Maximum cable size

[bernardgreen]

A "qualified" electrician is refusing the client's request to wire the final ring circuits with 4mm ?

The client ( a friend's son renovating a large property ) on my advice wants to pay for 4mm on the rings to reduce the voltage drop in cables to a minimum. The electrician is stating that as long as the voltage drop at full load is less than 2% the system is correct and any changes will make it not correct.

The electrician has calculated the voltage drop as being just less than 2% so insists 2.5mm is the correct and only cable to use.

As the client says a 2% drop in voltage is 4.6 volts which at full load ( say 30 amps on a 32 amp MCB ) is 138 watts being dissapated in the cable.

Have yet to discuss the 10mm feed to the range cooker.

 

[pjcomp]

The chances of a full load on a ring final circuit (or any other for that matter) are pretty remote and very transient, so there's never likely to be this 138A dissipation, and even if there was, why is that a problem for the customer? There is voltage drop everywhere all the time - the transmission losses on the national grid are enormous.

By all the technical specs, regulations and current best practice, 2.5mm T&E is the weapon of choice for a RFC, and the electrician's calcs apparently put the circuit within regs for this job.

I can understand any tradesman being resistant to the customer telling him his job, so I see a number of options: (a) customer employs electrician and lets him get on with it his way; (b) electrician (why the quote marks round the qualified, by the way, are you sugegsting he isn't?) does what the customer wants and charges for the extra price of cable and the extra effort of running 4mm instead of 2.5; (c) electrician walks away rather than work for someone under such circumstances.

A compromise could be that the electrician wires in 4mm but runs radials instead of rings ...

PJ

 

[bernardgreen]

why is that a problem for the customer? There is voltage drop everywhere all the time - the transmission losses on the national grid are enormous.

Paying for losses after the meter is one reason. Waste of power is the other

(why the quote marks round the qualified, by the way, are you sugegsting he isn't?)

Because from conversations with the client ( who has a sound knowledge of electrical theory ) suggest the "electrician" has learnt only the rule book and has little if any knowledge of the theory of electricity.

The compromise was the work will be given to another electrician who understands the client's needs and is prepared to use cables larger in order to reduce loss in cables due to avoidable voltage drops.

 

[ricicle]

As long as the client is prepared to sign the design part of the EIC then I can't see a problem.

 

[ban-all-sheds]

I wholeheartedly agree with the quote marks, because of this:

The electrician is stating that as long as the voltage drop at full load is less than 2% the system is correct and any changes will make it not correct.

He clearly doesn't have any real understanding - he's just doing electrics-by-numbers.

Because 2.5mm² complies 4mm² won't comply?

Kick him into touch and get someone with actual understanding .

 

[1john]

Prob looking for an excuse not to do the job for an interfering customer or maybe doesn't fancy having to chase in deeper boxes or try and get 2 x 4mm cables into sockets that wont take them.

 

[Adam_151]

Perhaps the electrician is worried about the increased fault touch voltage on a cirucit wired in 4mm/1.5mm as opposed to one in 2.5mm/1.5mm...

Not an issue as far as BS7671 is concerned.... you meet your 0.4 sec disconnection and thats that, but good enineering practice would be to keep it as low as possible

 

[bernardgreen]

Prob looking for an excuse not to do the job for an interfering customer or maybe doesn't fancy having to chase in deeper boxes or try and get 2 x 4mm cables into sockets that wont take them.

After being told he had lost the job because he was not aware of basic electric theory he phoned the client this morning ( Sunday ) to say that after re-calculating the system he could "legally use" 4mm for the final rings. He seemed to be begging to have the job given back to him.

 

[mikhailfaradayski]

something doesn't add up here.

Who would advise using 4mm T+E for multiple ring circuits?

How many 'rings' are there to be?

Is it really the electrician making these bizarre statements?

Did 'the electrician says' really happen? or would it be more accurate to say 'I have some second hand information, which goes a bit like - The customer, who is an interfering know-all busy-body, says that the electrician had said that......blah... blah.....'?

Under what circumstances would a RFC in 4mm be the best option, considering the available option of using radial circuits and bearing in mind the spirit of reg 314.1?

At what point would the designer consider that if it was benefical to use 4mm T+E, then using radials instead of RFCs would be a better option?

Also, if the customer was really going to be using 7.2kW for any significant period of time, is a RFC really the most appropriate design choice?

If the house really was that power hungry, then is 138W lost to the cabling going to make any difference to the customers overall energy bills? The power dissipated simply turns to heat anyway.

Does the customer intend to tell the plumber how and where he is to install any thermostat? If so, maybe you could point out that the power lost isn't really lost and will simply serve to warm the house very, very slightly, so causing thermostats to shut off marginally earlier?

Considering that a roll of 4mm T+E is about double the cost of 2.5 and it is a large house, i wonder how long the extra material costs and labour time would take to pay itself off from reduced power loss?

It all sounds a bit clumsy to me

I am making the assumption that this is a standard building that does not require anything other than a standard electrical installation, it will have no critical electrical services / life support systems, etc. installed so the 'tolerance to a single cable failure' which i have heard before, is not something which has any importance in this situation.

I would question the statement that the customer has a 'sound electrical knowledge'. I would say 'the customer has an erratic grasp of electrical theory and UK installation regulations'

Did the request for the installation of 4mm instead of 2.5 come before or after the spark had given a quote?

