Electrical Question: Cable Size?

[TongueThaied]

I hope somebody with expertise like Crossy can enlighten me on a lack of understanding with which I am wrestling. I am trying to get my electricity set up to build a new house. I will use all copper cabling – no aluminum. I am trying to determine the proper cable size from meter to maim panel, given my maximum load and the distance of the run. I have researched the Internet for hours, but 90% of what comes up is for the US. The most useful is for Australia or GB with 240V single phase. But none of the calculators seem to go high enough for my needs. The calculator on Crossy’s site (which is an awesome site, by the way!!), when values of 220V, 35 KW & 75 meters are plugged in, gives the result, “no suitable cables available.” There is also no single core cable choice with that calculator.

The charts are inconsistent. Crossy’s chart, the one I would like rely on, only goes up to a wire size of 10mm2, and that is not large enough. When looking at the various charts on the Internet, they don’t agree. One chart will give a 50mm2 cable a capacity of 168-213 amps, depending on cable type, while another chart sets it at 150 amps, and still another at 120 amps. I'm confused. Is it 200 amps or 120 amps? That's a pretty big spread. And then there is the voltage drop over distance calculation. One of the calculators I tried, using 240V (fixed), 150 amps and a distance of 75 meters, said I needed 70mm2 cable (but I assume that with 220V, the cable size would be even larger, right?).

I was assuming that the capacity of a cable is the amount of amperage it will carry regardless of the local system’s voltage. The ultimate measurement of electricity (volume, so to speak) is watts. Since V x A = W, a cable that would carry 100 amps at 240V would carry 24 KW. To put that same amount of KW through a cable at 220V, we would be talking about 109 A, and at 110V, 218 amps! Thus, for fixed amount of electricity (in watts) as the constant (usage), the cable size requirement seems to go up as the voltage goes down, right? If you are looking at a chart specifying wire size for ampacity at 240V, the cable size goes up for pumping same KW at 220V, and goes way up for pumping the same KW at 110V, right? I see it is a cost advantage to have a low amperage requirement to save money on cable size – one of the great benefits of 3 phase, or even "split phase." But I admit, the more I study this, the more divergent information seems and the more confused I become. The hell of it is, that it is all mathematics and with the correct inputs, I know there is only one right answer. But I can’t seem to get at it.

I suppose the ultimate question is: Is the wire size to amperage ratio fixed, and one ultimately can determine required wire size using any chart, then adjusting cable size by determining amperage by dividing MY maximum load (in KW) by 220V? (I understand the voltage drop over distance is an additional calculation).

Some background. I am planning the electrical for a new house. I previously wrote here about trying to choose between single and three phase electricity. The PEA office assured me I had three phase available, and, indeed, there are four wires on the pole. I decided on three phase. But when the PEA contractor came out, he said it was only single phase, explaining that one of the four wires was for street lights only. As it turns out, in addition to the street light wire, there is a neutral and other two lines. Every three houses in my village alternate between the two lines, three on one line, three on the other, etc. It appears that between the four wires, 3 phase is there; with attempted balance between houses and street lights, it just is not available to me. They said if I wanted to spend 300,000+ baht on a new transformer, poles and wire, I could get three phase. Well that’s not happening.

My village has only 50 mm2 wire on the poles. Now PEA informs me that the maximum size meter available to me is a 100 amp meter. They also now tell me that that maximum size cable that will fit that meter is 50 mm2. However, when I look at the Mitsubishi specs for that meter on line, it specifies the maximum cable size as 25 mm2.

My situation: Using Crossy’s “Maximum Demand Estimation in a Single Domestic Electrical Installation” I determined my adjusted maximum demand to be 165 amps (total amperage is well over 200). I know that seems like a lot, but I have 3-10KW water heaters, a 2KW washer that heats water, a 2KW dryer, a 2KW dishwasher, a 3KW oven, 7 air conditioners, a woodwork shop, a lot of outdoor flood lights, etc.

