Industrial heater wiring

[MrGwythers]

Hi I was wondering about how a 23kw 400v 32A electrical heater would be wired, e.g what mcb and what size cable?? With 32A commando socket.

From what I’ve learned in college, I’m a level 2 apprentice, when it comes to cable calcs the design current P/V=I that would give me 100A and the cable would have to be at least 16mm. Is this right ?

 

[Pete999]

Hi I was wondering about how a 23kw 400v 32A electrical heater would be wired, e.g what mcb and what size cable?? With 32A commando socket.

From what I’ve learned in college, I’m a level 2 apprentice, when it comes to cable calcs the design current P/V=I that would give me 100A and the cable would have to be at least 16mm. Is this right ?

You will need to know lots of information before deciding on cable size:
How long is the run
Installation method
Type of cable
Power Factor
ETC
Looks like you forgot to include the Square root of 3 in your cable calculations, is this a job or a College question?

 

[Lucien Nunes]

First, you correctly state that I=P/V, but as you also state P=23kW and V=400 then I=23000/400=57.5A, not 32A nor 100A as you claim. But that would be 400V single-phase. Ooops.

Start again with a 3-phase power calculation...

 

[MrGwythers]

You will need to know lots of information before deciding on cable size:
How long is the run
Installation method
Type of cable
Power Factor
ETC
Looks like you forgot to include the Square root of 3 in your cable calculations, is this a job or a College question?

It is theoretical not a job,
the run would be about 15m and either in Swa or sy cable and clipped direct. Why would you need a power factor ? And I haven’t got as far as doing square root of 3 in my cable calcs that’s level 3 stuff.

 

[MrGwythers]

First, you correctly state that I=P/V, but as you also state P=23kW and V=400 then I=23000/400=57.5A, not 32A nor 100A as you claim. But that would be 400V single-phase. Ooops.

Start again with a 3-phase power calculation...

Sorry my bad I worked it out for a single phase but the heater is still a 400v 23kw 32A industrial heater comes with a commando plug attached, so obviously would be plugged into a 32a commando socket is that not correct ?

 

[MrGwythers]

I believe I’ve worked it now after correcting the formula to 3 phase if it’s only 57A then you would use a 60 amp breaker and say 10mm armoured as cable rating would be 73A does that sound about right ?

 

[Darkwood]

Commando style plugs come in ratings higher than 32amp btw, don't assume its 32amp.

When you say the Kw of the heater are you sure this is an electrical rating and not the heating capacity of the unit... before we even start thinking out cable and fuses we need to clarify many points.

Is this homework or coursework?... if so we cannot spoon feed you the answers as this would defeat your learning curve but we are happy to nudge you in the right direction .

Are we to assume that the heating is by elements totaling 23kw per phase or is the 23Kw across the phases as can sometimes be the case in the real world especially when it's imported and ratings are not always translated to our common rating systems, lastly 400v on its own could be 2ph or 3ph so we also need to know that detail too.. if it is indeed a written coursework question then it lacks a lot of info so I suspect it is targeted to recent coursework you have done and we will have to take many assumptions for the info given.

 

[Lucien Nunes]

I think it's 23kW total - it makes sense in the context (check the current at 415V!)

I=P/(sqrt(3)Vline). Which is technically a bit naughty on a 32A plug if it's rated 23kW at 400V, not so if it's rated at 415V.

If it's coursework they should not be speaking of a Commando socket, that's a brand name not a type of connector.

 

[Darkwood]

@Lucien Nunes

I am probably over complicating it a bit as it will just be in line with his course work but the commando style plug been 32amp is ambiguous to me in how he has mentioned it, still wasn't sure if we are been told it is a 32amp plug or asking us if it will do on a 32amp plug, the 2 posts it is mentioned conflict with each other as it turned from a statement into a question.

I am surprised tbh how this question is laid out assuming he has translated it in a similar fashion for our input, there is much missing info if this was a real world scenario, I first thought it was real world and a large industrial gas burner with a big 22Kw motor on it or something but that's more me applying the industrial world to the little info given lol...

