cables size?

grousedogtom

Hi all
Need to run an swa cable and poly poly to the end of a long drive for future electric gates about 350 - 400 meters long, thinking a 4 square swa will do but need to know what size poly to run for this length. Many thanks.

padunne

There could be a fair voltage drop for 400m and 4sq isn't that big a cable. I nearly go 6sq just to be sure. As for the poly poly a problem can occur with interference.
If when you connect up a pair you get nothing or hiss you may need to double the pair ie put a blue with orange on either side. We got this once when we ran a poly poly a long way to a shed. Doubling the pairs had it working like normal again. Just something for ya in case it happens.

Buford T Justice

If you tell us the load that the gates will be taking, we can work out the gauge of cable you need to run.

2011

Fingers is 100% correct. The first step in any cable sizing exercise it to establish the size of the load. From this the protective device is selected and from that the minimum cable size. This size may then increase depending on other factors such as lenght of run, method of installation, ambient temperature, grouping factor etc.

Although I would guess that the load is small 400m will mean a large cable.

If the poly poly is for an analogue intercom I think you will have issues as most of these are designed for a max range of 100m

A digital intercom may be your best option. They can work over distances of over 1km.

grousedogtom

2 by 400 watt motors = 800 watts full load

Bruthal

grousedogtom wrote: »

2 by 400 watt motors = 800 watts full load

6 square cable with a 6 amp mcb on it would do that i`d say.

2011

The maximum permissible volt drop is generally taken as 4% (unless the manufacture states otherwise).

4% of 230 = 9.2 volts

If the motors have an output of 800 watts with an efficiency of 90% and PF of 0.88 (typical values) the current drawn will be in the region of 4.4A

A single phase 3 core 4mm sq SWA cable with XPLE insulation has a volt drop of about 11mA per volt per meter (table A52-J6 ET101). Multiply it all out and you get a volt drop over 400m of over 21 volts!

This is the equivalent of over 9% volt drop over the length of the cable. In addition to this will be the volt drop from the ESB meter to the distribution board (this should always be taken into account). The method of installation and other factors should also be taken into account as well as the earth fault loop impedance.

Increase the cable size to 6 sq. mm and the volt drop is almost 14 volts from the board to the gates. This equates to about 6%. Bear in mind that there may already be a 2% volt drop to the distribution board meaning that the overall volt drop is 8% !!

This is just a quick calculation, there are many other factors to consider to get a more accurate answer but I would be confident that a 4 sq. is not the correct size!

Bruthal

2011 wrote: »

The maximum permissible volt drop is generally taken as 4% (unless the manufacture states otherwise).

4% of 230 = 9.2 volts

If the motors with an output of 800 watts with an efficiency of 90% and PF of 0.88 the current drawn will be in the region of 4.4A

A single phase 3 core 4mm sq SWA cable with XPLE insulation has a volt drop of about 11mA per volt per meter (table A52-J6 ET101). Multiply it all out and you get a volt drop over 400m of over 21 volts!

This is the equivalent of over 9% volt drop over the length of the cable. In addition to this will be the volt drop from the ESB meter to the distribution board (this should always be taken into account). The method of installation and other factors should also be taken into account as well as the earth fault loop impedance.

Thought i said 6 square though. And taking the ESB meter to distribution board is over thinking, the run is so short on a 16 square cable (2 meters) compared to a 400 meter run to a set of gates drawing this load that the meter to board is negligible, its only 3-4 amps after all. Do we have to think of that when installing a shower with 10 times this load, why not take into account the 25/16 square concentric aluminium/copper cable from the ESB minipillar to the meter as well, and the ?meters from the ESB transformer to the mini pillar.

2011

Thought i said 6 square though

You did and I did the calculation for it too!

The OP said 4 sq.

M cebee

esb is allowed 10%
the installation is allowed 4% drop

but the voltage at the load has to be within the tolerance of the appliance

they obviously don't have any significant inrush or they'd never work over the distance

Bruthal

2011 wrote: »

You did and I did the calculation for it too!

The OP said 4 sq.

