Is Your Selected Cable Suitable?

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Manator

©Honorary Essex Boy™
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I have started this thread as a result of a question asked elsewhere on this forum. I have placed it here because I think most will gaain from it.

If you are doing the design for a new circuit it is relatively easy to calculate what size cable to install, I am not going to go into detail about the calculations but instead point out where and why other influences affect the cable of choice.

Why do we need to calculate the cable size? One of the main reasons to calculate any cable size it to ensure under normal use it is suitable for the loading. This can be measured as kw or amps, so first of all you need to know what load is to be supplied. Another important requirement is the voltage, all calculations these days are based on single phase 230v or three phase 400v.

We now know the voltage and the amps or kw loading. The next important factor is distance from supply to the load, if its a new circuit this can be measured directly, if it is already installed a simple continuity test could be used with the tables supplied in the regulations to calculate the actual or approximate length. The next factor for consideration is the allowed volt drop, this could be 3% or 5% dependant on what is supplied.

It is possible to calculate a cable size given the above information, however other factors would greatly effect the initial choice.

There are tables given in the regulations as to how and why these factors are used.

Example:

A  8.5kw shower unit is to be installed 10m from the distribution board, the supply is single phase at 230v

The cable leaves the distribution board on its own and is clipped directly, passing through no insulation.

The allowed volt drop is 5%

A simple calculation would show that a 4mm cable would be suitable for this application.

In real life this just would not happen though, many showers are installed on the top floor where attic ambient temperatures could exceed 50 degrees during the summer months. Many cables are grouped for part or all of their length and nowadays almost all will be in contact with insulating material in walls and in roof voids.

So how do these factors effect the cable choice.

The current carrying capacity of all cables are effected, the above example cable size could quickly change from 4mm to 10mm or more dependant on how and where it is installed. Remember we are trying to size the cable so that it is suitable for use and these factors are important in making our choice.

Grouping:

Almost all cables are grouped somewhere the most common place is when leaving the consumer unit, they could be on cable tray or basket or through holes drilled through joists. It is possible to size down a cable size if you run it separately from all other cables, this method is often used for large loads in industrial and commercial premises when space is available for doing so. Cables grouped together will effect the ability of any grouped cable to disperse any heat generated within that grouping.

Insulation:

Most modern houses are highly insulated these days, and it is possible to find insulation in walls and floors as well as the roof voids.

Insulation would hinder the cables ability to disperse heat, under large loads this could mean that a cable could overheat and thus damage the electrical insulation leading to failure of the cable or nuisance tripping of any protection devise fitted.

Ambient temperature:

This is often overlooked during calculations or a standard temp is used, in the example above I showed how a roof void could reach over 50 degrees during the summer months, this would probably pre heat the cable for the shower before any current is passed through it.

Installation methods:

The regulations give various codes for different installation methods however almost all cables are installed using various methods along its route. As a rule of thumb the when filling in any certificate would be to use the method that best describes the majority of the route. This does not mean that correction factors are not used.

Again take the example given above the cable is clipped direct for 6m of its run, the last 4m is under 200mm of insulation. The 4 meters would be subject to a correction factor, and not forgetting the ambient temp in the loft.

Once we have found a suitable cable that performs as it should, we then need to protect the cable from any damage caused or any faults that can occur. A breaker MCB, RCD or RCBO should be sized according to the correction factors for the cable and protect the cable from damage due to over heating for example. If the cable has a maximum carrying capacity of 20amps it would be no good fitting a 40amp protective devise, the cable would probably melt before the devise disconnected the load.

When sizing the protective devise we can use diversity and the test curve features of the devise. This is done at the design stage and can allow the devise to be fully utilised based on its curve characteristics. This would allow the cable to run at its maximum limit for a limited length of time providing no damage is imparted on the cable. The devise will only trip when that point on its curve is reached.

Experience teaches us that whilst it is simple to calculate a cable size.very often basics are forgotten or overlooked.

Disclaimer:

I have wrote this off the top of my head on a hangover Sunday morning. I may have missed something or omitted the obvious and anyone can correct me if a mistake has been made.

