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Armoured Cable Capacity


engineman

Question

Hello,  I hope this is the right place to ask this? I want to run a length of armoured cable 100 meters from a large old alternator to my workshops the line is fused to 100 amps per phase, how big a cable do I need to handle this 100 amps per phase?

 

I know nothing about how to work this out!  Any help would be gratefully received.

 

Thanks in advance.

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21 answers to this question

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Well that is nice to know that i'm not the only weird or odd bugger about!  My wife and son keep telling me I am  for what I play with!!  Anyway back to the matter in hand........It's a 4 wire alternator so 3 phase and Neutral, these heater banks are 3phase but cooled with a single phase fan!  (Yes thats what I thought!)  So I do have to have a 4 wire cable, well I think I do??  Any way I will leave it to you guys.  I have to have them 100 meters/yards away as there is no room in engine shed for all the heaters!   And it's hot enough in the engine shed as it is with out all the extra heat I will be producing!!! 

 

As for photos, how the hell do I get photos up on here????! 

 

Thanks for your continuing help

 

Steve

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You have to have done ten posts first, so what else you got?? We know all about welding sets, lathes and milling machines on here you know, not to mention boats and racing motorbikes..

 

john...

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Hello again..

 

Has everyone forgotten about you??

 

Ok, PROVIDED you have an OCPD [fuse or whatever] rated at 100A at the generator end to protect the cable, you need a 35mm 4 core cable assuming that it is going to be installed underground [either directly in the ground of in a duct] How the cable is installed you see, makes a difference, as it affects the cooling of the cable.

 

I have selected this cable size, assuming a thermoplastic cable designed to run at 70c. Most cables now are thermosetting ones, and can run at 90c, but this is a bit pointless, as most "accessories" are only rated for 70c.....

 

Arrangements for earthing and fault protection the others will advise you on, as their knowledge is FAR greater then mine.

 

One thing i do not have a clue about is earthing and fault protection for generators, because, one thing you have to achieve in the event of a fault, is the required "disconnection time" both in the event of an earth fault, and also in the event of a fault between live conductors.

 

The disconnection times as regards earth faults are as required by the regulations, [it will be 5 seconds for your circuit]

 

However, you also have to make sure that the OCPD operates fast enough in the event of an fault betweeen live conductors......

 

I think people tend to forget about this, as there are no set times, but so far as i can recall, all the calculations in the big book as regards the heating effect in the cables in the event of a fault, are based around a disconnection time of UP TO 5 seconds. If you exceed this, the cables COULD be damaged.

 

What concerns me, is that to check on this, you have to know what the prospective PEFC and PSSC are. I have no idea if a generator can put out enough fault current to operate the protective devices as intended. With the mains you would just measure the circuit impedance to check, not sure if you would get a valid reading with a generator..... No idea......

 

Over to the professionals i think!!!!

 

john...

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Sounds a fun project.  One we all want to see running on youtube.

 

The normal thing with a genset like this is earth the star point at the generator.

 

Do you really need to run it at full 100A load? if it's just for fun would a lower power load do?  It just seems you are going to burn a lot of fuel just to heat up a remote shed (that's nearly 70KW of heat to get rid of you know!!!!) 

 

Normally when sizing a cable you take into account volt drop so the voltage at the load remains within acceptable limits, so a long run of cable ends up with a bigger cable than needed just to handle the current. But in this case I wouldn't bother with that, just size the cable to carry the load current.

 

5 more posts and we can see some pictures, unless an admin comes along and upgrades your posting rights (please)

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Yes, sorry engineman, you have been somewhat forgotten!

 

We are all nuts on here, well many of us anyway.

 

35mm sq will be OK for current as Apprentice says.

As you are intending to draw the full power at the far end, then the voltage drop will be above that allowed under BS7671.

This will not really be an issue as it is only just outside limits, and there are ways to show it's OK, with some clever maths.

However, it's irrelevant for your use, plus the more energy the cable dissipates (within reason) the less the load banks have to! ;)

 

Are you planning to run this for minutes, hours or days at a time.

The reason I ask is that the cable will reach a thermal equilibrium, and we need to ensure that it does this within the limits of the cable to save you melting it

 

You need to be careful with the terminations because the Joule heating across a high resistance connection @ 100A will quickly cause deterioration.

