How to Transpose the Power Formula to Calculate Design Current

[@michaelwatkins6820]

From 06:25 is what I've found to be the best way to simplify transposing formulae. Now it all makes sense, now I can look at the questions and not get a headache. Thanks Joe 👍

[@tedlahm5740]

Thank you Joe. Always clear and concise.

[@JoeRobinsonTraining]

Thanks Ted!

[@MattWhitehead80]

Amazingly useful video. Just what I've been looking for.

[@beardedsparks2825]

Agree 100% on the triangle. I've seen learners trying to squeeze every formula into it. I don't like giving it as an option, but I have to sometimes.

[@JoeRobinsonTraining]

I still remember years ago one of my lecturers (a chap called Nigel) saying that the triangle is a crutch and I agree with him really. It's got its use but I want my learners to be able to transpose anything not just formulae that fit into the triangle mould and understand why the process works the way it does.... it's a lot to ask though!

[@vanderpant2116]

Great video Joe, nice that this one is being applied to a real life situation .... Hoping to see more of this as your learners develop the theory .... Loving every vid you upload 😀

[@JoeRobinsonTraining]

Thanks very much, stay tuned for more content!

[@pasalimani1478]

Hi, how can I Calculate the design current in a domestic single phase supply that has a power rating of 5 kW and operates with a power factor of 0.8. ?

[@JoeRobinsonTraining]

Ib=P/(Vxpf)

[@no_short_circuit]

Thanks Joe. I know you've explained this before but are the schools failing our kids when you have to go into so much depth on simple transposition - pity if that's the case. I've been studying a lot of this stuff in my retirement - why Ib for design current why not Id???

[@Marcel_Germann]

GrumpyGrampa I‘m from Germany and I can tell you two things: The thing with the school students is the same thing here. And in second Ib is harmonised, at least in Europe every trained electrician will know what that means. But in german it‘s easy to remember as Betriebsstrom (operating current).

[@no_short_circuit]

@@Marcel_Germann Thank you. I'm not an electrician but this stuff gives me something to keep my brain active in my retirement. I do despair about the level of basic maths in 16 years olds. Do you think that is where the Ib comes from? Id for design current seems more intuitive to me.

[@Marcel_Germann]

@@no_short_circuit There's no difference between the operational current and the designed one, it's more like an expectation than a design. Or there shouldn't be a huge difference between the actual operational current and the design. The designed current is a calculated value which you use as a basis to choose the correct sized wires and the correct fuses/MCBs to built the circuit that supplies the appliance. In that case a water heater for a shower which is a dedicated circuit, means this circuit provides power only to this single appliance, so you only need to design the circuit arround this device. And in the case of the shown water heater I already see the german service companies here grasping. The most ones don't permit single phase loads with more than 4600W (20A on 230V), because they're afraid of unbalanced loads. Most houses here have a three-phase connection, even the residential buildings, and you should distribute the loads equally on all three phases. The next step would be to use the 36.93A Joe calculated, that would at least require a cable with a conductor size of 6mm². I don't know which fuses you got in the UK, but here it would be a 40A, so 10mm² (depends on length of the cable, and how it is installed for heat dissipation) would be actually the better choice, especially if you install that cable in a wall which is insulated. The next smaller fuse size is 35A. This is actually too small, it wouldn't blow, but it would cause thermal issues in the fuse holder. There's a video out there made by John Ward: kzbin.info/www/bejne/jpvZoJyFarNnopo In a three-phase installation 3x230/400V the current calculation for the 8.5 kW would be: P = I x U x sqrt of 3 The sqrt of three is to correct the phase deviation of 120° between the three phases. That 120° come from the angle between the three coils in the generators (360° divided by 3 is 120°). So in that case the current would be round about 12.3A per phase, since there are no 12A MCBs, 13A would be a fit. As wiring I would install a cable 5G2.5mm² (L1, L2, L3, Neutral and earth), the neutral is not required in most water heaters of that kind, because they're purely resisistive and absolute symetrical loads. In a three-phase system the neutral only carries the differential current if the load is not perfectly balanced between the three lines. The circuit would be fused with 3x13A. The cable is also rated to be fused with 3x16A.

[@no_short_circuit]

@@Marcel_Germann Thanks Marcel - a lot to digest. I subscribe to John Ward but must have missed the video you suggest, I'll look at it later. As far as 3 phase is concerned that's something I'm just beginning to get my head round. Thank you very much for taking the time to give me such a comprehensive answer.

[@JoeRobinsonTraining]

I'm not sure if it can all be pinned on schools, I think it's a more complex issue including cultural attitudes, homelife, upbringing, etc. Most maths teachers I've known and met have been really passionate about their subject and want to do a good job. Tricky one really. Also I have no idea about Ib!

[@jagdishmloya]

I have a doubt, When we hold a battery in hand like egg(AA size, 1.5V), it actually get short circuited by body conductor but , it do not make any spark or heating effect at finger tips… why?

[@JoeRobinsonTraining]

Hi, it's because the resistance of your body is so high the voltage from the battery isn't high enough to push a large enough current through your body.

[@jagdishmloya]

@@JoeRobinsonTraining ok. Thank you.