Probably my mis reading as usual, but for a 400 meter run of cable or 1km, with a 3 or 4 amp load the run from the ESB meter to the board is meaningless and no different than if it was a 4 amp load in the attic, if it was an immersion drawing 13.5 amps upstairs this would not even be mentioned,

seaniefr

2011 wrote: »

You did and I did the calculation for it too!

The OP said 4 sq.

what about fault loop impedance? would'nt it be hard for a 6 amp mcb to pick up a fault over that distance given the resistance built up over such a run? Also could you not use a multi-core swa instead of a poly/poly?

2011

And taking the ESB meter to distribution board is over thinking

No it is a regulation!!

he run is so short on a 16 square cable (2 meters)

How do you know?? In some cases it is far longer. I know it is longer in my house!

Do we have to think of that when installing a shower with 10 times this load

If you are installing any cable you should ensure that it is sized correctly, yes. I have had to increase cable sizes for many reasons but mainly because of the size of the load and the length of the run. If you are designing an electrical installation this is what you must do to comply with the regulations. They are there for a reason, I am not making this up!

2011

Probably my mis reading as usual, but for a 400 meter run of cable or 1km, with a 3 or 4 amp load the run from the ESB meter to the board is meaningless and no different than if it was a 4 amp load in the attic, if it was an immersion drawing 13.5 amps upstairs this would not even be mentioned,

Look the chances are you are correct and the volt drop of the cable to the distribution board is so small that it can be ignored. But it is worth pointing out when you have not seen the installation. For all I know the OP intends to feed this from a sub board in a shed

Bruthal

2011 wrote: »

No it is a regulation!!


How do you know?? In some cases it is far longer. I know it is longer in my house!


If you are installing any cable you should ensure that it is sized correctly, yes. I have had to increase cable sizes for many reasons but mainly because of the size of the load and the length of the run. If you are designing an electrical installation this is what you must do to comply with the regulations. They are there for a reason, I am not making this up!

Ok so. Your putting in a 4 amp load in the living room, are you telling me you calculate the voltage drop on the 16 square ESB cable between the meter and the board? I know obviously it can be longer than 2 meters,, but your honestly telling me you would calculate this when putting in 7 or 8 100 watt lights?

2011

Also could you not use a multi-core swa instead of a poly/poly?

The only poly/poly cable I know of is one that is used for intercoms! I would suggest an SWA as this is an armored cable suitable for direct burying. I would guess that the cable is to be buried?

M cebee

robbie7730 wrote: »

Probably my mis reading as usual, but for a 400 meter run of cable or 1km, with a 3 or 4 amp load the run from the ESB meter to the board is meaningless and no different than if it was a 4 amp load in the attic, if it was an immersion drawing 13.5 amps upstairs this would not even be mentioned,

you add the VD on the tails to the VD on the 6sq swa

that's 63amp(fla) on the 16sq tails+ the 'design current' on the cable feeding the motor

that's the installation VD


if there's a problem with supply VD that's another story but must be considered

2011

what about fault loop impedance?

If the earth fault loop impedance is too high the protective device (MCB) will not operate within the permitted time. I have not done this calculation.

2011

M cebee wrote: »

esb is allowed 10%
the installation is allowed 4% drop

but the voltage at the load has to be within the tolerance of the appliance

Exactly and the installation is from the ESB meter to the end point of the circuit.

M cebee

ya the VD on the tails is calculated at 63amp loading

Bruthal

2011 wrote: »

Exactly and the installation is from the ESB meter to the end point of the circuit.

Well we need to think about it so, its easy to quote rules, if the 4 amp load is beside the distrubution board or a mile away makes absolutely zero difference, it will not change the volt drop on the cable from the meter to the MCB board one bit, thats why i suggested you would`t think about it when installing a shower in the house, and even if you did, what would you do, if the voltage drop is too high between the meter and the MCB board, no cable size between the MCB board and the load will remedy this.

Bruthal

M cebee wrote: »

ya the VD on the tails is calculated at 63amp loading

Ok and what is this VD so? On a 10 meter run of 16 square as example at 63 amps.

