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Topic Title: Size of neutral cable tail from supply to meter, a query
Topic Summary: Is neutral cable size adequate for a 3 single phase installation
Created On: 22 January 2013 01:37 PM
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 22 January 2013 01:37 PM
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bowmandj

Posts: 139
Joined: 04 October 2011

A church building I am working on has a three phase supply into the building with one phase used for upstairs and a second for downstairs.

The live feeds from the company fuse to the 3 phase meter is 16mm (even though regs tell us to use 25mm after the meter); and the neutral cable is common to both phases and is 25 mm.

The church is having IR patio style heaters installed and the installer has used the third phase for this to balance the load.

So my query is whether the single neutral of 25mm is man enough if we have three single phases rated at 100 amps.

The new heater phase is 20Kw (80 amps). The downstairs feed does kitchen and water heater (so can load to 12Kw (48 amps). The upstairs feed is lighter loaded but with spot lights on could reach 8Kw (32 amps).

So each phase will never exceed 100 amps but together it means the common neutral tail will have well over 100 amps on full load.

Is this right?
 22 January 2013 02:05 PM
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AJJewsbury

Posts: 11296
Joined: 13 August 2003

So each phase will never exceed 100 amps but together it means the common neutral tail will have well over 100 amps on full load.

Is this right?

Not quite.

As the currents in each line are out of phase with each other, they don't simply add up in the N - there's a degree of cancelling. If all three lines where equally loaded, you'd actually end up with ZERO amps flowing in the N. In the worst case (presuming no 3rd harmonics - which isn't going be an issue with simple resistive loads) is the N carrying the same as one of the lines.

(even though regs tell us to use 25mm after the meter)

The regs themselves aren't that prescriptive - 16mm2 is perfectly compliant if the cut-out fuse is 60A or 80A. Beware of all-purpose guidance being mis-interpreted.

- Andy.
 22 January 2013 02:27 PM
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bowmandj

Posts: 139
Joined: 04 October 2011

Originally posted by: AJJewsbury


As the currents in each line are out of phase with each other, they don't simply add up in the N - there's a degree of cancelling. If all three lines where equally loaded, you'd actually end up with ZERO amps flowing in the N. In the worst case (presuming no 3rd harmonics - which isn't going be an issue with simple resistive loads) is the N carrying the same as one of the lines.

(even though regs tell us to use 25mm after the meter)


The regs themselves aren't that prescriptive - 16mm2 is perfectly compliant if the cut-out fuse is 60A or 80A. Beware of all-purpose guidance being mis-interpreted.

- Andy.


Thanks Andy. After I posted I thought about it and remembered that the phase separation is 120 degree so there will be some anti phase flow. That is probably what the installer meant by balancing it, not just distributing the load over the phases but balancing the neutral flow as well. There could be a time when only one phase is loaded, the heating prior to a meeting before the kitchen and main hall uses power, and this is well within the neutral cable rating.

And yes you are right on the regs, I should have said that the 3 phase supply is fitted with three 100amp fuses.

Thanks

Derrick
 22 January 2013 04:45 PM
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AJJewsbury

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And yes you are right on the regs, I should have said that the 3 phase supply is fitted with three 100amp fuses

The DNO side often uses what looks like 16mm2 - I suspect they're actually rated to 90 degrees (rather than our usual 70) - so should be good for 100A if spaced.

Be cautious of the rating of cut-out fuses - while most of the fuse carriers are labelled 100A, that's no guarantee that there are 100A fuses inside. Some DNOs (YEDL for example) seem to have a policy of fitting maximum 80A fuses by default now, or even 60A if they see a need (e.g. undersized tails on the consumer side).

- Andy.
 22 January 2013 04:59 PM
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Jobbo

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You can calculate the neutral current with the following formula:

N current = Square root of /(L1^2 + L2^2 + L3^2) - (L1*L2 + L1*L3 + L2*L3)

Quickly stick it in excel and have a play with the figures

Regards

Jobbo
 22 January 2013 05:24 PM
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bowmandj

Posts: 139
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Originally posted by: AJJewsbury

And yes you are right on the regs, I should have said that the 3 phase supply is fitted with three 100amp fuses


The DNO side often uses what looks like 16mm2 - I suspect they're actually rated to 90 degrees (rather than our usual 70) - so should be good for 100A if spaced.

Be cautious of the rating of cut-out fuses - while most of the fuse carriers are labelled 100A, that's no guarantee that there are 100A fuses inside. Some DNOs (YEDL for example) seem to have a policy of fitting maximum 80A fuses by default now, or even 60A if they see a need (e.g. undersized tails on the consumer side).



- Andy.


