Rewire 1970's building

Idbatterim

Hi. Refurbishing a 1970's building, the old fuse boards will be replaced by modern trip switch. The wire appears to be in as new condition, an electrician has looked at it and said personally he wouldnt bother rewiring. Any thoughts?

Tow

You have a honest electrician.

2011

Idbatterim wrote: »

Hi. Refurbishing a 1970's building, the old fuse boards will be replaced by modern trip switch.

Good idea.

The wire appears to be in as new condition, an electrician has looked at it and said personally he wouldnt bother rewiring. Any thoughts?

It is very hard to give an answer without seeing the installation.
First off, when was the building last wired?

Wiring regulations have changed quite a bit since the 1970s, if it has not been wired since then additional wiring would be required to bring it up to standard. The fact that it does not have modern MCB type board suggest to me that it has not been wired in quite some time.

Here are a few questions for your electrician (I am assuming that this is a home):
1) Are there at least 2 dedicated socket circuits for the kitchen?

2) What earth cable to the earth electrode? It should be at least a 10 sq. mm cable

3) What size is supply cable to the main fuse board? It should be at least 16sq. mm

4) How many lighting circuits are there? There should be at least two.

5) Do all of the cables use modern insulation such as PVC? Older cables were used a natural rubber and should be replaced.

6) Will your electrician be able to certify the entire installation without installing any new cables? If this is possible then get him to proceed. Agree to make the final payment on receipt of the ETCI completion certificate.

dingding

Wuold there be an issue with the colour coding of the cable.

I would assume that a 1970's building would have red and black instead of blue and brown.

2011

dingding wrote: »

Wuold there be an issue with the colour coding of the cable.

I would assume that a 1970's building would have red and black instead of blue and brown.

It could be red and black alright, depends on when it was last wired.
Personally I could live with red and black if everything else was ok.

Idbatterim

thanks for the replies! building is a small ex hotel. yes cables have red and black instead of blue and brown? im 99% sure the material is pvc. Will put all the suggested questions to the electrician! there is an insane amount of boxes and fuses, particularly in the basement...

meercat

my advise is to rewire now
the building is 35years old and who knows what lies unseen
you may regret not doing this now while you are renovating
it should be minimal cost now rather than in the future

2011

Idbatterim wrote: »

thanks for the replies! building is a small ex hotel. yes cables have red and black instead of blue and brown? im 99% sure the material is pvc. Will put all the suggested questions to the electrician! there is an insane amount of boxes and fuses, particularly in the basement...

Most of my points above refer to a house rather than a commercial premises.
Your sparks may well be right, industrial/commercial installations are often wired to a higher standard than a domestic installation. As a rewire is not likely to be cheap it may be a good idea to get at least three quotations. It would also be interesting to see what the other contractors say about the wiring. Ask for all quotations to be submitted in writing detailing exactly what is being quoted for. Explain that you require an ETCI completion certificate for the installation.

You should also be aware that regulations have changed for emergency lighting and fire alarm systems in recent years. As part of your insurance these will need to be certified on a yearly basis.

Good luck with it.

Tibulus

Generally if the cable is PVC I would not recommend replacing them unless there are issues with fuses or MCB operating or evidence of damage to cables.The cost is just too great, not just the electricians cost but also replacingfloor boards, re-plastering etc.

I would check that the main earth cable (from board to outdoor electrode is10mm sq) and the same cable is used to earth all copper pipework.

An other essential for me would be install mains smoke alarms in parts ofthe house where bedrooms are located.

Also if there are alot of junction boxes in the basement it may be possibleto rewire that section of the property.

I would not be too concerned about the main conductor from the ESB cut outto the board being 16mm sq, this was an addition to the Irish regulations inthe last 15 years. I remember doing a calculation for a 10mm sq carrying 63amps (max load) and the maximum distance was 75m which the run is unlikely tobe.

Hope this helps.

2011

Tibulus wrote: »

I would not be too concerned about the main conductor from the ESB cut outto the board being 16mm sq, this was an addition to the Irish regulations inthe last 15 years. I remember doing a calculation for a 10mm sq carrying 63amps (max load) and the maximum distance was 75m which the run is unlikely tobe.

I don't agree with this statement.

If you look at table A52-J1 on page 289 of ET:101 you will see that the volt drop mV / amp /meter is 4.4 for a 10sq. mm single phase PVC cable.

Therefore a 63 amp current at 75 m will result in a volt drop of about 20.8 volts.
This would result in a 9% volt drop to the distribution board. The maximum permitted is 4% including the final circuit.

I accept that this would only apply with an unusually large load, but even at 40 amps the volt drop is excessive.

Remember that in addition to this there will also be a volt drop on the final circuit.

Tibulus

2011 wrote: »

I don't agree with this statement.

If you look at table A52-J1 on page 289 of ET:101 you will see that the volt drop mV / amp /meter is 4.4 for a 10sq. mm single phase PVC cable.