Okay, normally, I would never use anything like that. I don’t even like air conditioning and will rarely use it. I don’t wash with hot water. I don’t even bathe with hot water. In my normal day to day life, I doubt if I would ever even break 30 amps at any one moment. But let’s say my sister visited from the States and created the perfect electrical storm. She has every air conditioner on, she has a cake in the oven, she has a load of laundry in the washer on the hot water cycle and another load in the dryer, and she has left on every light in the house. While all this is going on, she’s taking a long hot shower in one bathroom while my wife is doing the same in the other, running two 10 KW water heaters and the pump. I’m out in the shop running rough wood through a planer, and all the outdoor security and yard lights are on. I don’t want to fry anything on my side of the meter during this horrible hour of over-consumption. It will probably never happen, but the potential is there.

And it fries my balls that in this country, you can’t have the meter on your house. The consumer is forced to pay PEA for the voltage drop between PEA’s designated meter location and the house. Because of this, it makes sense to minimize voltage drop.

The run from the meter is 76 meters with 34 meters in the air (going down from the line to the meter, then back up from meter to the top, then across the road to another pole, then down to the ground) and 42 meters underground.

So my questions:

What cable size for this run to accommodate, say, 150 amps and minimize voltage drop?

What is the minimum cable size and circuit breaker that can be used with a 10KW single phase water heater (Panasonic DH-10BM1T)?

Can I use the NYY underground cable in the air, too, or do I need to splice two cable types for this run?

Even though they will only provide a 100 amp meter, can I provide a 150 amp meter and compel them to use it?

Additionally, for reference, can you tell me the maximum amps @ 220V and meters for the following wire types and sizes, in ground and overhead?

50mm2 750V 70º PVC/PVC NYY TIS 11-2531 Table 6?

25mm2, same specs as above

35mm2, same specs as above

70mm2, same specs as above

I need the wire size answer without regard to the 100 amp meter size. I want to protect my side of the meter – not the meter. If I fry the meter, maybe PEA will listen to me.

If my cable size should be 50mm2, but the meter will only accommodate a 25 mm2 cable, can I just solder the ends of the 50mm2 cable, and then grind it down to match the 25mm2 diameter? Or will the PEA contractors refuse to use it?

Finally, part of the reason for a 76 meter cable run is aesthetics If I took the absolute shortest straight-line route, it would cut to 52 meters (with 26 meters above ground and 26 meters below ground). Would this change my cable size requirements for the above conditions?

One other thing, I could solve part of my problem and greatly reduce cable size (and expense) by having two meters and simply splitting my load between them (quasi US “split phase”). However, PEA refuses this. Because Thailand has graduated electricity rates (more expensive per KWH with greater usage), PEA sees two meters as a scheme to have higher consumption at the lower rate.

Any other ideas would be greatly appreciated. Like I said, since I am going to a lot of expense already, I want to be prepared for both the worse case scenario and future upgrades to the grid.

If my usage on one line throws their whole system out of balance they will have to upgrade eventually. Also, they have already been doing a lot of upgrades nearby. With kids working in the city, many villagers that previously had only a couple of fluorescent lights now have air conditioning in their uninsulated homes. It’s a lot of pressure on the grid.

My apologies for the verbosity. I have looked a dozens (maybe hundreds) of electrician and DIY internet boards. The most common complaint by electricians is not enough information. So I gave you everything I could think of. Right now, I’m very frustrated. It’s village life.

[Crossy]

The biggest single-phase domestic meter provided by PEA is 30/100.

If your maximum demand really comes out at 150+A you should be considering a 3-phase supply. Did PEA offer 3-phase?

You don't need 3-phase appliances but can treat it as 3 single-phase installations, you should then be able to use a more manageable 15/45 meter. If you go this way you're looking at 4-core 16mm² cable, PEA will expect you to use 35mm² for a 30/100 3-phase which is the biggest they will provide without you having your own transformer ($$$).

