Equipment Motors & Other Design Questions

[Blue Duck]

still sounds like homework to mee :|

 

[Doc Hudson]

Hello Sir is there something wrong to ask few questions and hear other opinions? Would you think when we say electrician do we mean that knows all the complex fields that this job is involved? I just repeat: my 2 questions were just to confirm if my calculations are right. If I am wrong and I shouldn't ask please let me know Thank you

There is nothing wrong with asking questions, that is one of the purposes of the forum. However there is no need to open a new thread for asking another minor variation about the same question, If everyone opened a new thread for every slight variation around a topic the forum would become cluttered and unreadable. Both of your threads are basically about verifying cable calculations, therefore they are best merged as one. 

Doc H.

 

[binky]

With an attitude like that valerub you can expect help to run out very quickly. I strongly suggest you apologise pronto.

 

[apprentice87]

I thought he was exceedingly polite to be honest..

Is not the forum about helping people?? If every time someone asks a question [no matter what their motives] they are told. "get an electrician" or just no help, we will not have a forum very long... We are supposed to be friendly here, not just a bunch of electricians running a "closed shop" whilst all congratulating eachother on how clever they are....

No newbies or others asking questions = no forum....

Remember that...

john..

 

[Guest]

I thought he was exceedingly polite to be honest.. Is not the forum about helping people?? If every time someone asks a question [no matter what their motives] they are told. "get an electrician" or just no help, we will not have a forum very long... We are supposed to be friendly here, not just a bunch of electricians running a "closed shop" whilst all congratulating eachother on how clever they are.... No newbies or others asking questions = no forum.... Remember that... john..

Hello SirThank you for your explanation

I only moved to another forum as you wrote the following: shall I move it to the student area?

My thought was I probably asked wrong question in a experience argue electricians forum that's why I went to 2330 electrotechnical forum as my question were probably more adequate but I realize the 2 topic have been combined.the only thing I am very sure I don't know how to use the site and for the answers I received I feel offended and next time I won't ask question becouse it seems wrong to ask questions. Anyway thank you

I thought he was exceedingly polite to be honest.. Is not the forum about helping people?? If every time someone asks a question [no matter what their motives] they are told. "get an electrician" or just no help, we will not have a forum very long... We are supposed to be friendly here, not just a bunch of electricians running a "closed shop" whilst all congratulating eachother on how clever they are.... No newbies or others asking questions = no forum.... Remember that... john..

thank you John I really appreciated. I get so much confusion to answer as I don't know how to use the site

 

[Guest]

There is nothing wrong with asking questions, that is one of the purposes of the forum. However there is no need to open a new thread for asking another minor variation about the same question, If everyone opened a new thread for every slight variation around a topic the forum would become cluttered and unreadable. Both of your threads are basically about verifying cable calculations, therefore they are best merged as one.  Doc H.

thank you for your answerI opened a new thread as I moved my second question to 2330 forum becouse you answer me writing the following:Shall I move it to the student area? As one thing I am very sure I get confusion to use the site that's why I thought I probably ask a question in a wrong forum? And I moved my second question in a different forum? I think was 2330 electrotechnical but I realize the second question for some reason went back where my first question is.anyway I won't ask anymore questions as I feel intimidate and "electricians" instead to answer (if they want to without any obligation) they argue and say things like I should ask apologize pronto.what for? Asking electrical question? Personaly I will not ask anymore Questions thank you.

 

[apprentice87]

Hi Valerub,

You say, "these machines will take 100Amps with one phase.so because I wire the subboard with a three phase the next formula is 100/1.732=57.73Amps the SWA will be run as method C (Clip direct) now I can select a cable size from the regs and it is 16mm2 this cable can take maximum load of 77 Amps.the earthing system will be connect to the earth (steal) of Swa and size will be 16mm2 copper conductor as per regs. the main cable that it came out from the intake is protect by 3 fuses (three phase) 100 amp each and will terminate to distribution board the main switch of the distribution board is a 4 pole 125amp rated. My question is would you please advise if my calculation is right also the distribution is 12 way and there is space for a three phase MCB the only thing is that L1 has 90 amp rating mcbs (final circuits) L2 is 160 (final circuits) amp rating and L3 is 140 amp rating (final circuits).Also would it be possible that distribution board will overloaded? what are the consequences? this is won`t be a job I just make it for my personal clarification thank you"

First off, I do not understand this bit "100/1.732=57.73Amps" The 1.732 thing is for working out the power transmitted by a three phase supply. So [ignoring power factor] power is volts x amps = power yes?? Well, in a three phase circuit it would be volts x amps x 1.732 = power. So you are misleading yourself there....

