Applying Diversity and Calculate the
Current Carrying Capacity and Voltage Drop for Cables and Flexible Cords
Applying diversity
1. Determine the premises; what type of premises are you installing the circuit in.
2. Calculate the design current: Ib applying the diversity from the table given in Appendix A – Table A2
Ib = Power (watts) / Circuit Voltage (allow for diversity)
Current Carrying Capacity of Cables
1. Establish the installation method; find this on page 66 of the on-site guide.
2. Use the design current allowing for diversity, the design current of the circuit is (Ib):
3. Select the size of the protective device (In) this will be either a semi enclosed fuse or MCB, they come in rated amp values of:
6, 10, 16, 20, 32, 40, 45, 50
The size of the protective device (In) must be greater than (Ib) but should not exceed it by more than 45%
4. The relevant factors are to be applied to calculate It(current carrying capacity of the cable)
It = In
Ca x cg x Ci x Cf
Where:
It = the tabulated value of circuit current
In = the rated current or the current setting of the protective device
Ca = the rating of ambient temperature ref: OSG Table F1 pg 140
Cg = the rating for grouping factor ref: OSG Table F3 pg. 142
Ci = the rating factor if the cable is surrounded in thermal insulation ref: OSG Table F2 pg. 141
Cf = the rating factor for semi enclosed fuse BS 3036 (0.725)
5. When finding the cable size ( C.S.A ) from the OSG you need two properties:
a. Tabulated current
b. Method of installation and type of cable
6. Once It has been calculated the size of the cable is to be selected form Tables F4 to F6 (On site Guide). The type of cable and installation method will determine which column is to be used, scroll down that particular column to find the cable size that will carry the tabulated current.
Voltage drop calculations
Voltage drop = mV x Ib x length
1000
1. To determine the voltage drop you need:
a. Cable type and method of installation
b. Cable CSA
This will determine the mV
Where:
Mv = look at table F4 (ii), F5 9ii) and table F6. The type of cable and installation method will determine which column is to be used, scroll down that particular column to find the mV for the established cable size.
Ib = the design current of the installation ( allowing for diversity)
Length = the length of the circuit in meters
(the division by 1000 gives the answer in volts)
Maximum voltage drop
3% for lighting circuits
5% for all other circuits
Compare calculated voltage drop, if calculated voltage drop is greater the maximum allowed, and then the cable size must be increased to accommodate the voltage drop.
Question 1: Given the following data, calculate the minimum cable size:
In a shop you are to install 2 cookers on a radial circuit:
Cooker 1 has a 6kW load at 230 V.
Cooker 2 has 3kw load at 230 V
The cables are 700C thermoplastic insulated and sheathed flat cable with protective conductor (copper conductors), installed in a stud wall with thermal insulation with the cable not touching the inner surface
The ambient temperature is 35 0C and the circuit is not grouped with other circuits. The circuit protection is provided by a semi-enclosed fuse.
The length of run is 45 m with permitted Voltage drop as 5%
Calculate the minimum size cable that is required for this installation
First apply diversity: appendix A
100% of the first appliance + 80% of the second. 26a + 10.4a = 36Amps = Ib
Find the installation method page 66 - Ref method 103
Apply the cable calculation: In = > Ib therefore - 40 amp semi enclosed fuse.
Calculate the tabulated current use appendix F:
It = In /(Ca Cg Ci Cf) 40 / 0.94 x 1 x 1 x 0.725 = 40 / 0.68 = 58.82amps
Using- It, the installation method and table F6 find the cable CSA
Cable = 16mm
Using the cable size and ref method calculate the voltage drop:
Using table F6 = (2.8 x 36 x 45) / 1000 = 4.5volts this is within 5 % of 230 = 11.5v
Try these:
Question 2: Given the following data, calculate the minimum cable size:
A ring circuit in a hotel at 230 V. The cables are thermoplastic 70 0C insulated cable, non armoured , clipped direct and run for a length of 16 m. Over current protection is provided by a type B BS EN 60898 circuit breaker.
The ambient temperature is 30 0C and the circuit is grouped with 4 other of the same/similar size. The circuit runs through thermal insulation of 300mm
The voltage drop maximum should be taken as 5%
Question 3: Given the following data, calculate the minimum cable size and MCB rating for the circuits:
You are to install 3 lighting circuits in a domestic property:
a. 10 lamps on circuit 1 at 20 meters
b. 20 lamps on circuit 2 at 30 meters
c. 17 lamps on circuit 3 at 15 meters
The cables are multi - core 700C thermoplastic insulated cable, non-armoured, run through conduit in a stud will for the given length.
The ambient temperature is 300C the over current device is a semi enclosed fuse.
The voltage drop maximum should be taken as 5%
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