Current carrying capacity of selected cable and correction factors.
Im looking at page 115 of the OSG Appendix 6 and im a little confused.
It says that the overcurrent device will be providing both fault and overcurrent protection. Is this referring to the MCB, the RCD or both? I assume it’s the mcb or the fuse.
Now it says the design current Lb must be established (ive covered this in another post). The overcurrent device rating Ln has got to be greater or equal to Lb. I understand this.
Then it says the current carrying capacity of the cable Lt is given by:
Lt is greater or equal to Ln / (Ca Ci Cg Cr)
Ca is correction factor for ambient temperature. Tables 6A1, 6A2
Im a little confused on this, can anybody help? Do people actually take temperatures into account? Do you have to go around with a thermometer and measure the temperature in a house and work it into your calculations? Then there is the difference in the tables, one is for protection against short-circuit and overload, and the other is when the overload protection device is a semi enclosed fuse to BS 3036. Im not sure how to know which table to use?
Ci is for thermal insulation. If a cable comes into contact with thermal insulation for more than 500mm it is degraded by 50%. What exactly does this mean? If a 1mm cable is used for lights, does that mean that if it runs through 500mm of thermal insulation you should run at least 2.5mm cable because you cant get 2mm? Or is this just used the the calculation for the calculation for the cable size as above? (on a side note, ive been told by a few NIC members that people don’t bother with this or any of these similar calculations and just use the standard cables always used for the job) If a cable is to be run through thermal insulation can you just run the cable in the cheap pvc capping/trunking and ignore the thermal insulation because the cable is not in contact with it?
Cg is for grouping. This again is a little confusing. As an example, if a rung circuit was run through joists, under floor boards and up the walls to the sockets, surely some of the cables would be touching and some wouldn’t. This would also be the same for the lighting circuit. Some would also be in the plastic capping what you put behind the plaster. What would I use from table 6C on page 118 of the OSG?
Cr is the correction factor 0.725 for semi enclosed fuses to bs 3036. im totally lost on this one?
Can somebody help me out on the above, and also give me a real life example of a situation and with the calculations worked out.
Im looking at page 115 of the OSG Appendix 6 and im a little confused.
It says that the overcurrent device will be providing both fault and overcurrent protection. Is this referring to the MCB, the RCD or both? I assume it’s the mcb or the fuse.
Now it says the design current Lb must be established (ive covered this in another post). The overcurrent device rating Ln has got to be greater or equal to Lb. I understand this.
Then it says the current carrying capacity of the cable Lt is given by:
Lt is greater or equal to Ln / (Ca Ci Cg Cr)
Ca is correction factor for ambient temperature. Tables 6A1, 6A2
Im a little confused on this, can anybody help? Do people actually take temperatures into account? Do you have to go around with a thermometer and measure the temperature in a house and work it into your calculations? Then there is the difference in the tables, one is for protection against short-circuit and overload, and the other is when the overload protection device is a semi enclosed fuse to BS 3036. Im not sure how to know which table to use?
Ci is for thermal insulation. If a cable comes into contact with thermal insulation for more than 500mm it is degraded by 50%. What exactly does this mean? If a 1mm cable is used for lights, does that mean that if it runs through 500mm of thermal insulation you should run at least 2.5mm cable because you cant get 2mm? Or is this just used the the calculation for the calculation for the cable size as above? (on a side note, ive been told by a few NIC members that people don’t bother with this or any of these similar calculations and just use the standard cables always used for the job) If a cable is to be run through thermal insulation can you just run the cable in the cheap pvc capping/trunking and ignore the thermal insulation because the cable is not in contact with it?
Cg is for grouping. This again is a little confusing. As an example, if a rung circuit was run through joists, under floor boards and up the walls to the sockets, surely some of the cables would be touching and some wouldn’t. This would also be the same for the lighting circuit. Some would also be in the plastic capping what you put behind the plaster. What would I use from table 6C on page 118 of the OSG?
Cr is the correction factor 0.725 for semi enclosed fuses to bs 3036. im totally lost on this one?
Can somebody help me out on the above, and also give me a real life example of a situation and with the calculations worked out.
