1
1livewire
hi
I have a question, it's a bit of a long one i'm afraid! It's from a book, and it's been niggling me for a while. Ok,
An example of circuit design
A consumer lives in a bungalow with a detached garage and work-shop.
The building method is traditional brick and timber.
The mains intake position is at high level and comprises an 80A BS
1361 230V main fuse, an 80A rated meter and a six-way 80A consumer
unit housing BS EN 60898 Type B circuit breakers as follows:
__________________________________
Ring circuit ............................ 32A
Lighting circuit ........................6A
Immersion heater circuit ...........16A
Cooker circuit ..........................32A
Shower circuit .........................32A
Spare way ...............................--
__________________________________
The cooker is rated at 30A, with no socket in the cooker unit. The
main tails are 16mm^2 double insulated PVC, with a 6.0mm^2 earthing
conductor. There is no main protective equipotential bonding. The
earthing system is TN-S, with an external loop impedance Ze of 0.3 ohms.
The PSCC at the origin has been measured as 800A. The roof space is
insulated to full depth of the ceiling joists and the temperature in
the roof space is not expected to exceed 35^0 C.
The consumer wishes to convert the workshop into a pottery room and
install an 8.6kW/230V electric kiln. The design procedure is as follows.
Assesment of general characteristics
The present maximum demand, applying diversity, is
-----------------------------------------------
Ring circuit ..............................32A
Lighting (66% of 6A) ................3.96A
Immersion heater circuit ............16A
Cooker (10A+30% of 20A) ........16A
Shower circuit ..........................32A
Total ......................................100A
-------------------------------------------------
Reference to the current rating tables in the IEE Regulations will
show that the existing main tails are too small and should be uprated.
So, the addition of another 8.6kW of load is not possible with the
present arrangement.
The current taken by the kiln is 8600/230 = 37.4A. Therefore, the new
maximum demand is 100 + 37.4 = 137.4A.
Supply details are single-phase 230V, 50Hz. Earthing: TN-S, PSCC at origin
(measured): 800A. Decisions must now be made as to the type of cable, the
installation method and the type of protective device. As the existing
arrangment is not satisfactory, the supply authority must be informed of
the new maximum demand, as a larger main fuse and service cable may be
required.
Sizing the main tails
1. The new load on the existing consumer unit will be 137.4A. From
the IEE regulations, the cable size is 25.0mm^2.
2. The earthing conductor size, from the IEE regulations, will be
16.0mm^2. The main protective bonding conductor size, from the IEE
regulations, will be 10.mm^2.
Some questions then,
1. When the author refers to: "The present maximum demand, applying diversity"
Is the author is refering to the design current Ib?
Ib < In < Iz
2. When the author refers: "The new load on the existing consumer unit will be 137.4A.
From the IEE regulations, the cable size is 25.0mm^2."
I can't find this value? When I look at table 4D2A p276 of the regs, I find
Reference method C, column 6, 138A (nearest value), 35mm^2.
thought; We don't apply a factor of 0.8 to the above do we? something about
measured values v operating temperatures?
help appreciated
book source: Scadden, B. Electric Wiring: Domestic, 2008
I have a question, it's a bit of a long one i'm afraid! It's from a book, and it's been niggling me for a while. Ok,
An example of circuit design
A consumer lives in a bungalow with a detached garage and work-shop.
The building method is traditional brick and timber.
The mains intake position is at high level and comprises an 80A BS
1361 230V main fuse, an 80A rated meter and a six-way 80A consumer
unit housing BS EN 60898 Type B circuit breakers as follows:
__________________________________
Ring circuit ............................ 32A
Lighting circuit ........................6A
Immersion heater circuit ...........16A
Cooker circuit ..........................32A
Shower circuit .........................32A
Spare way ...............................--
__________________________________
The cooker is rated at 30A, with no socket in the cooker unit. The
main tails are 16mm^2 double insulated PVC, with a 6.0mm^2 earthing
conductor. There is no main protective equipotential bonding. The
earthing system is TN-S, with an external loop impedance Ze of 0.3 ohms.
The PSCC at the origin has been measured as 800A. The roof space is
insulated to full depth of the ceiling joists and the temperature in
the roof space is not expected to exceed 35^0 C.
The consumer wishes to convert the workshop into a pottery room and
install an 8.6kW/230V electric kiln. The design procedure is as follows.
Assesment of general characteristics
The present maximum demand, applying diversity, is
-----------------------------------------------
Ring circuit ..............................32A
Lighting (66% of 6A) ................3.96A
Immersion heater circuit ............16A
Cooker (10A+30% of 20A) ........16A
Shower circuit ..........................32A
Total ......................................100A
-------------------------------------------------
Reference to the current rating tables in the IEE Regulations will
show that the existing main tails are too small and should be uprated.
So, the addition of another 8.6kW of load is not possible with the
present arrangement.
The current taken by the kiln is 8600/230 = 37.4A. Therefore, the new
maximum demand is 100 + 37.4 = 137.4A.
Supply details are single-phase 230V, 50Hz. Earthing: TN-S, PSCC at origin
(measured): 800A. Decisions must now be made as to the type of cable, the
installation method and the type of protective device. As the existing
arrangment is not satisfactory, the supply authority must be informed of
the new maximum demand, as a larger main fuse and service cable may be
required.
Sizing the main tails
1. The new load on the existing consumer unit will be 137.4A. From
the IEE regulations, the cable size is 25.0mm^2.
2. The earthing conductor size, from the IEE regulations, will be
16.0mm^2. The main protective bonding conductor size, from the IEE
regulations, will be 10.mm^2.
Some questions then,
1. When the author refers to: "The present maximum demand, applying diversity"
Is the author is refering to the design current Ib?
Ib < In < Iz
2. When the author refers: "The new load on the existing consumer unit will be 137.4A.
From the IEE regulations, the cable size is 25.0mm^2."
I can't find this value? When I look at table 4D2A p276 of the regs, I find
Reference method C, column 6, 138A (nearest value), 35mm^2.
thought; We don't apply a factor of 0.8 to the above do we? something about
measured values v operating temperatures?
help appreciated
book source: Scadden, B. Electric Wiring: Domestic, 2008
