Cable Size & Type To Power Our Pump House?

[Greenside]

When we install our permanent underground supply we will have to run power to our pump house which is about 30 metres away from the main building. A convenient trench exists and I was planning to run power, TV (the sat dish will be hidden away there), CCTV and an alarm sensor in two conduits which will end up about 70cm under the garden, Our temporary supply is currently routed into the pump house where we have a consumer unit which is grounded via a 2m rod connected with 10mm² cable. Through that we run:

• 1 x 2HP submersible pump which sits at the bottom of our borehole
• 1 x 400W Hitachi Pump for the domestic water supply (eventually for two buildings)
• 1 x 3HP pump for the irrigation system
• 1 Fluorescent strip light
• 1 x Double Outlet that gets used for power tools etc

I'm good to go with the TV cables and alarm which will share a conduit, but the electrician sent me off to get 40m of NYY-BCC twin 25mm² plus 40m of 10mm² to use as a ground plus the appropriate length of 2 inch conduit. When I saw the size (not to mention the price) of the cable, I figured that he was going somewhat over the top given the equipment we have installed, but about here my knowledge and confidence expires so I'm wondering if anyone can either tell me what size and type we should be using or how to calculate what I need.

Also, the main house system will (I hope) be properly grounded; what's the right thing to do about the ground we have installed at the pump house. Connect them together? Take it out and rely on the ground installed for the main house?

[Crossy]

OK. running load of 4,150 Watts (5 * 750 + 400) if all pumps together, * 4 for start current, on start if all the pumps start together = 16.6kW ( 75A ouch), 16mm² will just do the trick (about 4% volt drop on start).

Let's apply a little common sense here, since all the pumps are unlikely to start together let's use full load plus 4 x the biggest start current 4,150 + 2,250 = 6,400 W = 30A

6mm² would be JUST adequate if the supply is 220V and not 190V coz you're on a long feed so I'd be looking at 10mm² copper, use a 30mA RCD in the distribution board and a local ground stake at the pump.

Anyone check my sums please.

[Greenside]

Thank you Crossy, 10mm² looked a bit more like what I was expecting.

I see that a single cable is available with 2 x 10mm² cores - can I assume that I can run 2 x single 10mm² cables in my conduit since the cable is only available (economically) in 100m lengths?

I'm not clear on the issue of grounding, however. As per my OP, all the equipment in the Pump House is grounded via the bus in the consumer unit which is connected to a rod buried just outside the door so I'm guessing that qualifies as the local grounding you suggest, but should there be a third cable in my conduit connecting the Pump House ground to the one in my domestic installation?

There is an RCD in the Pump House consumer unit already and I'll check the rating tomorrow.

Edited July 1, 2011 by Greenside

[Crossy]

Provided you have a ground in the pump house and an RCD protecting it there is no need to connect to the main earth in the house.

TT installations such as this are not permitted by the Aussie regulations, however they are allowed in the UK (so are presumed safe) and I can find no Thai regulation prohibiting TT (indeed many if not most rural installations are TT anyway).

[electau]

Provided you have a ground in the pump house and an RCD protecting it there is no need to connect to the main earth in the house.

TT installations such as this are not permitted by the Aussie regulations, however they are allowed in the UK (so are presumed safe) and I can find no Thai regulation prohibiting TT (indeed many if not most rural installations are TT anyway).

Under AS3000 an MEN would be installed at the sub board supplying the pump. The MEN system is mandatory under AS3000 so the TT system is not permitted. However in Thailand both the MEN and The TT systems are permitted and may be mixed within an electrical installation.

[Greenside]

I installed 2 x 10mm cables in plastic conduit which is sitting in the trench waiting to be buried. Yesterday one of the guys from the contractor told me that I'd used the wrong kind of cable and that I should replace it to avoid later problems from moisture (far sighted for a local, but maybe he has a brother in the cable business!).

I put in THW cables - two single cables in a 1 inch conduit which is not only glued but has four 90 degree bends where it enters the pump house. He says I should have got NYY / VCT but now that I've been to Global House to get some, I see that not only is it a lot more expensive but I doubt if it will be able to do the 90 degree bends, given the size of the conduit so I'll have to replace that too and since some is already set in concrete it turns out to be a major job.

I am confused about the different types of cable available and will have to do some research next time the issue comes up but the big question now is how much of a risk is my present installation. The supply to the CU in my pump house will come from the CU in my house and both have RCDs in addition to MCBs but clearly I don't want to risk continual interruptions in the water supply if the wiring isn't up to scratch.

They're going to fill the trench this week, so bite the bullet now or what?

Edited July 31, 2011 by Greenside

[Crossy]

THW is intended for indoor, dry applications, NYY would be more suitable for an area that is potentially going to get damp. Your problem is going to be those bends, NYY minimum bend radius is around 15 x cable outside diameter.

Is it possible to change the bends to be fewer and more gentle? then if the cable does fail you can reasonably easily replace it.

In reality, the THW is going to be OK for a good while, particularly if you can keep the conduit free from damp.

[Greenside]

Thanks as ever Crossy for your speedy reply. Sounds like I'd better bite the bullet and try to figure a route that will take the NYY cable.