Now, you say you have 20 motors that all have a FLA of 5 amps. You would split them between phases to balance the load, so as Prodave explained, you could have 7 on one phase, 7 on the next and 6 on the last. This means you would be looking at about 35A per phase.

Now, you mentioned starting currents. This could be 10 times the running current, but, as long as you switch the motors one one or two at a time and not all 7 [on a phase] at a time i think things would be fine.

As your load per phase is 35A you need to have an MCB that can cope with this, So say a 40A one and as it is all motors, perhaps a "C" curve one would be best.

You must understand that you have to calculate your cable size on the basis of this 40A, NOT the 35A that the motors will draw. Cable size is based on the OCPD protecting the cable..

The main problems you will have are;

1, Volt drop. You need to get this below 5%. So 5% of 230v [you are using single phase 230V motors] is 11.5v so it would be 11.5A/35A = 0.32857 Ohms divided by 50 [your 50 metre run]  x 1000 = 6.57mV/A/m so you need to consult cable charts to find a cable that has a LOWER volt drop than 6.57 mV/A/m and can carry the 40A in the installation conditions you describe. [which i have forgotten!!] but i would suggest 10mm CSA cable...

Do not forget that you might also have to take into account the length of the cable feeding the DB that you are getting your supply from...

2, EFLI, You will need to measure Zs at the DB, find R1 + R2 for your submain, and calculate what Zs is going to be at your "new" board. This will have to be below about, [off the top of my head] 0.46 Ohms to suit your 40A "C" curve MCB, but best you read a current version of the regs!! This could well effect the size, or the type of cable you select...

I do not understand what you mean when you say;

"My question is would you please advise if my calculation is right also the distribution is 12 way and there is space for a three phase MCB the only thing is that L1 has 90 amp rating mcbs (final circuits) L2 is 160 (final circuits) amp rating and L3 is 140 amp rating (final circuits).Also would it be possible that distribution board will overloaded? what are the consequences? this is won`t be a job I just make it for my personal clarification thank you"

Hope this is helpful...

And that my friends, is how to treat newcomers to our forum and ensure it lives on........

john....

 

[binky]

I thought he was exceedingly polite to be honest..

Is not the forum about helping people?? If every time someone asks a question [no matter what their motives] they are told. "get an electrician" or just no help, we will not have a forum very long... We are supposed to be friendly here, not just a bunch of electricians running a "closed shop" whilst all congratulating eachother on how clever they are....

No newbies or others asking questions = no forum....

Remember that...

john..

the way I read  the reply to Doc H combining the 2 posts struck me as a bit sarcastic by Valerub. Might be just the way I'm reading it.....things don't always post as they are meant.

Other than that John, you know I'm very much of the opinion educatiing is more important then telling people off. In general Valerub has at least tried to answer his own question, and therefore merits some help.

 

[apprentice87]

Hi Binky!! Yes, i know that, But i do not think English is Valerubs first language, so i expect he is struggling a bit anyway..

john..

 

[ProDave]

And that my friends, is how to treat newcomers to our forum and ensure it lives on........

john....

Nothing wrong with that, but on the assumption this was a student exercise, I was trying to prompt the OP to think about the problem and work out the answer themselves, rather than spoon feeding them everything.

 

[Guest]

Hi Valerub,

You say, "these machines will take 100Amps with one phase.so because I wire the subboard with a three phase the next formula is 100/1.732=57.73Amps the SWA will be run as method C (Clip direct) now I can select a cable size from the regs and it is 16mm2 this cable can take maximum load of 77 Amps.the earthing system will be connect to the earth (steal) of Swa and size will be 16mm2 copper conductor as per regs. the main cable that it came out from the intake is protect by 3 fuses (three phase) 100 amp each and will terminate to distribution board the main switch of the distribution board is a 4 pole 125amp rated. My question is would you please advise if my calculation is right also the distribution is 12 way and there is space for a three phase MCB the only thing is that L1 has 90 amp rating mcbs (final circuits) L2 is 160 (final circuits) amp rating and L3 is 140 amp rating (final circuits).Also would it be possible that distribution board will overloaded? what are the consequences? this is won`t be a job I just make it for my personal clarification thank you"

First off, I do not understand this bit "100/1.732=57.73Amps" The 1.732 thing is for working out the power transmitted by a three phase supply. So [ignoring power factor] power is volts x amps = power yes?? Well, in a three phase circuit it would be volts x amps x 1.732 = power. So you are misleading yourself there....