The simple cable calculator only goes to 16mm², a better calculator for bigger supplies is here http://www.doncaster...able_calculator but the PEA required sizes will be fine for your 75m run.

Nothing to stop you using NYY on the aerial sections so it's all in one piece, but it's not going to be a cheap bit of cable

[TongueThaied]

The biggest single-phase domestic meter provided by PEA is 30/100.

If your maximum demand really comes out at 150+A you should be considering a 3-phase supply. Did PEA offer 3-phase?

You don't need 3-phase appliances but can treat it as 3 single-phase installations, you should then be able to use a more manageable 15/45 meter. If you go this way you're looking at 4-core 16mm² cable, PEA will expect you to use 35mm² for a 30/100 3-phase which is the biggest they will provide without you having your own transformer ($$$).

The simple cable calculator only goes to 16mm², a better calculator for bigger supplies is here http://www.doncaster...able_calculator but the PEA required sizes will be fine for your 75m run.

Nothing to stop you using NYY on the aerial sections so it's all in one piece, but it's not going to be a cheap bit of cable

Thanks, Crossy, that calculator is perfect, and the one I could not find! According to it, I need a 50mm² cable for a 150 amp, 220V single phase application with a 75 meter run (5% voltage drop).

I guess my post as too long to read completely. I apologize for that. Yes, I know I need 3 phase, but as I said, It is not in my village and PEA told me that to get it, I would have to start with 300,000 baht just for the transformer and go from there. I can't afford that. It just is not an available option where I live.

But it seems the 50mm² cable still might be a problem if it will not fit the 100 amp meter or PEA will not let me use it. I was planning to purchase and provide the cable myself. I know the 50mm² cable is pricey at around 195 baht/meter (I budgeted that). Are you telling me that if I am sitting there with 150 meters of 50mm² cable, PEA will NOT let me use it?

Also, as I said, I know that the 30/100 is the largest meter provided by PEA, BUT, if I source and purchase my own Mitsubishi MF-33E(U) 150 amp (max) meter, will they allow me to use it? That would solve my problem.

One other point of confusion are these 10kw single phase water heaters I mentioned. After extensive research, it seemed that I would have to use 10mm² cable (10K / 220 = 45.5 amps) (10 meter run). But this new calculator suggests I could actually get by with 4mm² cable and a 45 amp breaker. Is this correct? Big difference; big savings.

Thanks in advance.

Edited November 18, 2012 by TongueThaied

[Crossy]

PEA will NOT permit you to supply your own meter, indeed they may not even provide a 100A meter if they deem the current infrastructure cannot support it.

Nothing to stop you using 50mm² cable, if it won't fit in the meter terminals you can add a 25mm² pigtail to the end (don't clip off strands to make it fit).

For your 10kW (45A) heaters you will need 45A breakers and 10mm² cable, the Doncaster Cable volt drop calculator calculates volt drop, it does not give the minimum cable size for your current, use the cable rating tables for that.

Assuming you don't take 1 hour showers you could wire the heaters in 6mm² on 40A breakers and it will be just fine. A 40A breaker will carry 45A for over an hour before the thermal trip operates, the cable will get warm but will also be just fine (doing it this way is very common in the UK and is perfectly safe and legal).

Rating tables from Bangkok Cable are here http://www.bangkokca.../SECTIONEN.HTML

[David006]

PEA will NOT permit you to supply your own meter, indeed they may not even provide a 100A meter if they deem the current infrastructure cannot support it.

Nothing to stop you using 50mm² cable, if it won't fit in the meter terminals you can add a 25mm² pigtail to the end (don't clip off strands to make it fit).

For your 10kW (45A) heaters you will need 45A breakers and 10mm² cable, the Doncaster Cable volt drop calculator calculates volt drop, it does not give the minimum cable size for your current, use the cable rating tables for that.

Assuming you don't take 1 hour showers you could wire the heaters in 6mm² on 40A breakers and it will be just fine. A 40A breaker will carry 45A for over an hour before the thermal trip operates, the cable will get warm but will also be just fine (doing it this way is very common in the UK and is perfectly safe and legal).