[InterestedObserver]

Thanks as ever Crossy for your speedy reply. Sounds like I'd better bite the bullet and try to figure a route that will take the NYY cable.

Most international standards rate THW cable for wet or dry locations in raceway (conduit).

[InterestedObserver]

I put in THW cables - two single cables in a 1 inch conduit which is not only glued but has four 90 degree bends where it enters the pump house.

Are your 90⁰ elbows electrical, large radius or are they standard water pipe variety? Or did you actually bend the conduit?

Edited July 31, 2011 by InterestedObserver

[Crossy]

Thanks as ever Crossy for your speedy reply. Sounds like I'd better bite the bullet and try to figure a route that will take the NYY cable.

Most international standards rate THW cable for wet or dry locations in raceway (conduit).

This is likely what he has http://www.bangkokcable.com/catalog/BCC_CATALOG/THWEN.HTML

For Building wiring, installation on insulator or in raceway, dry location.

Like I said, if the conduit remains dry should be little problem, but the issue would be if any water was to remain in the lowest point.

EDIT It seems that Bangkok Cable's NYY is suitable for direct burial http://www.bangkokcable.com/catalog/BCC_CATALOG/NYY2EN.HTML which will remove the issue with getting it into the conduit.

[harrry]

Crossy, you considered all four pumps would not start at the same time. What would happen if the power cut as it is prone to do here and then came on again. Would that not have the pumps start at the same time if they had been running.

[thaibeachlovers]

Sorry, but had a chuckle at this post.

The pumps ( 2 ) at the MIL's house where I am at present are connected to the house by a much joined bit of what looks like ordinary 1.5mm housewiring cable ( can't read anything on the cable as so old and dirty ) suspended between house and a tree.

Earth- that'd be the day!

[Crossy]

Crossy, you considered all four pumps would not start at the same time. What would happen if the power cut as it is prone to do here and then came on again. Would that not have the pumps start at the same time if they had been running.

If all four start together that will pull a starting current of about 75A briefly. On 10mm cable you'll get a volt drop of about 15V during the start phase, should be OK to get the pumps running.

[thaibeachlovers]

Sorry to add to this thread, but can anyone advise whether I should add an earth to the MIL's pump, which has been working long time without? I checked out water pumps, and it seems easy enough to add one, as they seem to all have a casing bolt for that purpose. However, to do so, I'd have to partially dismantle the pump platform cover ( an old piece of roofing iron nailed on ), and given that it might collapse, is it worth the risk?

[klikster]

Like I said, if the conduit remains dry should be little problem, but the issue would be if any water was to remain in the lowest point.

EDIT It seems that Bangkok Cable's NYY is suitable for direct burial http://www.bangkokcable.com/catalog/BCC_CATALOG/NYY2EN.HTML which will remove the issue with getting it into the conduit.

Am I missing/misreading something or can burying that cable actually result is being able to use smaller cable? From that table:

6 mm rating - in ground - 61 amps

10 mm rating - in air - 62 amps

[electau]

Like I said, if the conduit remains dry should be little problem, but the issue would be if any water was to remain in the lowest point.

EDIT It seems that Bangkok Cable's NYY is suitable for direct burial http://www.bangkokca...LOG/NYY2EN.HTML which will remove the issue with getting it into the conduit.

Am I missing/misreading something or can burying that cable actually result is being able to use smaller cable? From that table:

6 mm rating - in ground - 61 amps

10 mm rating - in air - 62 amps

From AS3000, 6sqmm buried direct (unenclosed) is 50A. 10sqmm is 63A. The pump circuit circuit does not carry these currents continuously.

The maximum route length using these values would be dependant on the percentage voltage drop on the circuit.

30 meters from the main switchboard and a 2% voltage drop. 20 amps full load (FL) current you will require 10sqmm cable. For a 3% voltage drop 6sqmm would be the minimum.

　

　

　

[klikster]

From AS3000, 6sqmm buried direct (unenclosed) is 50A. 10sqmm is 63A. The pump circuit circuit does not carry these currents continuously.

The maximum route length using these values would be dependant on the percentage voltage drop on the circuit.

30 meters from the main switchboard and a 2% voltage drop. 20 amps full load (FL) current you will require 10sqmm cable. For a 3% voltage drop 6sqmm would be the minimum.

　

So Bangkok Cable are taking some liberties with the data?

Another thing I noticed was the roll lengths. 100 m vs 500 m standard lengths. Or will shops cut that to order?

[electau]

From AS3000, 6sqmm buried direct (unenclosed) is 50A. 10sqmm is 63A. The pump circuit circuit does not carry these currents continuously.

The maximum route length using these values would be dependant on the percentage voltage drop on the circuit.

30 meters from the main switchboard and a 2% voltage drop. 20 amps full load (FL) current you will require 10sqmm cable. For a 3% voltage drop 6sqmm would be the minimum.

　

So Bangkok Cable are taking some liberties with the data?

Another thing I noticed was the roll lengths. 100 m vs 500 m standard lengths. Or will shops cut that to order?

Different cable standards, these may be based on NEMA ratings.

Most cable suppliers will cut 500 meter drum lengths to order, they may charge you a cutting fee.