 

[bernardgreen]

something doesn't add up here.

Who would advise using 4mm T+E for multiple ring circuits?

I did, and the client, using basic electrical theory, agreed.

Is it really the electrician making these bizarre statements?

Yes, the electrician said that 4mm was not permitted under the rules for a ring final.

Did 'the electrician says' really happen?

Yes

The customer, who is an interfering know-all busy-body, says that the electrician had said that......blah... blah.....'?

No the client is an intelligent person with experience of renovating buildings, he also has a thorough understanding of electricity. He is also well aware that a single loose connection in a radial presents a risk of overheating and possible fire before the fault is discovered.

Considering that a roll of 4mm T+E is about double the cost of 2.5 and it is a large house, i wonder how long the extra material costs and labour time would take to pay itself off from reduced power loss?

The lengths of the cables from CU to sockets took the calculated voltage drop on 2.5mm very close to the maximum permissable. The "electrician" took this to mean he HAD to use 2.5mm and that to use 4mm would be "illegal".

The main requirement was not cost but reliability and safety.

I would question the statement that the customer has a 'sound electrical knowledge'. I would say 'the customer has an erratic grasp of electrical theory and UK installation regulations'

The answer has been given

Did the request for the installation of 4mm instead of 2.5 come before or after the spark had given a quote?

Before during initial discussions

 

[holmslaw]

..

 

[mikhailfaradayski]

Bernardgreen it seems that, from your posts, that you have been speaking to the customer, not the electrician directly. With the greatest of respect, how can you be certain of exactly who said what? Don't you feel as though there may be room here for a little bit of chinese whispers?
It does seem a tad odd that any practicing electrician would say oversizing a cable was illegal.

Could it not have been the case that the electrician said 'a RFC wired in 4mm is not referred to in the regs and on the whole would be un-necessary' and then the customer modified the story slightly when relaying it to you?

Did 'the electrician says' really happen?

Yes

see above

The customer, who is an interfering know-all busy-body, says that the electrician had said that......blah... blah.....'?

No the client is an intelligent person with experience of renovating buildings, he also has a thorough understanding of electricity.

If he is accustomed to renovating properties for resale and a profit, i would imagine he would be smart enough to do a break even analysis of costs based on increased material costs against energy lost to cabling. How many years do you think it would take for the installation of 4mm to become cost effective?

He is also well aware that a single loose connection in a radial presents a risk of overheating and possible fire before the fault is discovered.

He should seek the services of an electrician who can make terminations correctly, then have the installation inspected at reasonable intervals, then the customer would not need to worry about such a scenario.

Considering that a roll of 4mm T+E is about double the cost of 2.5 and it is a large house, i wonder how long the extra material costs and labour time would take to pay itself off from reduced power loss?

The lengths of the cables from CU to sockets took the calculated voltage drop on 2.5mm very close to the maximum permissable. The "electrician" took this to mean he HAD to use 2.5mm and that to use 4mm would be "illegal".

If that were the case, then i would suggest that the installation of that RFC was a bad idea anyway, maybe reducing the size of the ring and installing additional circuits would be more appropriate to comply with 314.1. IMO the idea behind the floor area limitations on final circuits mentioned in the OSG is mainly to negate the need for full circuit calculations.

Does the customer know the maximum permissable volt drop for any given circuit?

What floor area is to be served by this RFC? Does the customer understand why there is a notional value for maximum floor area served?

The main requirement was not cost but reliability and safety.

ah, sorry, its just that your response to pjcomp earlier in this thread would suggest that the main consideration was focussed on the cost of lost energy, at that point there was no mention of any safety related issues.

I would question the statement that the customer has a 'sound electrical knowledge'. I would say 'the customer has an erratic grasp of electrical theory and UK installation regulations'

The answer has been given

Did the request for the installation of 4mm instead of 2.5 come before or after the spark had given a quote?

Before during initial discussions

Just out of curiosity, how big s this house?
What do you think is a realistic level of power usage? You mention 138W dissipated when the circuit is running at 30A.
What if a more realistic average value of 6A were to be used? Would the customer still feel there is benefit to increasing his material costs?

 

[bernardgreen]

The client requested that 4mm was used.

The "electrician" said 4mm would contravene the regulations so he could not use it.

The first electrician was wrong,

The second electrician agree that the client was not wrong in having 4mm for the rings.

It does seem a tad odd that any practicing electrician would say oversizing a cable was illegal.

Not if the "electrician" had been taught to follow the rules and calculations without any exception.

The house is long and narrow and the CU is at one end and the kitchen and utility room at the other end. From the picture I estimate about 40 feet from CU to those rooms.

 

[ban-all-sheds]

Not if the "electrician" had been taught to follow the rules and calculations without any exception.

He would never have been taught that you weren't allowed to use a cable which was bigger than the minimum required (subject to it fitting, of course), only that you weren't allowed to use a smaller one.

He said what he did because he doesn't understand what he's doing.

Your quote marks are apposite - the guy is not an electrician, he's just pretending to be one. He probably genuinely thinks he is one. But on that as on other things he is wrong.

 

[mikhailfaradayski]

Do you think that the customer has done a break-even analysis of the costs in relation to the additional material cost against the cost of lost energy?

I would think that he should have done, given his apparent advanced knowledge of electricity.

I wonder how many years his 4mm option will take to pay for itself?