One for the electrical boys, please ...

We are in the process of taking some modern commercial premises, in which the landlord is dividing down a 2k sq ft unit into two 1k sq ft units, one of which will be ours. The electrical supply comes into the unit that will not be ours, so the landlord is putting a new supply into our bit for us as part of the deal. The currently installed supply is three phase, with each phase company-fused at 100A, as far as I can see. We have approached NPower for a meter, and one of the questions on the application form refers to the type of meter required. The two three phase options given are "Three Phase Whole Current" and "Three Phase CT".

I have not the slightest idea what the difference between these is, or which it will be that we need ?? I have checked with the landlord to make sure that it is a three phase supply that he is bringing over to our unit, and he says that we will "need a three phase meter for a 40kVA supply" First question then. If each phase is currently fused at 100A, does that mean that the incoming supply is 25kVA per phase i.e. 75kVA, and that by dividing it between the two newly created premises, that is how he arrives at 40kVA ?

Our demand will be fairly high. There will be two electrical fryers, each loading at 2 x 9kW, so I guess if they are all on and heating up together, that would be a demand of 36kW. There is also a ceiling mounted airco / heater unit that I don't know the specs for . It's not very big - maybe 2 ft square. Other than that, the demand will be moderately low - some fume extraction which is only a few amps, a couple of small fridges and freezers, lights (not more than 1kW) and small odds and sods for a normal ring main. So, given the *potential* for everything being on together, does that mean that the supply he is proposing giving us, is going to be adequate for our needs ?

Another question on the form is "Voltage - please delete" The three options given are 415V 11kV and 33kV. I assume that the correct answer there is 415V as it's a three phase supply direct into the premises ?

Help please, lads ... !! d:-)

TIA

Arfa

Reply to
Arfa Daily
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Whole Current means the whole current passes through the meter, i.e.

100A on the supply means 100A passing through the meter.

CT is current transformer, i.e. there's a transformer between the supply and the meter. So with 1 20:1 ratio 1000A on the supply would mean 50A passing through the meter, which would be calibrated accordingly. Cheaper than providing a meter that will read kilo-Amps.

I suspect that at 3 x 100A you'd use Whole Current metering unless there's a good reason not to.

In a commercial kitchen or similar, 32A general purpose ring finals are of little use - they're too easily overloaded and too inconvenient if everything goes off at once.

Owain

Reply to
Owain

Yes, unless you really want your own 11kV substation in the corner of the room.

Owain

Reply to
Owain

Just a query in passing - IIUC you can get industrial machinery (such as lathes, drills etc.) which run on 3 phase directly - which would obviously require three phases.

If you have normal 230V kit do you have two supplies of 230V split between the three phases when you install a 3 phase supply?

Cheers

Dave R

Reply to
David WE Roberts

No, a 3-phase supply will come as 4 wires: L1, L2, L3 and N.

Each phase ( L1, L2, L3 ) to N is a standard 240v supply. Phase-to-Phase is 415v.

( Give or take the EU harmonisation voltages etc.. )

So you can feed 3-phase motors etc off L1, L2, L3. They typically won't require the N.

The dist board will have a 3-phase incomer, and 3 L bus-bars. 3-pole breakers will connect to each of the L to feed 3-ph loads. Single-pole MCBs will connect to only one L, and the single-phase wiring will then use the N bar, just like a domestic CU.

Reply to
Ron Lowe

Have you got 3 phase appliances? It is worth noting that 3x100A fuses on a 3 phase supply does not give you 300A of available power for 230V appliances (no matter how well you share the load between the fuses) as the neutral is only rated at 100A.

I have not done 3 phase for a while (other than just connect up a few machines) but 40kVA sounds like he has just multiplied 400 (the nominal voltage between phases) by the 100A fuse. ISTR that it should be multiplied by sq rt of 3 to get the maximum power.

With a bit of (bad) luck Dennis should be along shortly to explain everything as he is a f***ing know all.

Reply to
ARWadsworth

Not if the OP is only using 230V appliances.

Please pay attention to what is written.

Reply to
ARWadsworth

I really hesitate to differ, but are you saying that, if each 230 V appliance was taking 100A, there would be 300A in the neutral?

Reply to
newshound

No there would not be 300A in the neutral.

Reply to
ARWadsworth

I calculate it that if you have all three phases drawing 100A, the neutral would be carrying only stray currents, but if only one phase was loaded, the neutral current would be equal to the load current. With two running, it's something on the order of 1/ (square root of 2) multiplied by the combined load current, so in this case about 141A maximum with full load on two phases. With three phases running in balance, it's close to zero. It's a vector sum, and the worst case is with two phases running at full load, and the other one drawing zero load. For other combinations, it depends on the loads on each phase.

Reply to
John Williamson

I was having a brain dead moment. My apologies to Harry.

Depending on the balancing of the system there could be a higher current in the neutral than 100A and that is what I was thinking of.

I stand corrected. I was wrong and I admit it.

Reply to
ARWadsworth

Do pay attention, he has two 9 kW fryers, they are not going to be 230V. Even the dishwasher I put in was three phase and that was only 7 kW. And no I don't know enough about 3 phase to answer the OPs questions.

Reply to
dennis

Find the specs for that aircon unit. I'd expect it to use a substantial fraction of the heat it needs to get rid of - and on a hot summer's day that will be the whole 36kW and a bit. I'm no expert (I merely suffer these things in computer rooms) but I wouldn't be at all surprised to find it eats another 18kW.

Seems to me that may exceed your power limit.

Splitting this lot evenly over three phases isn't going to be possible. Some of the time you'll have load on one of the phases only, sometimes two, and only occasionally all three.

Andy.

Reply to
Andy Champ

In article , dennis@home writes

No thanks. re-plonked due to morph.

Reply to
Mike Tomlinson

40 Kva is approx 53 amps/phase,so he is trying to split the available power equally between the two units. At that level you want the "whole current" and 415v options.
Reply to
mark

Yes I think so, up to 72kVA per MPAN(meter point access number) which is

111A per phase whole current metering is allowed beyond that it gets complicated with paying for the supply depending on time of day etc,

To avoid this I decided to split our units into two separately fed sections each with 100A 3ph.

AJH

Reply to
andrew

You forgot Harry ...

Reply to
geoff

No, I didn't.

And I was wrong. I offered my apologies to Harry. What more can you do? I was wrong and I admitted it.

I will not try to make excuses, I will eat humble pie.

Reply to
ARWadsworth

Surely it's the supplier's or metering company's decision what type of metering to install? (Consumer chooses tariff, supplier installs appropriate metering equipment of their choice.)

Round here (the old Eastern region) they tend to use whole-current metering up to 3 x 200 A and CTs for larger. If you find CTs on smaller supplies it's generally where old meter equipment hasn't been updated.

Reply to
Andy Wade

I was waiting for the penny to drop. . .

What you do have to watch though is non-linear loads (old iron-cored fluorescent ballasts, for example) where the 3rd and 9th, etc. harmonics add in in the neutral, even when the load is perfectly balanced. The

17th ed. requires double-size neutrals in some circumstances. Checking neutral current with a clamp meter (preferably true-RMS reading) is always a wise precaution on 3-ph work.
Reply to
Andy Wade

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