Now, you say you have 20 motors that all have a FLA of 5 amps. You would split them between phases to balance the load, so as Prodave explained, you could have 7 on one phase, 7 on the next and 6 on the last. This means you would be looking at about 35A per phase.

Now, you mentioned starting currents. This could be 10 times the running current, but, as long as you switch the motors one one or two at a time and not all 7 [on a phase] at a time i think things would be fine.

As your load per phase is 35A you need to have an MCB that can cope with this, So say a 40A one and as it is all motors, perhaps a "C" curve one would be best.

You must understand that you have to calculate your cable size on the basis of this 40A, NOT the 35A that the motors will draw. Cable size is based on the OCPD protecting the cable..

The main problems you will have are;

1, Volt drop. You need to get this below 5%. So 5% of 230v [you are using single phase 230V motors] is 11.5v so it would be 11.5A/35A = 0.32857 Ohms divided by 50 [your 50 metre run]  x 1000 = 6.57mV/A/m so you need to consult cable charts to find a cable that has a LOWER volt drop than 6.57 mV/A/m and can carry the 40A in the installation conditions you describe. [which i have forgotten!!] but i would suggest 10mm CSA cable...

Do not forget that you might also have to take into account the length of the cable feeding the DB that you are getting your supply from...

2, EFLI, You will need to measure Zs at the DB, find R1 + R2 for your submain, and calculate what Zs is going to be at your "new" board. This will have to be below about, [off the top of my head] 0.46 Ohms to suit your 40A "C" curve MCB, but best you read a current version of the regs!! This could well effect the size, or the type of cable you select...

I do not understand what you mean when you say;

"My question is would you please advise if my calculation is right also the distribution is 12 way and there is space for a three phase MCB the only thing is that L1 has 90 amp rating mcbs (final circuits) L2 is 160 (final circuits) amp rating and L3 is 140 amp rating (final circuits).Also would it be possible that distribution board will overloaded? what are the consequences? this is won`t be a job I just make it for my personal clarification thank you"

Hope this is helpful...

And that my friends, is how to treat newcomers to our forum and ensure it lives on........

john....

John

It is well explained.

I have few questions and everyone can feel free to make me a better understanding.

if you have a three phase distribution board 100 amp service the DB is 12 way.

The DB physically has still 12 single spaces to fit 12 MCB or RCBOs or 4 three phase MCBs, but the other 24 existing circuits by rcbo`s or mcbs they have a mix rating of 32 amp each or 20 amps. 

If I do a calculation I can say L1 is 140amp current rating L2 125 amp current rating and L3 is 90amp current rating. I know that DB will not take that power at the same time as it won`t cope if it happen.

So the 100 amp service DB  even if I have still space to add more circuits, still not best practice to do so as the DB already exceed 100 amp in each phase(current rating).

Also In a large building can not be design a cheap installation as it is difficult to calculate diversity.

Anyway if this is the case what will happen if that DB will try to drown all that current?

If you notice to the installation you have three fuses 100 amp GG 88 or other similar that now I don`t remember the exactly name plus the SWA will terminate in to the DB protect by the main switch that can be 3 or 4 pole and it`s rated 125 Amp.

In my knowledge the 125 amp should be tripped first as it is more sensitive of the gg fuse even if it has greater current rating than gg fuse what is your opinion in this question?

Also why every  distribution board that are 100 amp service are all exceeding 100 amp rating?

my second question is I seen a 200 amp busbar 3 or 4 tails of different size we say 100 mm 16 mm and 10 mm go to the different boards or isolators. My question is if the 100mm cable is drowing 100 amps what about the 16mm or 10mm why the current does not transfer even to other two whilst connect to the same busbar?

I tested personally recently with ohmmeter and I do not understand why the current does not go to the other two.

I know for fact that if that current will drown to these small cable they were on fire.

what is your expertise on this question?

thank you very much

 

[ProDave]

The main switch has a current carrying capacity of 125A. That does not mean it is an mcb and will trip if that current is exceeded.

You could argue from your analysis of the existing loads, that it would be better to put more of the machines on L3 to try and balance the building consumption a bit, but that would increase the load on L3 almost certainly forcing a larger size cable.

 

[Guest]

The main switch has a current carrying capacity of 125A. That does not mean it is an mcb and will trip if that current is exceeded.

