Cable size meter to building 72M

[regedit]

I've got an 'electrician' wiring a 10 room building and just want to check cable size needed to run 72m from the meter to the building. At the building the cable will then split off to each room via its own meter. Each room has its own CU supplied with 10mm cable (rooms have 1 aircon max 20,000 BTU and hot shower, say 4000kw).

What size cable should he be fitting for the 72m run from the electric meter point to the building ? Cable will run underground in protective pipe.

Thanks.

[Youlike]

Thai cable is not the same quality so it's very hard to calculate the real thickness.

 

And also those transformers mounted on poles, they explode regularly or they start melting and dripping liquid metal.....it's not the same quality....

 

Best would be to order it from a reliable country with real standards and real electricians.

[Yellowtail]

1 hour ago, Youlike said:

Thai cable is not the same quality so it's very hard to calculate the real thickness.

 

And also those transformers mounted on poles, they explode regularly or they start melting and dripping liquid metal.....it's not the same quality....

 

Best would be to order it from a reliable country with real standards and real electricians.

 

Or just move there. 

[Yellowtail]

How big is your meter?

 

How big are the meters and CUs for each room?

 

 

 

 

[maxpower]

2 hours ago, Youlike said:

Thai cable is not the same quality so it's very hard to calculate the real thickness.

 

And also those transformers mounted on poles, they explode regularly or they start melting and dripping liquid metal.....it's not the same quality....

 

Best would be to order it from a reliable country with real standards and real electricians.

 

The woodworker will blow a vessel when he sees this post, you'll be made to stand in the corner for at least two hours.

 

PS

Before the gurus arrive it might be a good idea to know how many phases are available.

[Crossy]

How big is the intended meter?

How many phases?

 

Making some assumptions.

 

About 6kW per room = about 30A.

 

10 rooms x 30A = 300A so 100A per phase on 3-phase. A 30/100 3-phase supply would be the ticket taking into account diversity.

 

75m @ 100A and 3-phase would be 35mm² copper or 50mm² aluminium.

 

But do verify with your supply authority, they may have different requirements.

[regedit]

17 hours ago, Crossy said:

How big is the intended meter?

How many phases?

 

Making some assumptions.

 

About 6kW per room = about 30A.

 

10 rooms x 30A = 300A so 100A per phase on 3-phase. A 30/100 3-phase supply would be the ticket taking into account diversity.

 

75m @ 100A and 3-phase would be 35mm² copper or 50mm² aluminium.

 

But do verify with your supply authority, they may have different requirements.

Thanks Crossy. Will check with electric authority.

Meter isn't fitted yet - they put in a temporary single phase one for the construction.

How to determine size of intended meter - are you saying should be 30A based on above ?

[bankruatsteve]

How will the rooms be configured?  AC?  Hot shower?  Kitchen(ette)?  30A per room seems like maximum demand potential that might be very unlikely to occur.  Reality might be 50A for the entire building for a peak load.  The incomer cables should be sized for worst case but 30/100 3-phase seems a bit of overkill to me.

[Yellowtail]

 

47 minutes ago, bankruatsteve said:

How will the rooms be configured?  AC?  Hot shower?  Kitchen(ette)?  30A per room seems like maximum demand potential that might be very unlikely to occur.  Reality might be 50A for the entire building for a peak load.  The incomer cables should be sized for worst case but 30/100 3-phase seems a bit of overkill to me.

 

From the OP, 10 rooms, each with it's own meter. I think it safe to assume each room is a longer term rental. 

 

As such, all or most AC likely at the same time, and all or most hot showers working in the morning and in the evening. 

 

Likely each room also has kettle, microwave, refrigerator perhaps a hot-plate and assorted electronic.

 

Better to error on the side of too much service than too little.  

[Crossy]

1 hour ago, regedit said:

Meter isn't fitted yet - they put in a temporary single phase one for the construction.

How to determine size of intended meter - are you saying should be 30A based on above ?

 

Check what they can provide without you going to the expense of your own transformer. They may only be able to provide a 15/45 3-phase (depends where you are).

 

In reality, as @bankruatsteve notes 30/100 may be overkill, but if it's available it will cost only slightly more.

