Discuss cable sizing for 400a 175m 4 core in the Electrical Forum area at ElectriciansForums.net

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matt12321

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hey,

ive been asked to work out a cable size for someone and not sure on the sizing for the amps. the information received is that its to be armoured, 4 core and the distance is 175m and needs to carry 400amps. is there a cable big enough or will they have to run two in parallel.

thanks
 
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Guest123

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  • #2
Plenty of cables capable of carrying 400A.

I've just installed supplies to some roof mounted chillers rated at 400A at a university, a shorter run than yours granted, but they were 150mm 4c.

Obviously you need to factor in all the applicable cable factors, installation method, nature of load, temp, grouping etc etc to arrive at the size you need but a single cable will suffice IMO.
 

darkwood

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As with everyone asking these questions... you've supplied a quarter of the info needed, how can we even start to weigh it up if we dont know install methods, ambient temp, nature of the load (i.e. high inertia motor) etc etc ..... if you havent be given the info then i suggest you ask alot of questions about the install before posting them our way....
 
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matt12321

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  • #4
buried direct, no grouping, ambient temp 30, soil res 2.5
 
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Guest123

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buried direct, no grouping, ambient temp 30, soil res 2.5
I was just about to add, the only problem you might have is if the cable is buried directly in the ground.

I think you may need parallel conductors for this.
 
B

Bobster

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What are you using to protect the cable?
 

topquark

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As above plus is it single or three phase supply? If three phase is that 400A current requirement per phase?
 
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matt12321

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  • #8
300 a breaker i think, 3 phase supply and 400a across all 3 phases
 

topquark

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Three phase + neutral + earth .... no bonding to be done or are you hoping to use the armour for earth?
 
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matt12321

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  • #10
three phase and neutral and running a separate earth and no, no bonding
 
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Guest123

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300 a breaker i think, 3 phase supply and 400a across all 3 phases
You could have stated that from the beginning, save me going off on one!

400A per phase is totally different to 400A TP.
 
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matt12321

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  • #12
sorry lenny.

so any ideas after all the info on the size and if i can run one single or 2 in parallel.
 

topquark

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Well 2 x 50mm would give you a VD of around 5.75V .... you still haven't told us what's running on it though!. To achieve that VD with one cable you'd be up at 120mm (assuming I have my thinking head on).
 
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matt12321

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  • #14
i dont know what it will be running, he didnt say. what size earth will 120 mm need
 

topquark

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i dont know what it will be running, he didnt say. what size earth will 120 mm need
Then you need to ask him. Inrush current etc may need to be taken into account (unless he's already done that within the "spec" you've been given). As for the earthing requirements, what do the regulations tell you with regards to sizing?
 

darkwood

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300 a breaker i think, 3 phase supply and 400a across all 3 phases
I would be inclined to confirm this, its not really the way 3ph current is usually expressed, it also seems overkill fitting a 300A front end because of the implemented costings.

3ph 400amp is usually 400amp per phase
If what you say is correct then you are telling us 133amp per phase but with a 300amp front end protection this would all be ringing alarm bells as to what you exactly is required as i would suspect 3ph 160amp/200amp system would vastly reduce costs of both materials and installation while still giving a comfort zone.

So il re-iterate we have a serious lack of info especially regarding nature of loading, also as already mentioned your opening post was requestion info completely different to that of which you described later in the thread.

Im can only assume this is well out of your comfort zone and you may be panic questioning as some questions you have asked are clearly explained within the regs..... we cant know the additional earthing needed until we know:-

(A) The nature of the load (dedicated circuit, sub-mains(multiple ways)).
(B) The correct specs of what your asking which will give us a cable size, then we can attain if the armour is acceptable as the earthing alone.
(c) Does anthing need Bonding (Gas/Water steel etc) as this will need to be included when working out earthing.

I would personally in your case get someone of experience on board as a little mistake in design could be costly and IMHO we can only head you in the correct direction but a to give you a detailed answer would require a detailed spec and/or onsite visit.
 

topquark

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300 a breaker i think, 3 phase supply and 400a across all 3 phases
When you stated "across all 3 phases", I took that as meaning total current requirement at supplied location, IE 400A in total. I can see your statement could be read in two ways and you could equally have meant 400A per phase.
 
E

Engineer54

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  • #18
Over a 175m cable run, which is to be direct buried, i would always go for a parallel cable installation. It will cost less, it will be easier to handle and install, and give superior current carrying capabilities, and last but not least can also reduce your volt drop limitation...

Now if were talking 400A , a single cable based purely on current carrying capability is a 4 core 150mm SWA cable (408A). The parallel equivalent cable is 2 X 70mm 4 core SWA cable, which between them can carry 506A. You may however need to go with 2 X 95mm (608A) cables to cover any volt drop limitations on the 2 X 70mm cables...

I haven't done any calculations here, the current figures i have inserted are also only from memory, but they won't be far out if wrong... lol!!! As you can see, it makes sense to parallel rather than to struggle with a single larger size cable, and will generally come with benefits too ...lol!!
 
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