Terminal Box with Fuse

[Elekk]

Is it possible to run a larger (35mm2) cable from panel, and put a fused terminal box in the field just before the first light. And from that terminal box I can take smaller 10mm2 cable for looping from light to light? If yes, which company makes fused terminal box?

(A bigger cable at the start of run is required to avoid voltage drop and also because the cable is bunched with several other cables.)

 

[sparkytim]

How are you going to terminate 10mm conductors into a light?

If your larger conductor is only required for Volt drop requirements then just protect the circuit with the correct size Fuse/MCB for the smallest conductors used in the circuit.

 

[Elekk]

Thanks Sparkytim

 

[NozSpark]

Size the MCB for the load and then size the cable is ok for current carrying capacity and volt drop

 

[SLIPSHOD & SLAPDASH]

Just wondering what the project is ?

 

[Evans Electric]

35mm cable for the lighting !!!!!!! Thats some job there , Elekk !!! OMG!!! What are you using for the sockets?? Biggest cable I've ever used for lighting final sub circuit was 4mm feeds to the ends of the bays then looped fittings with 2.5 mm

That was a storage warehouse for Bass Charrington.

---------- Post Auto-Merged at 23:28 ---------- Previous post was made at 23:26 ----------

Is it possible to run a larger (35mm2) cable from panel, and put a fused terminal box in the field just before the first light. And from that terminal box I can take smaller 10mm2 cable for looping from light to light? If yes, which company makes fused terminal box?(A bigger cable at the start of run is required to avoid voltage drop and also because the cable is bunched with several other cables.)

If its just for VD I don't think you need to to fuse down .

But 35mm !!! you must have about 800 X 5ft fluoros on that circuit.

 

[Doc Hudson]

Just wondering what the project is ?

Indeed. Also I just noticed you have a new Avatar Slipshod, what is the small writing underneath your axe?

Doc H.

 

[steptoe]

as used during the french revolution???????????

 

[Evans Electric]

Was that not Madame Guillotine Steps?

 

[steptoe]

I think it may have been Deke, but surely she may have needed assistance on times?

 

[SLIPSHOD & SLAPDASH]

Indeed. Also I just noticed you have a new Avatar Slipshod, what is the small writing underneath your axe?Doc H.

It says, this is a picture of an axe

 

[apprentice87]

Hi Elekk,

I think you are looking at things the wrong way round. You do not size the overcurrent device to suit the cable, you size it to suit the design current of your circuit. So, first off, decide what the design current is going to be. You may well be able to "build in" a bit of "diversity" here, as that will help too.

Anyway, you mention 38A, so presumably that is the current you want your cable to carry.

As the overcurrent device obviously has to be rated higher than the design current [as otherwise as soon as you switch it all on the thing will trip/blow] you will need the "next size up" as it were.

If your design current is 38A, then you would use a 40A MCB of a suitable type for the load.

Now then, the "correction factors" for grouping etc etc, are not applied to the cable, they are applied to the overcurrent device protecting the cable.

So next you need to know the "correction factors" you need. As your cable in a group of 12 others, making 13 in all, i would say that the CF is .41 provided that is, that they are all in free air and not in ducts of whatever, and that no other CF's apply.

So now, you need to divide the rating of your overcurrent device by whatever correction factors are applicable. So, 40A divided by .41 is 97.5 so now you need to look up in the charts, whatever size cable is required, that has a tabulated current carrying capacity of 97.5Amps when using the applicable installation method.

This is going to give a huge cable for just 38A so could you run it on its own?? Then you could use a more sensibly sized cable as you would not have to divide the 40A by .41 to come up with over 97A and you .could just have a 40A rated cable, although, as you say you might need a huge cable anyway for volt drop purposes

Apart from this, you will also have to check volt drop [as you have already done] and then also calculate what r1 + r2 is going to be for your length of run, and add it to Zdb as measured at your panel, and make sure that the resultant ELI is low enough to operate your chosen overcurrent device in the required time.

Next you should use the adiabatic equation [or look in the charts] to make sure that your cable provides an adequate size cpc, and that therefore, the cable will not cook itself in the time it takes for the overcurrent device to operate in the event of a fault to earth.

You will also need to check that your overcurrent device is rated to handle the max PFC [PEFC OR PSCC] at the point it is installed.

Have a look at page 255, 256, and 257 in the BRB. They explain all this better than me!

Is this thing three phase or single, and what is the length of the run??

john.

 

[Chr!s]

Hi john

Hi Elekk,I think you are looking at things the wrong way round. You do not size the overcurrent device to suit the cable, you size it to suit the design current of your circuit. So, first off, decide what the design current is going to be. You may well be able to "build in" a bit of "diversity" here, as that will help too.

Anyway, you mention 38A, so presumably that is the current you want your cable to carry.

As the overcurrent device obviously has to be rated higher than the design current [as otherwise as soon as you switch it all on the thing will trip/blow] you will need the "next size up" as it were.