Rating tables from Bangkok Cable are here http://www.bangkokca.../SECTIONEN.HTML

Nothing to stop you using 50mm² cable, if it won't fit in the meter terminals you can add a 25mm² pigtail to the end (don't clip off strands to make it fit).

don't understand the reasoning on this....a pigtail must be joined and thence impedance.etc....care to explain??

[Crossy]

Nothing to stop you using 50mm² cable, if it won't fit in the meter terminals you can add a 25mm² pigtail to the end (don't clip off strands to make it fit).

don't understand the reasoning on this....a pigtail must be joined and thence impedance.etc....care to explain??

The reason for using a pigtail rather than trimming the strands is simply that the outer jacket of a 50mm² cable won't fit through the cable gland on the meter so you could expose bare copper.

The main reason for using over-sized cables is for limiting the volt drop rather than actual current rating (50mm² THW is good for nearly 200A) so using a short length of cable that fits but is still capable of carrying the load current is the way to go.

[TongueThaied]

Thanks, Crossy!!! As always, your information is priceless. After over 100 hours digging through the Internet, I found a lot of voltage drop calculators, but never one quite as appropriate to the need and user friendly as the one you provided. I also never ran across the Bangkok Cable catalogue, which is the one my local dealer uses (paper). It will help me immeasurably as I begin to purchase all the materials for this house.

You also saved me a lot of money and annoyance regarding the 10KW heaters, and gave me confidence in the smaller cable size. Besides which, I have found that 45 amp breakers are not in abundance. the 40 amp breakers will be much more palatable. Thanks for that!

You have no idea how much your replies have helped me (how much frustration they have relieved). This morning, I'm off to the PEA office to see if they will allow me the 100 amp meter.

[Crossy]

This morning, I'm off to the PEA office to see if they will allow me the 100 amp meter.

Good luck with your quest for 100A

Do your planned heaters have adjustable power? We have Redring 7kW heaters but they're all set to 4kW and we never have an issue with the water being too cold, obviously you may need a bit of extra oomph in winter if you're up north / in the mountains

[electau]

First you should contact the PEA in your area and advise them of you proposed requirements.

You will need to calculate your max demand first. In Thailand 63A for a residential installation is ample. This is the setting of a fixed setting MCB for the main switch.

The standard metering is 45A and is the responsibility of the PEA.

The consumers mains should be a mimimum of 16sqmm and may be required to be larger to take into consideration voltage drop depending on the route length of the circuit. Also the PEA may stipulate a minimum size. One would suggest limiting voltage drop in mains to about 3%.

Minimum cable size for 3% drop for a 80 meter run would be 25sqmm for single phase connection. to the main switch board.

As you live in a village your options are limited. You should design your requirements around a 45 to 63 amp max demand. You do not need 10kW water heaters, use solar thermal or a 3 to 4 kW heater. Consider using LPG for cooking.

An MCB will carry short term overloads 1.2 x rated current of device continously and must trip at 1.45 x rated curent within the conventional tripping time ( 1 hour).

MCBs are available 40A and 50A.

　

　

Edited November 20, 2012 by electau

[TongueThaied]

This morning, I'm off to the PEA office to see if they will allow me the 100 amp meter.

Good luck with your quest for 100A

Do your planned heaters have adjustable power? We have Redring 7kW heaters but they're all set to 4kW and we never have an issue with the water being too cold, obviously you may need a bit of extra oomph in winter if you're up north / in the mountains

They have agreed to install the 30/100 meter, so I should be in good shape. I'll use 50 mm² cable for the runs with 25 mm² cable pigtails for the meter. I am assuming "30/100" means sustained loads of 30 amps with limited additional loads up to 100 amps. I have never really seen it explained. Normally, with our habits, we will always be below 30 amps. But if a visitor comes and turns on all the air conditioners and takes a hot bath, well, there you go.

