2/3 ratio with ring circuits
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Ring circuits are not commonly used in the US for less than MV designs (conjecture).
The concept is that each conductor being a complete loop (i.e. both starts and ends at the circuit's source), the load current is split between the two directions which the current travels. This permits using a smaller conductor size than that of a radial circuit (i.e. conductors connected to the circuit's source at only one end).
Sizing of a ring circuit ocp would be dependent on the subjected standard. I am not immediately aware of any such allowance in the NEC.
Last edited:
broadgage
Senior Member
- Location
- London, England
Here in the UK "ring final circuits " are commonly used for 230/240 volt domestic appliances in homes and similar places.
This would not be allowed in the USA as it violates the Code article that prohibits small conductors in paralell.
A UK ring circuit normally consists of a ring or loop of 2.5mm cable (similar to #14) both ends of the ring are connected to the same 32amp breaker, thus giving a total capacity of about 7KW.
The 2.5mm cable has a rating (dependant on conditions of installation) of from 20 amps to 27 amps.
It will be seen the that the OCPD has a greater rating than the cable, this being allowed because the current divides between the two ends of the ring, the OCPD must NOT be twice the cable rating since the current is unlikely to divide equaly.
The plugs fitted to appliances are fitted with small fuses normally of 3 amps or 13 amps.
Any number of outlets may be connected to a ring circuit, but the floor area served should not exceed 100 square metres.
Most homes have 2 ring circuits, though a larger or better specified home would have more than this.
This would not be allowed in the USA as it violates the Code article that prohibits small conductors in paralell.
A UK ring circuit normally consists of a ring or loop of 2.5mm cable (similar to #14) both ends of the ring are connected to the same 32amp breaker, thus giving a total capacity of about 7KW.
The 2.5mm cable has a rating (dependant on conditions of installation) of from 20 amps to 27 amps.
It will be seen the that the OCPD has a greater rating than the cable, this being allowed because the current divides between the two ends of the ring, the OCPD must NOT be twice the cable rating since the current is unlikely to divide equaly.
The plugs fitted to appliances are fitted with small fuses normally of 3 amps or 13 amps.
Any number of outlets may be connected to a ring circuit, but the floor area served should not exceed 100 square metres.
Most homes have 2 ring circuits, though a larger or better specified home would have more than this.
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