M cebee

no idea but the VD on the tails assumes it's fully loaded up to 63amp

while only the small design current is used for the VD on the swa

that's my understanding anyhow

Bruthal

M cebee wrote: »

no idea but the VD on the tails assumes it's fully loaded up to 63amp

while only the small design current is used for the VD on the swa

that's my understanding anyhow

yes but its simple to to understand that a 4 amp load along a 400 meter 6 square cable fed from the MCB board , or the same load on a 1.5 square cable a meter long will both have the exact same effect on the ESB cable between the meter and MCB board, thats my point, as in it makes no difference, and because it was a 400 meter long cable, it was suggested the ESB mains had to be taken into account, im just wondering why? Im not trying to test anyone. We learn every day dont we.

2011

There is a volt drop on every cable ever made. The total volt drop in an installation is the sum of the volt drop on each section of cable from the ESB to the motors.

Total volt drop = volt drop to the distribution board (as Robbie points out this may be small) + volt drop on the SWA

Bruthal

2011 wrote: »

If the earth fault loop impedance is too high the protective device (MCB) will not operate within the permitted time. I have not done this calculation.

If the cable for the long run is the propper one for the load then the loop impedence should be within tolerence, another reason why the MCB is as close to the load as possible, regardless of the size of the cable. I.E. we hear all the time the MCB is to protect only the cable, but that is a very simplistic idea. In this case above, its a 6 amp MCB for a 6 square cable, where as to protect the cable a 32amp one would be used.

Bruthal

2011 wrote: »

There is a volt drop on every cable ever made. The total volt drop in an installation is the sum of the volt drop on each section of cable from the ESB to the motors.

Total volt drop = volt drop to the distribution board (as Robbie points out this may be small) + volt drop on the SWA

Yes no getting away from that. But we all probably heard that a circuit is only as good as the smallest CSA, but this is not true at all, far from it. If it was true then a 20 amp fuse on a 2.5 square cable would suggest the CSA of the fuse (far smaller than the 2.5 cable) would render the 2.5 square pointless.

4 amps on a 16 square would be negligible, regardless if its 63 amp load that is used in the calculation according to mceebe. If 63 amps is used then its a fixed VD for a given tails run. As i said, a shower might take 40 amps, and because its in the house on a short cable, no mention of the tails VD is made, only when its a 400m cable run to gates.

In fact, if the VD on the tails is too high, then whats the remedy? The 16 square from meter to MCB board is also the size from mini pillar to meter. So it seems pointless calculating the voltage drop to the meter from MCB board. As in the difference between the meter and MCB board is likely to be neglegible unless the run is very long from meter to board even with 63a on it.

Bruthal

2011 wrote: »

There is a volt drop on every cable ever made. The total volt drop in an installation is the sum of the volt drop on each section of cable from the ESB to the motors.

Total volt drop = volt drop to the distribution board (as Robbie points out this may be small) + volt drop on the SWA

Its an interesting one all the same. I dont disagree with anything your saying really, just more curious to other peoples thinking on it all.

2011

Robbie if you look at my post you will see that I did my calculation on the SWA only. I did not take the VD on mains cable into account I just mentioned that it existed as it is relevant in some cases.

Besides the chances are that the mains cable will have more than 4 amps flowing through it as it is supplying many other circuits (I would imagine). If the mains cable has 20 A flowing through it the volt drop for the mains cable will be based on 20 A, not 4.

Bruthal

2011 wrote: »

Robbie if you look at my post you will see that I did my calculation on the SWA only. I did not take the VD on mains cable into account I just mentioned that it existed as it is relevant in some cases.

Besides the chances are that the mains cable will have more than 4 amps flowing through it as it is supplying many other circuits I would imagine.

Yes it will have more than 4 amps on it which is why i suggest it should be the distribution board the supply voltage should be taken from. But 16 square 10 meter run at 63 amps will have about 1 volt drop. But now we assume the meter now has 230v at it, yet the meter may be at the end of 10 meters of 16 square concentric cable from an ESB mini pillar, so why should the meter be the 230v point, maybe someone is getting my point somewhere.

Bruthal

seaniefr wrote: »

what about fault loop impedance? would'nt it be hard for a 6 amp mcb to pick up a fault over that distance given the resistance built up over such a run? Also could you not use a multi-core swa instead of a poly/poly?

The fault impedence should be fine if the cable is within the 5 percent volt drop for the load scenario and the nearest size MCB is used.

mceebe seems to have all the data on this. Is it a different requirement when its outside the house?