestic installation. Remember a C type MCB of the same rating requires a lower earth fault loop to operate as quickly as the B type user fault conditions (with everything else being equal). Therefore it may require a larger cable size to meet the disconnection time requirements. This graph shows a few different fusing characteristics. However C type MCBs deal with inrush currents better than B type. So in an installation such as a workshop C type MCBs would be preferable as they would be less likely to trip when something like a 110V transformer is plugged in.

3) One the protective device (MCB or fuse) has been selected the cable can be selected. This much have a higher continuous carrying capacity than the protective device. Annex 52C provides current carrying capacities for various cable types for different methods of installation.

4) Calculations should be carried out to ensure that the volt drop for the selected cable will not be excessive (4% of nominal voltage). Annex 52E provides mV/A/m tables which can be used to calculate volt drop. As the cable run increases for a given cable size the volt drop increases due to the resistance of the cable. This is compensated for by increasing the cable size (a lager sized cable will have a lower resistance).

5) Ensure that maximum earth fault loop impedance values are not exceeded for the appropriate disconnection time (generally 0.4 seconds). This can be checked in Tables A61C.

I hope this helps

gctest50

Steve wrote: »

I can't see any camera drawing 4A @240V, you need to check that. Also, most will be happy with min 100V so bear that in mind. .75 swa would probably do .

Might need to allow for lights and/or heaters ( defrosting ) in the future

Wouldn't be great having to rip it all up again

D.24

Brilliant stuff 2011, thanks for taking the time to write such a detailed explanation much appreciated, Cheers

2011

I should add that other factors can impact on cable size such as:

* Ambient temperature
* Grouping factor
* The nature of the load. Nonlinear loads such as VSDs and UPS can result in a larger cable size being required than a resistive load of the same size (and power factor).
* The type of cable.

Interestingly I have just discovered that there is some very inexpensive (<€4) and free cable sizing software available. This might be worth trying.
I have just downloaded a 17th edition cable sizing app

Risteard81

Just be aware that the 17th Edition cable sizing software will use a suggested maximum volt drop of 3 percent for lighting and 5 percent for power. The 4 percent for everything went out with the 16th Edition.

2011 wrote: »

I should add that other factors can impact on cable size such as:

* Ambient temperature
* Grouping factor
* The nature of the load. Nonlinear loads such as VSDs and UPS can result in a larger cable size being required than a resistive load of the same size (and power factor).
* The type of cable.

Interestingly I have just discovered that there is some very inexpensive (<€4) and free cable sizing software available. This might be worth trying.
I have just downloaded a 17th edition cable sizing app

2011

Risteard81 wrote: »

Just be aware that the 17th Edition cable sizing software will use a suggested maximum volt drop of 3 percent for lighting and 5 percent for power. The 4 percent for everything went out with the 16th Edition.

Interesting point, however this isn't really an issue. From my experience of using Amtech (I would think that any comparable software would be the same): The designer must inset the maximum percentage volt drop allowable on the cable being sized. This would almost always be a value of less than the overall value permitted by the applicable regulations as there is already a volt drop up to the point that the cable is being connected to. Remember the volt drop on the cable being sized is only part over the overall permissible volt drop calculation.