 

The UK network operators use something like 2kW (10A) per home when sizing supplies but that relies on diversity over hundreds of dwellings (each home would likely have a 60 or 80A incoming fuse). With only 10 units the possibility of everyone running the A/C and shower at the same time is much greater.

  

[regedit]

On 9/11/2020 at 12:26 PM, Crossy said:

 

Check what they can provide without you going to the expense of your own transformer. They may only be able to provide a 15/45 3-phase (depends where you are).

Yes, turns out they will prove 3 phase 15/45 meter. Am I still looking at 35 sqmm cables ?

Also, are the phases are setup with 3 rooms on 1, 3 rooms on 2 and 4 rooms on 3, assuming all rooms expected to have same consumption ? 5 rooms are on an upper floor.

[Crossy]

15 minutes ago, regedit said:

Yes, turns out they will prove 3 phase 15/45 meter. Am I still looking at 35 sqmm cables ?

Also, are the phases are setup with 3 rooms on 1, 3 rooms on 2 and 4 rooms on 3, assuming all rooms expected to have same consumption ? 5 rooms are on an upper floor.

 

You should be able to drop to 16mm² copper or 25mm² aluminium subject to approval by the PEA. The aluminium is less likely to wander off in the night ???? 

 

Try to balance your loads across the 3-phases although precise balance isn't necessary or practical in a domestic environment. For example put your main bedroom A/C on one phase then the other 4 bedrooms split beween the remaining two, 2 rooms on each.

 

Try to manage the power outlets and lighting so that all areas get at least something if a phase goes out (the most common failure mode).

[bankruatsteve]

6 minutes ago, Crossy said:

For example put your main bedroom A/C on one phase then the other 4 bedrooms split beween the remaining two, 2 rooms on each.

Crossy, in this case the OP says he has a 10 unit (apartment?) building.  5 rooms down, 5 rooms up.  Divy the phases by rooms.  ????

[Crossy]

Just now, bankruatsteve said:

Crossy, in this case the OP says he has a 10 unit (apartment?) building.  5 rooms down, 5 rooms up.  Divy the phases by rooms.  ????

 

Yeah, I missed that bit of the history ????

 

Each room with it's own single-phase board. Outside/public area lighting split beween the phases.

 

[regedit]

How is conduit size determined ? Will run underground,  assuming 3 phase on 30/100 meter, 4x32mm cables, 72m run. Thanks.

[Crossy]

1 core x 35mm² NYY (which you need for underground) is about 16mm in diameter.

 

For a fill-factor of 40% you're looking at conduit of 50mm diameter minimum (bigger would make life easier). At that size your contractor will have to pull all 4 cables at the same time, you'll never manage to pull them one at a time.

 

Also, do consult with your local PEA office, some have been known to not allow underground from meter to building.

 

[regedit]

We can go with 4 core from meter to the building. At the building (after a main breaker switch) II need to split the supply into 10 lines to supply 10 individual meters (before going to the 10 individual consumer units in each room). What is best way (thing to use) to split out the supply from the incoming cable to the 10 different units. Assume not twisting wires together...!

[ThaidDown]

52 minutes ago, regedit said:

We can go with 4 core from meter to the building. At the building (after a main breaker switch) II need to split the supply into 10 lines to supply 10 individual meters (before going to the 10 individual consumer units in each room). What is best way (thing to use) to split out the supply from the incoming cable to the 10 different units. Assume not twisting wires together...!

Again, check with local PEA if 4 core is allowed or 4 x 1 core is required.

For distribution bus bar in a suitable box.

eg

https://www.lazada.co.th/products/5-bus-bar-i1345874368-s3375120071.html?

or similar.

[Yellowtail]

Would you not want individual breakers for each of the ten secondary meters?

 

 

Actually, I think if you do not have a breaker for each meter, you would have to use wire sized for the main breaker to feed the secondary meters...

 

 

[bankruatsteve]

42 minutes ago, Yellowtail said:

Actually, I think if you do not have a breaker for each meter, you would have to use wire sized for the main breaker to feed the secondary meters...

Can't think of a reason why that would be needed.  Main breaker in CU after meter and sized for the cable seems proper to me.

[kamalabob2]

The OP could walk into the PEA with his approved building plans, with his land papers, and they will be helpful on his REAL options. I have never seen them refuse proper underground cables in HDPE, and I sure thought PEA had nationwide standards. There are MINIMUM cable size recommendations on the PEA website.