If your design current is 38A, then you would use a 40A MCB of a suitable type for the load.

Now then, the "correction factors" for grouping etc etc, are not applied to the cable, they are applied to the overcurrent device protecting the cable.

Id disagree, it all depends on what your trying to achieve, you may apply them to It,Ib,In.

So next you need to know the "correction factors" you need. As your cable in a group of 12 others, making 13 in all, i would say that the CF is .41 provided that is, that they are all in free air and not in ducts of whatever, and that no other CF's apply.

Again, this may be very inaccurate and may lead to oversized cables, several key factors need to be considered before applying Cg. Alternativle ERA utilize UFG for derating cables.

So now, you need to divide the rating of your overcurrent device by whatever correction factors are applicable. So, 40A divided by .41 is 97.5 so now you need to look up in the charts, whatever size cable is required, that has a tabulated current carrying capacity of 97.5Amps when using the applicable installation method.

This is going to give a huge cable for just 38A so could you run it on its own?? Then you could use a more sensibly sized cable as you would not have to divide the 40A by .41 to come up with over 97A and you .could just have a 40A rated cable, although, as you say you might need a huge cable anyway for volt drop purposes

Would you not calculate for volt drop first?

Apart from this, you will also have to check volt drop [as you have already done] and then also calculate what r1 + r2 is going to be for your length of run, and add it to Zdb as measured at your panel, and make sure that the resultant ELI is low enough to operate your chosen overcurrent device in the required time.

Next you should use the adiabatic equation [or look in the charts] to make sure that your cable provides an adequate size cpc, and that therefore, the cable will not cook itself in the time it takes for the overcurrent device to operate in the event of a fault to earth.

You will also need to check that your overcurrent device is rated to handle the max PFC [PEFC OR PSCC] at the point it is installed.

Have a look at page 255, 256, and 257 in the BRB. They explain all this better than me!

Well, youve not done too bad for a apprentice

Is this thing three phase or single, and what is the length of the run??

john.

 

[apprentice87]

Hi Chris,

1, You say that, as regards what the CF's are applied to;

"Id disagree, it all depends on what your trying to achieve, you may apply them to It,Ib,In"

Yes, i know there ARE circumstances when you would apply them to Ib instead of In, but i was generalising a bit. Not sure when you would want to apply them to It though..

2,You say that, in regards to "grouping" that;

"Again, this may be very inaccurate and may lead to oversized cables, several key factors need to be considered before applying Cg. Alternativle ERA utilize UFG for derating cables"

Yes, I would agree, as there is grouping and grouping if you see what i mean. The way the cables are arranged in the group, has a big bearing on things, as does especially the loading on the individual cables in the group, as, the grouping CF's assume that all the cables in the group are fully loaded. Indeed, some of the cables [those carrying less than 30% of their rated load] may be ignored altogether as regards grouping, but again, i was generalising.

3, You say that;

"Would you not calculate for volt drop first?"

Well, yes, i would, in fact, it would, after determining the design current, be the very first thing i would do, as then you could save yourself doing a lot of calculations, and then falling at the last hurdle as it were. I was just trying to explain things in a logical easy to follow step by step example!!

Finally, you say; as regards explaining things;

"Well, youve not done too bad for a apprentice"

Thank you very much for that!!!! That actually meant quite a lot to me. You see, I really really do try. I will freely admit that i have a GREAT deal more to learn, a hell of a lot more to learn in fact. I am only really a beginner, but i do try!!!!!

You see, I do not want to be one of those that just does things "because that is how it is done" I want to FULLY understand exactly WHY it is done that way, and to have a proper understanding of the theory behind it all too.

I do not want to be electrician of the century or anything, just to be confident in my own mind, that i know both what i am doing, and also why i am doing it.

My motto is; "if it is not right, then it must be wrong"

I know maybe i am a bit picky, or maybe a bit of a perfectionist, and i am VERY hard on myself when i comes to judging what i have done, but that is just the way i am!!

Thanks again for your comments, as they are all constructive, and those are the sort i like!!

Thanks Chris.

john..

 

[Chr!s]

"Id disagree, it all depends on what your trying to achieve, you may apply them to It,Ib,In"Yes, i know there ARE circumstances when you would apply them to Ib instead of In, but i was generalising a bit. Not sure when you would want to apply them to It though..

You apply them to it to find your Iz

Thank you very much for that!!!! That actually meant quite a lot to me. You see, I really really do try. I will freely admit that i have a GREAT deal more to learn, a hell of a lot more to learn in fact. I am only really a beginner, but i do try!!!!!

Your welcome! Im a relative newbie myself so lots to learn too!

My motto is; "if it is not right, then it must be wrong" I know maybe i am a bit picky, or maybe a bit of a perfectionist, and i am VERY hard on myself when i comes to judging what i have done, but that is just the way i am!!

Nothing wrong with that, in fact its how you need to be.

 

[apprentice87]

Thanks Chris!!