You could argue from your analysis of the existing loads, that it would be better to put more of the machines on L3 to try and balance the building consumption a bit, but that would increase the load on L3 almost certainly forcing a larger size cable.

The main switch has a current carrying capacity of 125A. That does not mean it is an mcb and will trip if that current is exceeded.

You could argue from your analysis of the existing loads, that it would be better to put more of the machines on L3 to try and balance the building consumption a bit, but that would increase the load on L3 almost certainly forcing a larger size cable.

The main switch has a current carrying capacity of 125A. That does not mean it is an mcb and will trip if that current is exceeded.

You could argue from your analysis of the existing loads, that it would be better to put more of the machines on L3 to try and balance the building consumption a bit, but that would increase the load on L3 almost certainly forcing a larger size cable.

Yes that is true thank you for your expertise

 

[sprocketflup]

Apprentice - theres a bit of a difference between a DIYer asking why his bathroom light trips the MCB now hes put all the blacks together and all the reds together, and the situation we have here

 

[Qui-Gon Jinn]

I have sensed a distrurbance in the Force.

Please play nice my Padawan, else your Jedi Master will be forced to teach some lessons.

Mr valerub, you seem somewhat confused Sir.

Also, it seems that English is not your first language, this my Padawan is not an issue.

However, please do space out your posts to make them easier to read, this will assist the others in assisting you.

I have edited your post above in the hope that it will be easier for others to read.

I will request by private message one of our members who is a Jedi in his own field to assist you if he will in explaining some of the concepts you are struggling with.

Do continue to post.

However my Padawan, Mr valerub please make your posts shorter and spread them out more so that answers can be shorter and more quickly written to each smaller question, thus the other Padawan can more easily assist you.

I will be watching this thread, and, if discussions or answers get out of hand I will be alerted, and following a recent Staff convention, there has been some alignment of thought, Mr Apprentice is close to being aligned with the thought of the Moderators & Admins.

There has also been some alingment of moderation style, so perhaps Mr Smith will not be required so often, as even a Jedi Master can learn new skills, and this Master, has certainly learned skills from Mr Smith.

Welcome to the Forum Mr valerub.

 

[binky]

Ok, attempt at very simplified answer :

imagine a swimming pool full of water and consider that national grid. FRom ths pool you have a large pipe connected to your main distribution board. From this main board you have smaller pipes connecting to the smaller boards. At these boards are an assortment of different size taps. If the taps are turned off, no water flows. It doesn;t matter how much water is in the swimming pool, nothing will flow.

Now you turn on all the small taps at one of the small boards, water will now flow. If the pipe supplyig the board is big enough, water can flow as fast as the taps will allow. Now if the taps are bigger than the supply pipe, then water will not flow fast enough for the taps to work properly and maybe only a trickle of water will flow out of the taps. This means you need a bigger pipe.

Water flowing through a pipe does have friction with the pipe that can restrict flow of water. Likewise electricity flowing through a cable encounters the resistance of the cable and this causes 'voltage drop'. If the cable is too small the voltage drop is too high and the machinery on the end of it won't work properly. When electricity encounters a resistance it also creates heat, if the cable is too small and the voltage drop too high, it makes the cable hotter than the insulation can work with, and the insualtion melts causing the cable to fail. To prevent this from happening we use MCBs or fuses to prevent more electric from flowing through the cable than it can take wthout melting. Ideally the cable will be big enough for the total load on the end of it. The longer the cable the higher the resistance, so the more chance of the cable getting too hot. So for a longer run of cable we use a larger cable size to reduce the resistance and keep the voltage drop within limits. Calculating cable sizes and MCB sizes is therefore very important.

Hopefully that helps ray

 

[apprentice87]

Hi Valerub,

As has been pointed out, the mainswitch feeding a DB is not usually an over current protection device, and is merely a switch. You do have boards though where the incomer IS an OCPD to protect the board from overcurrent. Here is an example.

https://www.cef.co.uk/catalogue/products/1932536-12-way-250a-tp-n-mccb-panelboard-grey

As you can see these boards use a MCCB [moulded case circuit breaker as an incomer] to protect the board.

You point out that you might have a "125A" board, but if you add up all the MCB's they come to more than this. This is because the makers have allowed for diversity and assume that not all circuits will be loaded to the maximum at the same time. Indeed, if they were, you might well have problems with adjacent MCB's overheating and tripping prematurely.