Therefore a 63 amp current at 75 m will result in a volt drop of about 20.8 volts.
This would result in a 9% volt drop to the distribution board. The maximum permitted is 4% including the final circuit.

I accept that this would only apply with an unusually large load, but even at 40 amps the volt drop is excessive.

Remember that in addition to this there will also be a volt drop on the final circuit.

This is a common mistake that people make, the ETCI regulations use very conservative values and are only intended as a rough guide for Electricians.



If a consultant were to use this method they would not be in business for long.



The method which I used for calculating is based upon the actual cable data available from suppliers, Anixter websiteis an excellent source of such material.





My calculation is as follows:





1. SystemData



Cable Size: 10mm sq

Conductor: Cu

Distance: 75m

Load: 14,500 W

Voltage: 230V

Power-Factor: Unity

Phasing: Single Phase





2. CableData (From Cable Supplier)



Max Current CarryingCapacity: 82 Amps

Conductor DC resistance At20degC: 1.83 mΩ/m

Cable Reactance: 0.0835mΩ/m





3. CircuitCurrent



=P/(V*PF)

=14,500/(230*1)

=63.04 Amps





4. PercentageLoading



=(Inom/Imax)*100

=(63.04/82)*100

=76.87%





5. VoltDrop



=(Inom*PH)*({mΩ/m} +j{x mΩ/m})*D

=(63.04*1)*(.00183+j.0000835)*75

=8.65 Volts





6. Notes



A: This volt is excessively high and close to the maximum limit, however still complying with regulations. The likelihood of the cable operating at 63amps is low and any occurrence would only be for short periods. Most likely when electric shower is operatingat same time as electric cooker. Also 75m is the absolute maximum distance and in normal domestic installations would nerver actually be this length.



B: The cable is loaded at 76.87%, generally less than 80%is considered good practice. However the cable is operating close to thislimit.



C: The load for the calculation is assumed to be 14.5KW atunity power factor. For a general domestic installation the ESBN would use a 12KVAtransformer. However a 16KVA can be requested for large houses, heat pumps,farms or small businesses.

2011

Tibulus wrote: »

This is a common mistake that people make, the ETCI regulations use very conservative values and are only intended as a rough guide for Electricians.

The ETCI regulations not a guide, there is a legal obligation to comply with them.

However the ET:101 values also align with this cable datasheet:

http://www.batt.co.uk/upload/files/currentratingstable4d5_1312876143.pdf

If a consultant were to use this method they would not be in business for long

From personal experience I would have to disagree.
I will not sign off on anything that does not comply with ET:101
Part of complying is issuing an ETCI certificate, which in this case would not be possible.

The method which I used for calculating is based upon the actual cable data available from suppliers, Anixter websiteis an excellent source of such material.

Yes, I found a datahseet by Anixter here:

http://www.eland.co.uk/documents/SWA%20Cables%20-%20BS5467.pdf

On page 5 they provide a higher volt drop value of 4.7 mV/A/m (for a 230VAC single phase load on a 10 sq. mm XLPE/PVC/SWA/PVC cable).
Again this aligns very nicely with the values provided in ET:101

My calculation is as follows:

Conductor DC resistance At20degC: 1.83 mΩ/m

The cable resistance is per core.
Therefore you will have a volt drop out to the load on the phase core and a volt drop back on the neutral core.
Total core length is 150m.
This is one of a few reasons that your volt drop calculation is incorrect.

A: This volt is excessively high and close to the maximum limit, however still complying with regulations.

Maximum permissable volt drop is 4% of nominal voltage.
Normal practice is to limit the volt drop to between 1.5 and 2% to the distribution board. The remaining volt drop will be on the final circuit.

If the volt drop was 8.65 volts as you suggest this would only allow a 0.55 volt drop on the final circuit to keep within the 4%.
This would mean that very larger than normal cables would have to be used for all final circuits.
This is not likley to be finincially viable or practical.

The likelihood of the cable operating at 63amps is low and any occurrence would only be for short periods. Most likely when electric shower is operatingat same time as electric cooker. Also 75m is the absolute maximum distance and in normal domestic installations would nerver actually be this length.

I agree that a 63A load would be abnormal.
Even when I used a design current of 40A in my post above the cable still would not comply with the regulations.

B:The cable is loaded at 76.87%, generally less than 80%is considered good practice. However the cable is operating close to thislimit.

This really depends on the application and the client.

The last number of projects that I have worked on and the one I am on now we size for a minimum of 125% of the load when installing cables to distribution boards or MCCs (we use ERACS software for cable sizing).
This is to allow for future circuits.

Bruthal

Tibulus wrote: »

2. CableData (From Cable Supplier)



Max Current CarryingCapacity: 82 Amps

Conductor DC resistance At20degC: 1.83 mΩ/m

Cable Reactance: 0.0835mΩ/m


5. VoltDrop



=(Inom*PH)*({mΩ/m} +j{x mΩ/m})*D

=(63.04*1)*(.00183+j.0000835)*75

=8.65 Volts

The 1.83 mΩ/m is probably per meter of conductor. So it would be double for a cable.