That is a good point, the fuses are sealed so I cannot check the rating of the fuse inside. The seal is stamped The Eastern Electricity Board and an inspection label on one of the tails is dated 1995 and was from Eastern Electricity Plc.. The meter may have been checked since then but it does not look as if the supply fuses have been inspected in 17 to 18 years!! That does not mean they are 100 amp but it does mean that they have missed any new trends and/or DNO policies :-)

Derrick

Edited: 22 January 2013 at 05:31 PM by bowmandj
 22 January 2013 05:30 PM
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bowmandj

Posts: 139
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Originally posted by: Jobbo

You can calculate the neutral current with the following formula:

N current = Square root of /(L1^2 + L2^2 + L3^2) - (L1*L2 + L1*L3 + L2*L3)


Quickly stick it in excel and have a play with the figures

Regards

Jobbo


Thanks Jobbo, I will do that. Funny how that formula rings something in my head dating back to 1969 at Carshalton College when doing my C&G's.

Regards

Derrick
 22 January 2013 05:42 PM
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sparkingchip

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How about spreading the heaters across all the phases to try and get close to 54 amps on each phase? Then it might be a balanced load, but only when it is cold, dark and everything is turned on.

Andy
 23 January 2013 10:51 AM
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bowmandj

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Originally posted by: sparkingchip

How about spreading the heaters across all the phases to try and get close to 54 amps on each phase? Then it might be a balanced load, but only when it is cold, dark and everything is turned on.

Andy


I did say that to the installer but he wanted to minimise the voltage difference between heaters. He wanted them all at 230 rather then the 400 that difference phases would bring. He has the warning signs to indicate that heater and spotlight are different phases but they are quite a distance apart. The heaters are within arm reach distance so he wanted them at the lower voltage. Also a single phase install with one new CU was cheaper then load spreading which would have meant two other CU changes and lots of wiring stretching (lots of old single wire for the lights and separate earth line).

Regards

Derrick
 23 January 2013 11:13 AM
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AJJewsbury

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He has the warning signs to indicate that heater and spotlight are different phases but they are quite a distance apart.

Your installer is a bit out-of-date. The requirement to label '400V between adjacent enclosures' disappeared quite a while ago. Nowadays the label is only required if the voltage exceeds 230V to earth (rather than between lines).
- Andy.
 23 January 2013 02:41 PM
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chevalier

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In principle you should agree the additional load with the DNO. You can also ask the DNO to confirm the fuse sizes. They will almost certainly need to visit to find out themselves, then ask if they can up rate the 16 to 25 tails while they are there...
 23 January 2013 04:03 PM
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bowmandj

Posts: 139
Joined: 04 October 2011

Originally posted by: chevalier

In principle you should agree the additional load with the DNO. You can also ask the DNO to confirm the fuse sizes. They will almost certainly need to visit to find out themselves, then ask if they can up rate the 16 to 25 tails while they are there...


I will do that as the 3 ph meter is an old mechanical dual rate meter (the church originally had storage heaters). I think the installer assumed that as the church had storage heaters a few years ago then the loading for the IR heaters was okay.

But a good tidy up of the supply board would be in order, remove the mechanical clock and old porcelain bits and have a new Landis compact meter.

They can okay the install at the same time.

Regards

Derrick
 23 January 2013 04:27 PM
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OMS

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I think the installer assumed that as the church had storage heaters a few years ago then the loading for the IR heaters was okay.


He probably missed the fact that the sto-rads would be "on" when the rest of the building was "off" - midnight mass notwithstanding

Regards

OMS

-------------------------
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 23 January 2013 08:34 PM
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slittle

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The local church for the village I grew up in has a 60 amp tns supply. I know this now because we do a bit of work in there from time to time (the vicar plays bowls with my father)

Anyway, every carol service when I was small the lights were turned off and one of the carols was sung by candle light and no organ(I think it was away in a manger), after which all of the lights were thrown back on and the organ fired up for "in the bleak midwinter". Year on year we would get to the second verse only for the cut out fuse to melt and every thing stop. Bleak it was in mid winter with no lights, no heating and no organ. Looking back, I'm guessing it was a combination of inrush from the lighting and start up from the blower motor.

No service upgrade available without a serious dig and across a church yard that's not really viable ;-)


Stu
 23 January 2013 09:08 PM
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Frankmul

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Originally posted by: AJJewsbury


As the currents in each line are out of phase with each other, they don't simply add up in the N - there's a degree of cancelling. If all three lines where equally loaded, you'd actually end up with ZERO amps flowing in the N. In the worst case (presuming no 3rd harmonics - which isn't going be an issue with simple resistive loads) is the N carrying the same as one of the lines.


- Andy.


I have an excel sheet here which may show what happen with the neutral current
http://www.wuala.com/frankmul/...tion%20rev%202.0.xlsx/
Statistics

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