[regedit]

2 hours ago, kamalabob2 said:

The OP could walk into the PEA with his approved building plans, with his land papers, and they will be helpful on his REAL options. I have never seen them refuse proper underground cables in HDPE, and I sure thought PEA had nationwide standards. There are MINIMUM cable size recommendations on the PEA website.

Electricity supply is from the Sattahip boys... and when we visited they were of no help, except saying get an electrician who can tell you the specs, and come back when you have a house number and only then would they come out to survey. AFAIK you can't get a house number until you have electrical supply...

[regedit]

I'm going to put breakers for the 10 consumer units to be able to disconnect supply without having to enter the rooms. Breakers will be in a locked control room. Same for water.

[Crossy]

2 hours ago, regedit said:

I'm going to put breakers for the 10 consumer units to be able to disconnect supply without having to enter the rooms. Breakers will be in a locked control room. Same for water.

 

I'd put a large 3-phase board in your utility room and use one breaker per room. An easy 1 box solution.

 

Do you intend metering each room? Probably worth it for monitoring if not billing. A lot of options on aliExpress including ones with RS485 or wifi.

 

[regedit]

Crossy, can you help me understand what you mean by one large board - do you mean one CU covering all 10 units/rooms instead of one CU in each unit/room ? Plan is to have meters for each room, located on the utility room wall. Thanks.

[Crossy]

10 hours ago, regedit said:

Crossy, can you help me understand what you mean by one large board - do you mean one CU covering all 10 units/rooms instead of one CU in each unit/room ? Plan is to have meters for each room, located on the utility room wall. Thanks.

 

Sorry, I wasn't as clear as I should have been, Madam was in a rush to go shopping, let's try again.

 

Start with a 3-phase board in your utilities room with the incoming supply and main MCCB.

That board has an MCB per room (preferably 2-pole**).

Each MCB feeds the meter for that room and then off to a consumer unit in the room itself.

 

Size each MCB to protect the cable to the room so you can get discrimination with the incoming RCBO in the room and avoid being called out to turn the juice back on after a trip.

 

** 3-phase boards that take 2-pole MCBs seem to be as rare as hen's teeth, we had to have similar units custom made for one upgrade project as the original ABB boards had been discontinued. Talk to your contractor and local electrical specialist outlet.

 

Hope that's slightly clearer.

 

[regedit]

On 11/14/2020 at 5:03 AM, Crossy said:

Start with a 3-phase board in your utilities room with the incoming supply and main MCCB.

That board has an MCB per room (preferably 2-pole**).

Each MCB feeds the meter for that room and then off to a consumer unit in the room itself.

 

Thanks Crossy. 

For the earthing, would it be better to run all 10 earth cables back to this one board and then one (16mm ?) cable down to a single 2.5m rod, or is there any benefit to splitting the earths, say 5 clamped directly to 2 rods ?

[bankruatsteve]

5 minutes ago, regedit said:

Thanks Crossy. 

For the earthing, would it be better to run all 10 earth cables back to this one board and then one (16mm ?) cable down to a single 2.5m rod, or is there any benefit to splitting the earths, say 5 clamped directly to 2 rods ?

IMO you should run 2 bus (1 per 5 rooms) for the earth from the CU and then just connect to that for each room.

[Crossy]

Your inspector will probably want to see a rod and MEN link at the big board but if you also have access to the building steel then bond that to the rod as well (our roof steel tests better than the rod due to our multiple 16m piles). 

 

I would run individual grounds back to the main board for integrity.

 

But as @bankruatsteve you could bus several rooms together, if you go that way please use proper bolted connectors, no twisted joints.

[regedit]

On 11/16/2020 at 4:44 PM, Crossy said:

 if you also have access to the building steel then bond that to the rod as well (our roof steel tests better than the rod due to our multiple 16m piles)..

You mean run a cable from the roof steel to the rod (so 2 cables will finally be attached to the rod) ? 5 of the earths will pass through the roof space so adding one more attached to the steel down to the control room will be doable.

 

What size cable for CU to rod and roof steel to rod ? Mains supply will be on 4x32mm.

 

Thanks.