I have not used a calculator for sizing any cables in this text so even that may be wrong, I advise all who read this to study the tables and do their own cable calculations.

 
grouping always gets me - the number of times I've looked into trunking runs that are jammed with cables and people have been chucking new circuits into over time  :shakehead

 
Thanks Adrian, I have lost count of the times I have met other sparks who believe all they learned at college does not mean anything in the real world. Calculations are important especially if you are designing the circuit. It is also vital in understanding why it has been installed by others.

A designer may use a lower rated cable for a certain use because he has carefully assessed the diversity and calculated that the cable would suffice given the circumstances of use. To be fair in most domestic installations the best option is to give a fair amount of headroom. In other words go larger than required. In industrial situations its all down to costs, and getting a cable to do what is required for the minimum amount of outlay.

 
Just to add to what Manator just wrote; I have lost could of the number of idiots that have said to me; [in a none electrical field though]

"bits of paper do not mean anything, it is whether you can do the job that counts"

I would re-phrase that, to state what they REALLY mean; " I have not actually passed any exams and so you have "touched a nerve"

You can have all the experience you like; You will STILL be complete rubbish, dangerous in fact, unless you ALSO possess the "underpinning knowledge" [to use a modern term!] to know WHY you are doing what you are doing.

Anyone that believes "all they learned at college does not mean anything in the real world" might as well not have gone in the first place. They are just as dangerous as 5WW's more so in fact, BECAUSE THEY TRULY BELIEVE THAT THEY KNOW WHAT THEY ARE DOING.

Only thing i would say is though Manator, where you say;

"A breaker MCB, RCD or RCBO should be sized according to the correction factors for the cable and protect the cable from damage due to over heating for example"

Do you not mean that the OCPD [MCB etc] is selected according to the load, and then the correction factors are applied to the CABLE in order to arrive at the correct size.. [of cable] and not the other way about...

Good post manator...

john...

 
Good post John, and a very good informed question.The cpd and cable should be calculated hand in hand. It is of no use selecting a cable to find that the cpd will not operate as expected.The size of the MCB, RCD or RCBO has to be used at the start of calculations, this would give you negative results when using cables not suitable.

Say we choose a 32amp type B breaker, we calculate the cable given all the de-rating values, the load, and the volt drop and distance. We still need to know if the breaker will break the load under any fault or overload. Sometimes cables are installed given all the factors and still fail because the cpd is not sized accordingly.

In my example I think the load is around 37amps, so the cpd would be chosen as 40amp. The cable must be able to withstand any extremes until the cpd disconnects from service. 4mm could do this but would you want to push a cable to its limits on a family home on a daily basis?

 
Last edited by a moderator:
Supply voltage = Volts

Ib - design current = Amps

Protective device type = BS

In - protective device rating = Amps

Length of run of cable = Metres

Power factor =

Cable Type :

Installation Method :

Ambient Temperature =

Number of circuits including this one =

Length of cable in thermal insulation =

Apply Correction factors:

From TABLE 4C1 : Cg =

From TABLE 4B1 : Ca =

From TABLE 52.2 : Ci =

Protective device factor : Cc =

It = tabulated current carrying capacity

It = In / (Cg x Ci x Ca x Cc)

It = / ( )

It = Amps

From TABLE Cable selected = mm²

TABLE For mm² ; mV/A/m =

mV/A/m corrected for power factor = mV/A/m x Power Factor =

Volt drop = (mV/A/m x Length x Design current) / 1000

Volt drop = ( ) / 1000

Volt drop = Volts

(Maximum permissible volt drop (regulation - 525) = Volts)

Calculated Cable size = mm², Minimum Earth conductor size = mm² (Table )

Maximum Cable Length = Metres

I have nicked this from another post a while back (Dont ask Me which, I have a hard enough time remembering what day of the week it is these days)

I would also say, once you have done your cable calc, run it through the adiabatic equation.

Andy

 
I think you copied a post I did on the IET forums, I was trying to show a young upcoming star how to calculate his belt size. Just before he stood before a parliamentary sub committee.

 
I have just done as Manator has suggested as well as Andy and looked

at the influence of ambient temp plus grouping.  Even before looking at

the 4 metre of 200mm thick insulation the outcome was a considerable

increase in cable size over the 4mm intially stated.  8kW for a shower is

considered puny by some and a thorough examination of the intended

run for the cable may reveal that the rating of the shower as the maximum

allowable may be less than the customer's expectations.

 

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