Cable Rating for Shower

Recalculate for a PME supply with an external Ze of .292 ohms.

Reply to
ARWadsworth
Loading thread data ...

Huh? What is the relevance of the the supply's earth impedance?

The impedance of the live wires of the supply will (I hope) be so low that even if the house were to draw 100A, the supply voltage at the house won't dip much. I really don't know what order of magnitude drop to expect, but would have thought no more than 1V, and if so, the supply impedance would be 0.01 ohm, or 0.005 ohm on each conductor.

If the neutral conductor is then in effect parallelled up with earth at 0.292 ohm, it would improve (reduce) the supply impedance and hence voltage drop by less than 1%.

Reply to
Ronald Raygun

Hi,

Some real world figures and limits:

My TN-C-S supply has a Ze of 0.18 Ohms (that's quite low BTW, short of having the transformer in your garden). Despite this, my lamps flicker when the electric water heater comes on at 37 odd amps - due to a voltage drop at the CU terminals of around 7V.

For a 100A load, the voltage would drop 18V!

Now, as this is a TN-C-S, the L-E impedance is the same at the cutout as the L-N impedance. The max permitted Ze (and therefore Z(L-N) for TN-C-S is 0.35 Ohms.

Don't forget, that includes the impedance of the source (the transformer), the possibly quite long, despite being chunky, cable in the road plus anywhere from a few to a few 10s of metres of weedy concentric stuff that may well be a step undersized by BS7671 but as it is mostly buried in the ground (cooling) and different regs apply, is technically fine.

Reply to
Tim Watts

The supply cable is copper and has the same resistivity as T&E cable. Your supply could easily be a few 100 m away from the ring in the street. For a PME supply the earth is the neutral and a line impedance of upto 0.35 ohms is allowed. That is a greater impedance than the 40m of 6mm T&E used for the shower.

Voltage drops of 5% or more are not uncommon on the supply terminals.

Reply to
ARWadsworth

Let's not confuse earth impedance with line impedance.

If Ze (earth impedance, the impedance between consumer earth and "real" earth) is allowed to be up to 0.35 ohm, and the earth path is via the supplier's neutral conductor, that doesn't imply that the impedance of said neutral conductor will be as much as that, it only means that the impedance of the connection bewteen real earth and the neutral conductor at the supplier end (plus that of the neutral conductor itself) should not exceed 0.35 ohm.

If the impedance of the neutral conductor itself were as much as 0.35 ohm, then since the phase would generally be the same type of wire, the overall supply line impedance would be 0.7 ohm, and you'd be dropping 28V at the CU when the house draws 40A.

Generally I would presume that the line impedance of any particular supply connection would be scaled to the size of load the supplier expects the consumer to present. A house with no oil/gas heating would have electric heating, and could easily have 3kW heaters on simultaneously in each of half a dozen rooms, plus a 10kW shower running. Even if we don't count the turkey in the oven, that's still 28 kW, or over 120A at nominal 230V.

If the line impedance in this example were as much as 0.35 ohm, the supply would drop by over 42V, which is hardly acceptable. And 28kW is hardly an outrageous load to be reckoning with for a domestic supply. Some houses (not in this country) have instant water heaters for filling baths (like a shower heater but with much higher water volume throughput per minute) and these are typically rated at up to 33kW (but at that size generally connected to a 3 phase supply).

Agreed, it would be, but I'd hope that that size of line impedance would be extremely rare.

Fair enough, I was unaware it would typically be as bad as that. But not 18% as in the above example, I hope!

Reply to
Ronald Raygun

The Ze measurement is the total the impedance of all elements of the fault path (i.e. L to E) as measured at the origin or the installation in the property. So the "up to 0.35 ohms" allowed for a PME system includes the impedance of the phase and Protective Earth and Neutral (PEN) conductors. Hence with PME, the Ze is also the same as the supply impedance, and also gives you an indication of the prospective short circuit current at the origin.

With TN-S supply impedance will tend to be lower than the earth loop impedance.

No, the 0.35 is the total round trip from phase to PEN - so the drop at the installation would be half that.

I am not sure planning of supply capacity comes down to analysis of each houses requirements. It will probably be done at a larger grouping - so a village with no gas may get a higher overall provision than one with etc.

Given many properties have 60A supplies, 28kW is fairly outrageous! ;-)

IME much below 0.2 ohms is relatively rare outside of fairly dense towns...

(OOI, our supply impedance is 0.24 ohms - overhead supply - probably no more than 100m of cable to the pole mounted transformer. Previous place as 0.25 ohms, higher property density, and probably 150m to the (more substantial) sub station)

Reply to
John Rumm

The line impedence is the external restance of the L and N supply cables. For a PME supply this is the same as the L - PE (ELI).

The 0.35 ohms is the combined impedance of the Live and the Neutral supply cables.

Or 20% more than a typical UK domestic supply of 100A.

28kW is an outrageous load for any length of time for a domestic property. I still have a 60A supply to my house and I saw a house last week with a 30A supply.

It is fairly common. Tim's supply of 0.18 ohms is very typical.

An example that needs redoing with a total external resistance of 0.35ohms. Not 0.35 ohms for both L and N (so a 9% supply voltage drop at 100A).

Of course the supply voltage to a house can change throughout the day by quite a few volts without you turning anything on in your house depending on others use of electricity.

Cheers

Reply to
ARWadsworth

Generally UK supply impedance is not noticed. The exception is when someone connects a huge cheap induction motor saw (without soft start) or an "oil drum welder". I think most DNO still have a supply condition where if you connect a 1hp+ motor (or 3kW

  • load) you technically have to apply in writing for permission. No- one does, but they would laugh their ass off if you connected a 28kW load because I suspect the town never mind the street would dim. DNO apply quite a stiff diversity figure to a bunch of houses.

DNO commonly use a BS1362 main fuse (DNO cutout) - a 60A fuse will allow 96A to be drawn continuously, a 100A fuse will allow 160A to be drawn continuously. Hence it is rare for the main fuse to fail and 30A

40A 60A ratings are commonly found yet rarely a problem. One issue is loop-in (shared) supply houses which both have 60A & 60A (or worse 80A & 60A) fuses because even with 60A & 60A that permits 192A to be drawn on what is typically a circa 135-140A supply cable.

Italy & france have much smaller supply capacity. In Italy you have interlocks where you "have electric dryer or cooker but not both at the same time". In France you can have a larger supply, but at a premium cost which IIRC is charged all year round - hence France electric heating uses oil filled aluminium radiators.

If a UK house has a lot of storage heaters (not common, but some do in rural areas) the voltage drop from say 240V to 222V or 216V has a very material impact on the charge the heaters achieve and comfort levels. It is still within spec though.

Reply to
js.b1

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.