Quite how the makers work out the diversity i do not know, it might vary between makers, and there are standards for this, but i have seen boards where it says something like " Recommended diversity factor 66%" In other words, as i understand it, you could load up all the MCB's in the board to 66% of their maximum, PROVIDED that this does not exceed the rating of the board itself.

I do not quite understand this bit; "my second question is I seen a 200 amp busbar 3 or 4 tails of different size we say 100 mm 16 mm and 10 mm go to the different boards or isolators. My question is if the 100mm cable is drowing 100 amps what about the 16mm or 10mm why the current does not transfer even to other two whilst connect to the same busbar?"

If the different cables go to different boards, then the current will depend on the loads. If you mean the cables are "in parallel" then it depends on the impedance of each cable as to the proportion of the load it carries. BUT, it is not as simple as that, funny things can happen with cables in parallel, but that is getting a bit to advanced for me.....

john...

 

[Guest]

Ok, attempt at very simplified answer :

imagine a swimming pool full of water and consider that national grid. FRom ths pool you have a large pipe connected to your main distribution board. From this main board you have smaller pipes connecting to the smaller boards. At these boards are an assortment of different size taps. If the taps are turned off, no water flows. It doesn;t matter how much water is in the swimming pool, nothing will flow.

Now you turn on all the small taps at one of the small boards, water will now flow. If the pipe supplyig the board is big enough, water can flow as fast as the taps will allow. Now if the taps are bigger than the supply pipe, then water will not flow fast enough for the taps to work properly and maybe only a trickle of water will flow out of the taps. This means you need a bigger pipe.

Water flowing through a pipe does have friction with the pipe that can restrict flow of water. Likewise electricity flowing through a cable encounters the resistance of the cable and this causes 'voltage drop'. If the cable is too small the voltage drop is too high and the machinery on the end of it won't work properly. When electricity encounters a resistance it also creates heat, if the cable is too small and the voltage drop too high, it makes the cable hotter than the insulation can work with, and the insualtion melts causing the cable to fail. To prevent this from happening we use MCBs or fuses to prevent more electric from flowing through the cable than it can take wthout melting. Ideally the cable will be big enough for the total load on the end of it. The longer the cable the higher the resistance, so the more chance of the cable getting too hot. So for a longer run of cable we use a larger cable size to reduce the resistance and keep the voltage drop within limits. Calculating cable sizes and MCB sizes is therefore very important.

Hopefully that helps ray

Thank you

Hi Valerub,

As has been pointed out, the mainswitch feeding a DB is not usually an over current protection device, and is merely a switch. You do have boards though where the incomer IS an OCPD to protect the board from overcurrent. Here is an example.

https://www.cef.co.uk/catalogue/products/1932536-12-way-250a-tp-n-mccb-panelboard-grey

As you can see these boards use a MCCB [moulded case circuit breaker as an incomer] to protect the board.

You point out that you might have a "125A" board, but if you add up all the MCB's they come to more than this. This is because the makers have allowed for diversity and assume that not all circuits will be loaded to the maximum at the same time. Indeed, if they were, you might well have problems with adjacent MCB's overheating and tripping prematurely.

Quite how the makers work out the diversity i do not know, it might vary between makers, and there are standards for this, but i have seen boards where it says something like " Recommended diversity factor 66%" In other words, as i understand it, you could load up all the MCB's in the board to 66% of their maximum, PROVIDED that this does not exceed the rating of the board itself.

I do not quite understand this bit; "my second question is I seen a 200 amp busbar 3 or 4 tails of different size we say 100 mm 16 mm and 10 mm go to the different boards or isolators. My question is if the 100mm cable is drowing 100 amps what about the 16mm or 10mm why the current does not transfer even to other two whilst connect to the same busbar?"

If the different cables go to different boards, then the current will depend on the loads. If you mean the cables are "in parallel" then it depends on the impedance of each cable as to the proportion of the load it carries. BUT, it is not as simple as that, funny things can happen with cables in parallel, but that is getting a bit to advanced for me.....

john...

John thank you very much it is well explained 

 

[Mr Smith]

jedi will be already of the knowing

mr smith is here and watching as always

although posting not

mr smith does not see any member having been overly inconsiderate in their responses as yet

although

the tone of the posts suggests this may be a risk

 

[kerching]

Valerub.....

Simple analogy...

Think of the current being ."sucked" by the appliance....it sucks what it needs to survive as long as it is healthy.

The appliance is NOT force fed the current.

A bit basic...but accurate enough