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As Dave says,

We REALLY want to see a video of this,  :D

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We are looking for something like this:

 

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Well, let's help the guy get it going then he can get his video up! ;)

 

BTW, engineman, keep posting because when you hit 10 posts you can post pictures & hopefully links to your previous endeavours! ;)

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was playing with cable calculator on Doncaster cable web-site. Using Thermoset cable seemed to significanlty reduce cable size required = cheaper.

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Yes, it might from the point of view of current, but the voltdrop will be much worse. The table i looked at [ the one in the regs] gave the 70c, 35mm, 4 core, a rating of 98A when buried direct, a mA/V/M of 1.1 and so a volt drop, of eleven volts, or about 2.75%

 

Seemed ok to me??

 

john..

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Well engineman has now reached 10 posts, so he should find he's able to post pictures and links now.

 

There are at least a dozen of us eager to see a picture or even a video of this machine.

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Hi engineman,

How is the cable to be run, as in, will you bury it, clip it to a wall etc.?

This makes a difference in the current carrying capacity of the cable.

You say it's running from an alternator, is this d.c. or a.c. output.

You said fused @ 100A per phase, so I am guessing 3 ph a.c.?

Do you want to consider the requirements of BS 7671 so that the install will be compliant, and,in the event of an issue, your insurance company can't wriggle out of the situation?

Or, are you just interested in a cable that will carry 100A without melting?

BTW, this might sound harsh, but it's not meant that way, I just need to understand a little more of what you have and want to achieve.

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What Sidey said  ..plus  ... are you actually going to use power up to 100A per phase ...in a workshop  ?????  

 

When you say that to electricians we start thinking of a small engineering manufacturers ...capstan lathes , power presses...air compressor...10  -  20 employees etc etc.

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Hello and sorry for the late reply.  Its the silly season (Harvest time) for me as I fix things for farmers who seem to brake there toys a lot more at this time of the year!!  Thank you to all who have posted to my original question regarding the cable.

I have owned the engine in question for around 20 years but only found and bought a suitable alternator in May!  The engine an Allen semi diesel built in 1926 is a hot bulb and requires the use of large compressed air blow lamps to heat up the bulbs on top of the cylinder heads like pro Dave's picture above.

 

The alternator is the hard bit to find in the UK as there value for the copper was very high so almost none survive!!  Anyway found one in Sweden and shipped it back!  Now I have to install the engine and alternator in the shed (which I have been building for the last year!) Now I know what cable to buy (thanks to you guys) I will now get it ordered (£900 odd pounds) unless anyone knows of it cheaper? 

 

I wil try and upload a picture in a day or so when time permits.

 

Thanks again and watch this space.

 

Steve

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Sounds good to us lot!!

 

john..

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...I wil try and upload a picture in a day or so when time permits...

 

YES PLEASE!

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I'll do you a picture of me mangle,  :D

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Hello The Snake and Spark 2014, Thank you for your replies to my question. I have this old 1920's 3 phase alternator working at 400 volts AC which is coupled to a large 1920's Blow lamp start two stroke  semi Diesel engine, I am planing to put this old set up on full load and to do this I will be drawing 100 amps per phase, using 3 phase heater banks which I can build up in 15Kw stages.  The cable will be in the ground so I won't be able to see what its doing except at each end!  

 

This is only a play thing but still has to be safe! I am reasonably confident about connections and earthing etc, I have to buy the cable and I dont know what size to ask a price for??  Plus i dont want to blow a big hole in the ground and have to dig the bugger up again! 

 

Sorry to seem weird but I play with theses sorts of Engines and Loading one right up is something I have been wanting to do for some 30 years now, as running engines light does not have the same effect!!!

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No!! No!!!! Not weird at all!!! We like that kind of thing on this forum!!!!!

 

Why do you want the load bank 100 yards away from the engine though??

 

What other engineering type stuff you got?? Oh, and photos!!!!!!!

 

Not sure the earthing will be as simple as you imagine, but these lot on here will know what to do!!

 

This alternator, is it 3 or 4 wire?? What i mean is, do you just have the 3 phases coming out, or a neutral too???

 

Oh, one more question.. This cable.. Do you care about volt drop at the load end, or do you just want to be able to connect the load bank to the alternator, this might make quite a difference to the size of cable you need..

 

john...

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How refreshing to see a question that isn't

What is the reg fir this as i am working away and not got the book with me

I have taken down a light and there are a lot of wires

The gas man says.......

I have removed a switch and there are a lot of wires

My electrician has been abducted by aliens and i have not got a certificate

Etc tec tec

We LIKE ODD stuff!

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Most of us on here are ODD FFS,,,,,  

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