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Two high-power appliances on a single 40A RCD
Former Community Member
I have an electric shower installed on a 40A RCD, in a room adjacent to my kitchen. The shower is only used in an emergency - i.e. when our gas boiler is unable to provide hot water to our main bathroom. I would like to take a spur from this 40A connection to use for a new double oven, which is rated at 32A. Can anyone advise on a safe and legal way to do this, ensuring that only one of the two appliances can be connected at any one time?
Consider carefully the OP and the real circuit. It has a 40A CPD and if installed correctly a 10mm2 cable. But lets assume it has a 6mm cable buried in a wall or somewhere clipped direct. The shower is obviously satisfactory and probably takes 37A. The oven could potentially take another 16A or so and you are screaming (or actually Alcomax) that the cable could melt. Could it? A 40 A breaker actually trips quickly at at least 52A so our customer should never get a trip. The circuit has 3 loads with diversity, the shower probably 15 mins max then nothing (it is used occasionally as a gas standby) and 2 ovens of perhaps 4kW total maximum. The ovens heat up then are on and off for a few minutes at a time. The cable will never reach 70C conductor temperature, in fact have you ever seen a melted PVC cable in the fixed installation? So you have a simple diversity problem and there is no particular danger. The risk of real cable melting needs overcurrent operation at 2x rating for a long time, and that is hours. I do understand that diversity is poorly understood but giving advice is tricky and there are reasonable limits to what one should say. "Its not allowed by the regs" requires solid proof and understanding of the whole BYB, in many ways it is not absolute in the way that many think. This is particularly true of intermittent loads and diversity, and also CPD ratings and circuit configurations. The danger is almost all from long term fairly small overloads, which is not true in this instance.
By example take the cables in your street. They are probably protected by a 600A fuse and feed many houses. You may have a 10mm2 service if you have an old house (I used to some years back). A short on the service cable can be quite spectacular because the short circuit rating is quite suspect at a 5kA fault. If each house takes power and it exceeds about 1kA for a reasonable period the 600A fuse will blow, but it takes time. The street cable will not really be warm. If they all have off peak heating and 800A flows all night (not unusal) the cable will be pretty hot by morning. Is our little used shower a problem, very unlikely.
I'm afraid things are not quite as simple as the OSG.
Consider carefully the OP and the real circuit. It has a 40A CPD and if installed correctly a 10mm2 cable. But lets assume it has a 6mm cable buried in a wall or somewhere clipped direct. The shower is obviously satisfactory and probably takes 37A. The oven could potentially take another 16A or so and you are screaming (or actually Alcomax) that the cable could melt. Could it? A 40 A breaker actually trips quickly at at least 52A so our customer should never get a trip. The circuit has 3 loads with diversity, the shower probably 15 mins max then nothing (it is used occasionally as a gas standby) and 2 ovens of perhaps 4kW total maximum. The ovens heat up then are on and off for a few minutes at a time. The cable will never reach 70C conductor temperature, in fact have you ever seen a melted PVC cable in the fixed installation? So you have a simple diversity problem and there is no particular danger. The risk of real cable melting needs overcurrent operation at 2x rating for a long time, and that is hours. I do understand that diversity is poorly understood but giving advice is tricky and there are reasonable limits to what one should say. "Its not allowed by the regs" requires solid proof and understanding of the whole BYB, in many ways it is not absolute in the way that many think. This is particularly true of intermittent loads and diversity, and also CPD ratings and circuit configurations. The danger is almost all from long term fairly small overloads, which is not true in this instance.
By example take the cables in your street. They are probably protected by a 600A fuse and feed many houses. You may have a 10mm2 service if you have an old house (I used to some years back). A short on the service cable can be quite spectacular because the short circuit rating is quite suspect at a 5kA fault. If each house takes power and it exceeds about 1kA for a reasonable period the 600A fuse will blow, but it takes time. The street cable will not really be warm. If they all have off peak heating and 800A flows all night (not unusal) the cable will be pretty hot by morning. Is our little used shower a problem, very unlikely.
I'm afraid things are not quite as simple as the OSG.
