Amps/kilowatts use on radial

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Old 06-12-08, 03:07 AM
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Amps/kilowatts use on radial

During recent works our electrician noticed that half of the power points in our house are on a radial. The radial was originally on a 32amp fuse and for safety reasons the electrician has fitted a 20amp fuse, However, he pointed out that anything in excess of 20amps could trip the fuse. I notice the socket to our computer has a 13amp fuse. Do we need to avoid using sockets that are in excess of 20amps, or does it relate more to the number of kilowatts used by each electrical item at a time? We have had works carried out and are about to move back in to the part of the house where the radial is.
 
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Old 06-12-08, 05:18 AM
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Welcome to the forums! Where are you located? What type breaker box do you have, and are you working off 120 AC 60 Hz or other voltage?
 
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Old 06-12-08, 01:41 PM
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From the description of 'radial', 32A fuse and 13A fused plug, this is probably UK wiring, at 220V and 50Hz.

Common circuits in the UK are 'ring' circuits, where the circuit leaves the OCPD (fuse or breaker) panel, loops around to various loads, and then goes back to the OCPD panel. In essence, from each load point, there are _two_ paths back to the panel, and these two paths are effectively in parallel between the load and the panel.

When installed as a ring, it is quite common for the circuit protection to be greater than the amp rating of the individual conductors, with the assumption being that some of the current would follow each path. While this sort of installation is legal and code compliant in the UK, it is a code violation under US rules.

To answer the original question, the rating of a socket or fused receptacle is the amount of current that the individual unit is capable of safely supplying. This does not mean that the receptacle _must_ supply that amount; it is simply a maximum. If you were to connect a single 1A lamp to that receptacle, than all that would be used is 1A.

In order to avoid blowing the 20A fuse, you much use less than a total of 20A, for _all_ the loads connected to that radial circuit. If you have a bunch of 13A receptacles, all with 1A loads on them, then you will be fine. But if you have 2 13A receptacles, each with 12A of load, then the circuit as a whole will be overloaded.

If you know the watt ratings of connected devices, you can figure the approximate amp loads by taking watts/voltage. So a 220V 2500W kettle would require approximately 11A, and a 20A 220V circuit can supply approximately 4400W. (There are details that I am intentionally not going into; these are just approximations.)

-Jon
 
 

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