 

[Pete999]

It is theoretical not a job,
the run would be about 15m and either in Swa or sy cable and clipped direct. Why would you need a power factor ? And I haven’t got as far as doing square root of 3 in my cable calcs that’s level 3 stuff.

It is theoretical not a job,
the run would be about 15m and either in Swa or sy cable and clipped direct. Why would you need a power factor ? And I haven’t got as far as doing square root of 3 in my cable calcs that’s level 3 stuff.

PF yes my error you have quoted KW I initially thought it was KVA you were quoting sorry for the confusion, so you haven't done any 3 phase calcs yet then? As Lucian stated I=P/root(3)Vline

 

[MrGwythers]

It is not coursework or a job, a friend owns a gym and has 2 shitty heaters in it and he was told about putting 3phase heaters in, and I looked at his supply which had a 3 phase board in. I was just trying to learn about 3phase heaters a bit more just so I’d know how to wire them up possibly in the future when I’m qualified, I know I’m only level 2 but I want learn as much as possible. I have uploaded a picture of the type of heaters I was looking at. Like I said this Is all theoretical and part of my learning curve.

 

[telectrix]

when it says that you need a 32A 3 phase sullpy. that's 32A per phase. so in total you have 7.3kVa per phase, or 22kVA in total.

 

[Pete999]

PF yes my error you have quoted KW I initially thought it was KVA you were quoting sorry for the confusion, so you haven't done any 3 phase calcs yet then? As Lucian stated I=P/root(3)Vline

when it says that you need a 32A 3 phase sullpy. that's 32A per phase. so in total you have 7.3kVa per phase, or 22kVA in total.

Have you got a bad hand Tel?

 

[MrGwythers]

So the heater I was looking at runs at 22A according to the technical info, which is why it says it needs a 32A supply. And that the 23kw is the heat output so can anyone explain the difference in this and other appliance wattages e.g if you were to work out a cooker circuit amperage you would use the wattage?

 

[Spoon]

the run would be about 15m and either in Swa or sy cable and clipped direct.

All the SY cables I have seen have "SY control cables are not suitable for fixed wiring applications requiring compliance with the requirements set out in BS7671."
If you are looking at SY cable in RS Components then the description "and is suitable for fixed installations" is incorrect and they are in the process of changing it.

 

[Lucien Nunes]

So the heater I was looking at runs at 22A according to the technical info, which is why it says it needs a 32A supply

Eh? I can't see anything about 22A. The correct current, as given by the formula both Pete and I have supplied you, is 32-33A according to whether you take the supply as 230/400V or 240/415V. As it is a resistive heater, the power factor is almost exactly unity and hence work from the true power rating in kW rather than the apparent power in kVA. There's also no need to consider efficiency, i.e. whether the stated 23kW is the input or output power, because an electric fanheater is inherently ~100% efficient.
I=P/(sqrt(3)Vline)
I=23,000/(1.73x415)
I=32A
I've considered the line voltage to be 415V as that is probably how the elements are rated. In fact they may well be 240V elements wired in star, with the star point floating. You might like to consider the implications of that setup - three elements rated 7.3kW at 240V connected in star - with the star point also connected to neutral. Now you have the equivalent of three single-phase heaters, one on each phase, and you can calculate the current simply as I=P/V.

 

[MrGwythers]

I’m talking about the specifications stated on the heater website and in this picture I’ve uploaded where it states the amperage is 22a, it also states it’s 400v fitted with a 32a rated commando plug, so a 32a commando socket is needed.

 

[Lucien Nunes]

Then the 22A is a mistake. At 22A the power input would be: P=Vline.I.sqrt(3) =400 x 22 x 1.73 = 15.2kW.
Hmm, there's an error on the internet!

 

[Richard Burns]

I would suspect the 22A was a typographical error, however another website states the power consumption is 20 units per hour which would indicate, assuming constant operation, a 20kW heater??