paralell cable connection

[honglien]

hi everybody!
I need your help for my issue, that is cable connection in paralell.
I have to supply for a load through an MCCB-3P-400V-500A. I used a Cu/XLPE/PVC/DSTA/PVC-3x240mm2-1x150mm2, but it was not cable to carry whole load. so I used one more Cu/XLPE/PVC/DSTA/PVC-3x70mm2-1x50mm2.
1. Is it ok to connect these two cables in paralell?
2. How can I connect 3 through-type Current Transformers in this case? Where CTs are connected before MCCB.
Thanks alot!

 

[davidbeach]

Nope, you can only parallel matched cables. The 70mm² cable will have a higher impedance and will carry less than its share of the current, leaving the 240mm² cable overloaded.

 

[waross]

This installation will not meet North American codes.
If you are outside of North America, and not subject to stringent codes, you may parallel dissimilar cables. The cables will share the current in inverse proportion to the impedances of the cables. You should look up the impedances (NOT the resistances) and determine the percentage of the load that each cable will carry. Then check the cable ampacities to be sure that no cable is overloaded. Verify the currents with a meter after installation. It is still not a good idea.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

Honglien,
You would need to pass both cables through the CT, if the hole is too small perhaps a short section of bus.

Warross,
Code issue aside would the impedance miss-match self correct as the overloaded cable heats up, raising the impedance. Does NEC even allow you to use conductors in parallel without special approval?
Regards
Roy

 

[davidbeach]

No special approval required; just need to be 1/0 or larger and identical characteristics. I certainly wouldn't count on the mismatch being self correcting.

 

[jeebusmn]

NEC 310.4
It has to be terminated the same, have the same length, same material, area, insulation and be 1/0 or larger. Each cable has to be sized handle the full load or each has to have its own protection

 

[ScottyUK]

Each cable has to be sized handle the full load or each has to have its own protection

I agree about same length, cross section, material, route, etc for paralleled conductors but this is a new one for me.

So in an installation with, say, 4x 500mm² single core cables per phase where any one of those conductors is far too small on its own to carry the load current you say that each core needs its own protection? I am really surprised. I've never seen, or even heard of, an installation like that anywhere.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

ScottyUK, you are absolutely correct; it doesn't require that. There's no way in he!! that you can do a cable run over about 600A (and in practical terms over 380A) without paralleling conductors. The whole paralleled run is protected at or below its ampacity. The NEC does require full sized (but need not exceed the phase conductor size) grounds in each parallel run.

 

[itsmoked]

Full size grounds? You mean if a phase is (4) 4/0 cables you need (4) 4/0 grounds and you need one for each phase? That appears to be what you've stated.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jeebusmn]

You guys are right. That second sentence I think I pulled from some recommend text for parallel feeders to reduce the voltage drop.

 

[davidbeach]

Keith, no, if the feeder requires a 4/0 ground based on circuit ampacity then each conduit must include a 4/0 ground. If the circuit requires a 500 ground but you are building your circuit out of parallel sets of 350, then you needn't go beyond 350 with your ground.

 

[itsmoked]

Ok. Thanks for the clarification.

Keith Cress
kcress -

http://www.flaminsystems.com