Yes the Panasonic heaters have settings for 6, 8 and 10KW. I don't live in the mountains and rarely even use hot water myself. It is unlikely I will ever use these heaters on full power for any amount of time. But it cost little more to get these heaters over, say a straight 6KW, and I liked the versatility. I am installing a bathtub, but will endeavor to fill it from a solar heated tank just outside. I just abhor putting in infrastructure that will not support potential load. I suppose in this case, it might be a lot like insisting on a roof that will support a snow load!

Thanks for your input Electau. I know what is "ample," but I already have the 10KW Panasonics. Also, I understand that 63 amps is a typical high load for a Thai house, but I am not Thai and I come from a land where 200 amp (@ 110V -- equivalent to 100A @ 220V) is standard. I have already calculated load, and the demand by the air conditioners alone is 30 amps (using 75% of connected load). You throw in a couple of appliances, some lighting and my workshop and their are times where the 63 amp limitation is easily blown. Since I am building from scratch, why on earth would I want to build in a bottleneck?

I have been unable to find a large single phase panel, so I will probably split my load between two 50 amp subpanels. I was thinking that I will bring the 50 mm² main feed into a small primary panel, which will house a cut-out and MOV, then split the line between two 50 amp ELCB's. which will then feed the two 50 amp subpanels. There will be another small subpanel in the shop. I could use one panel and a lot fewer breakers than I plan, but I plan to put each appliance on its own circuit (including each AC & WH) (that's 16 breakers right there, not counting the shop), each room's lighting will be on a separate circuit, etc. I might be in Thailand, but I am going to wire my house to western standards. And the oft given advice to use "a qualified electrician" in my village is like telling me to "go ride a unicorn."

Which is why I thank you all profusely for the help, because if I don't get it right; it won't be right. If any of the above seems flawed, please shoot me down. One can often only learn by being wrong and recognizing it.

Edited November 26, 2012 by TongueThaied

[Crossy]

Actually a 30/100 is calibrated at 30A and can sustain 100A continuous, peak to 200A won't injure the meter but the local supply will probably burp.

We have done exactly what you suggest, incomer to a main panel with two 50A outgoing breakers feeding sub-panels, 1 for aircon and water heating, the other for lights and power.

Our 'Power' panel also handles the generator transfer switch and the output from our big UPS. All panels are ABB 16 way units with DIN rails, easy to adapt to your purpose.

MOVs and an 'incoming supply' lamp in the main panel.

[Theboy]

Just out of interest does anyone know what meters are available in Thailand ? Obviously 30/100 are but what size are the smaller ones ?

[Crossy]

Just out of interest does anyone know what meters are available in Thailand ? Obviously 30/100 are but what size are the smaller ones ?

Standard sizes from PEA are 5/15, 15/45 and 30/100.

[electau]

The max demand in amps for an electrical installation is for design purposes. Your actual demand in amps is what you consume at any point and will vary depending on your load at the time.

This peak demand may only exist for a short time. It will be in most cases be considerably less than the calculated max demand.

The PEA supply a transformer to supply this load and to other consumers in many cases.

The diversity of most LV residential loads from a single transformer is between 3 and 5 kW each residence. so a 100kVA transformer could supply up to 20 normal residences.

What is the size of transformer that will supply you?

The fact that the PEA wil supply you with a 30/100A meter does not mean that they can supply 100A you should look at peak loads of 30A in practice.

Edited November 27, 2012 by electau

[electau]

If the transformer cannot supply the peak load of your installation and other installations connected to it low voltage problems on the distribution network may be encountered. 50sqmm mains will not solve that problem. A larger transformer will.

Edited November 27, 2012 by electau

[electau]

You actually answered your own question, you state that your peak load will rarely go over 30A.

Install a 63A MCB as the main switch and you have maximum demand by limitation.

With additional loading above 32A use one or two items of equipment at a time.

Load management is the answer to your problem. ie. Diversity.

　

Edited November 27, 2012 by electau