Sunday, January 15, 2012

Brew Kettle Modification

Brew Kettle with  2 x 2kW elements
I recently upgraded all my equipment with the move to all grain brewing but it didn't take me long to notice the areas of improvement. The most obvious and simple upgrade is to add a second heating element to the brew kettle. While that was simple, I found the investigation to identify the maximum amount of power that can be drawn from a double socket more challenging. I noticed a huge variance in comments across the internet with a lot of people making the assumption that if you have a double socket then, in theory, it should be able to power two 13A appliances at the same time. I found this to be a fair, albeit inaccurate, assumption to make and one would hope that the electrician had chosen the appropriate gauge wiring for such a scenario. 

Breaker switches for kitchen sockets
I decided in the end that there was too much speculation and investigated my circuit breakers. Thankfully they are very well labelled so there is no random manual tripping of switches to see what is what. The kitchen has two double sockets and after manually tripping the switches in the breaker box confirmed that they were wired to separate breaker switches. 
A quick look at the manufacturers datasheet confirmed that the 'B20' on the breakers represented 20A so this implies that it can in theory provide 4.6kW of power (20A*230V) before tripping. Each element in the kettle is rated at 2kW so both of them could feasibly run on a double socket provided no other appliance shared that link. 
Unsurprisingly the other appliances in the kitchen also shared these links, it is just all hidden in the walls and presses. The fridge, microwave, extractor fan and washing machine are spread between the two lines.  

Current Derating Curve for 'B Type' 20A MCB

On a typical brew day, the fridge and extractor fan are definitely going to be on and while both of them have relatively low power consumption, they both have very high inrush currents when starting. Both units can use up to 1kW for a few seconds when cutting in. This might only happen once with the extractor fan but it is going to be happening several times an hour with the fridge. To see how the breaker deals with these temporary surges, one must consult the manufacturers datasheet again. Unlike a current rating which is 20A, we are looking for a 'derating curve' which will give an indication as to how the breaker performs under loads greater than its specified range. 

We will assume that the 2 kettle elements, the fridge and the extractor fan are all being powered from the same breaker switch so this would give the following nominal power consumption:

Kettle Element 1: 2kW
Kettle Element 2: 2kW
Extractor Fan: ~300W
Fridge: ~120W

This gives a total of 4.42kW during periods when all of them are running which is within the range of the breaker. Even if the fridge and extractor fan both turned on at the exact same time, the most power that would be drawn wouldn't be much more than 6 or 7kW for a few seconds. Taking the worst case scenario into account, 7kW (~31A) is almost double the rating of the breaker so we will round it up to 40Amps. Now look at  the derating curve, 40A is a multiple of two of IN (Nominal Current). Tracing this value into the curve shows that the breaker can handle 40A for a time between 10 to 40 seconds. The inrush current situation would only last for a few seconds so the breaker is well capable of dealing with such an occurrence. I found this on a forum which is a reference to the rating of a double socket. 
BS1363-2:1995 requires for double socket outlets that both socket outlets have loads applied via test plugs, 1 test plug having a load of 14 amps whilst the other has a load of 6 amps, making a total load of 20 amps on the cable supplying the double socket outlet. The double socket outlet is then subjected to this loading for a minimum continuous period of 4 hours or longer until stability is reached with a maximum duration of 8 hours (stability being taken as less than 1 degC rise within 1 h). The test is passed if neither the terminals / terminations, nor the accessible external surface, increase in temperature by more than 52 degC.
So some sockets could get as hot as 70degrees C (20deg ambient + 52deg) and pass. A double socket is capable of safely driving a dual element brew kettle but personally I'm not happy putting that much power through one cable for 4 or 5 hours on a brew day. I will split the kettle elements between the 2 double sockets in the kitchen and be comfortable in the knowledge that it is well within the ratings in my apartment wiring infrastructure.

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