Manator
©Honorary Essex Boy™
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.
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.
