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Hi I have been asked to size a few fuses up for some machines to protect against short circuit, i have been given the total kW which is 230 and its 3 phase 400V. What other information would i need? I Have had a quick guess at 450A put im not sure how to work it out.
any help welcome

thank you
 
Hi I have been asked to size a few fuses up for some machines to protect against short circuit, i have been given the total kW which is 230 and its 3 phase 400V. What other information would i need? I Have had a quick guess at 450A put im not sure how to work it out.
any help welcome

thank you
https://www.google.co.uk/search?q=3+phase+calculations+amps&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&ei=B6FWV4-kI8GAaLDVh8AB
[url]http://ncalculators.com/electrical/kva-calculator.htm


[/URL]
 
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Welcome to the forum :)

What are the nature of the machines, are they resistive loads, inductive, a combinations, if inductive then what kind, if a motor then how is it controlled, how is it started, whats the start duty on it etc....

Some machine are very simple to work out where are others can be complex, if the machines are new then there is a requirement for the company selling the machines to provide the data like front end fusing.
 
That's my calculation of design current based on your figures, assuming a PF of 1. But I'm just completing my Level 2 so I'm not saying this is correct.

Fuse Sizing {filename} | ElectriciansForums.net
 
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That's my calculation of design current based on your figures, assuming a PF of 1. But I'm just completing my Level 2 so I'm not saying this is correct.

Correct on working out load.

For sizing the fuses inrush needs to be taken into account. Also what the fuses are protecting there may be the need to fit semiconductor fuses.

From there, the usual in sizing a cable/ cables suitable. Meeting VD and Zs requirements along with environmental requirements.
 
ye it is just a 3 phase motor and a small oil pump. I have very limited information i think, i cant work out how they got to 450A. Yea it is 230KW total
 
Hi

Please take this comment politely , are you really competent to be carrying out this type of work, an error could have serious consequences .

Cheers
 
ye it is just a 3 phase motor and a small oil pump. I have very limited information i think, i cant work out how they got to 450A. Yea it is 230KW total
You lost me here, your 'OP' - opening post said you guessed at 450amp but now you suggest someone else gave you that figure?

What size is the motor and type of motor is it?
What is its starting method and starting duty?

This will be beyond your teachings tbh and is an area where general Electricians make very expensive mistakes, if the installation instructions suggest 450amp supply fuses then thats what you need to put in, if like myself you are able to design the required cable and fusing for the machine without such info then you often learn that some manufacturers cover there --- by upsizing requirements. Im in no way suggesting this is the case with your install but as you are not experienced in this field then the safe bet is to use basic design to the front end which has been suggested.

It common in these installs for the O/L exclusion rule to be implemented on cable sizing as the machine may be classed as a fixed load but care has to be taken here on high inertia loads and Zs limits when reducing cable sizes under the OCPD rating.
 
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sorry was suppose to be "They had a guess at" and "but im not sure". What is the basic information i need to figure it out i will be able to find some more information out from there.
 
sorry was suppose to be "They had a guess at" and "but im not sure". What is the basic information i need to figure it out i will be able to find some more information out from there.

There is no basic information route here, the characteristics of the machine namely the motor in this case can mean this is a complex design to get it right or may just be a straight forward KW rating of machine to fusing for a standard duty motor with standard characteristics like starting and no' per start per hour.

It takes a lot of experience and knowledge in this field to recognise what you are looking at and the requirements of the machine in particular the inrush ,duration and frequency of starts, imagine such a large machine nuisance tripping and the loss of production to such issues, you as a designer will be liable to any such issues, like I said before we need to establish how this machine works, all these points mentioned can have massive implications on design. Example - A large inertia load may take longer to start up and by standard calc's the cable would end up undersized and the OCPD would probably trip frequently, is the motor controlled by DOL, soft start, star-delta, resistive banks, VSD etc all have their unique starting demands and will require different if any allowances on the upstream OCPD.

I don't want to try scare you totally away from this here as it may be a simple calc needed but I would need to know exactly how the machine works, I also want to make sure that you are aware this kind of work and design is alot more advanced than you are willing to allow for judging by your posts. If there is no-one other than yourself to design this then I would recommend bringing someone experienced to design this or walk away, the possible comeback from a small mistake can take many small companies down, and your insurance may not cover you for such installs if you haven't declared 3ph and machinary in your cover, and if you are covered they still have a angle to deny cover if you were out of your depth.
 
sorry was suppose to be "They had a guess at" and "but im not sure". What is the basic information i need to figure it out i will be able to find some more information out from there.

I think we need some more information about what it is exactly you are asking - the details are coming in sparse. Is this a college question you've been asked to answer (in which case all the required details should have been provided) or is this for a job. I'm guessing it's for a job based on your posts, in which case my two questions would be:

1) Who is this mysterious 'they' who are having a guess
2) Why are they expecting a trainee electrician to come up with the answers?
 
think they just wanted to try make some use out of me while i am waiting to see all the equipment, just gave me the information and told me to go work it out lol.
 
sorry was suppose to be "They had a guess at" and "but im not sure". What is the basic information i need to figure it out i will be able to find some more information out from there.

Write down every question you have been asked in the above posts and try to answer them. This will help you a lot.
 
think they just wanted to try make some use out of me while i am waiting to see all the equipment, just gave me the information and told me to go work it out lol.


2 lines of thought here then, they are humouring you or just want you to work out the basic way you were taught or they don't actually realise themselves... I would just work out for a standard motor start and duty based on the info you were given, if you want to shock them then ask them the questions I put to you ;)
 
That's my calculation of design current based on your figures, assuming a PF of 1. But I'm just completing my Level 2 so I'm not saying this is correct.

Fuse Sizing {filename} | ElectriciansForums.net

Based on the information provided then yes you are correct, but the information Is somewhat lacking so overall your answer may not work out in the real world.

The PF is unlikely to be 1 on a machine of that size, but your guess is as good as anyone's until information is provided.

Likewise there may be efficiency to factor in to the calculation.
 

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