1 or 2 dedicated 120VAC circuits for my CNC? - Page 10

You'd have to contact Belden for exact answers, but here's my speculation...

The typical NEC-based ampacity chart covers single-conductor wires.  What you're looking at below are multi-conductor cables... and there are a bunch of specifications for multi-conductor cables.  And the max ampacity depends on numerous factors - insulation type, thickness, number/size of individual conductors, specifications to be met, etc.

The numbers you posted appear to be the manufacturer's ampacity rating for the environment in which the cable is operating.  In the first case Belden considers 30c to be the 'standard' environmental temp for that specification cable, and 25c for the other.  And the spec could be read as "this cable can handle up to 8 amps per conductor when operating at 30c"

Note that one of the cables is "PLTC" rated (power limited tray cable) and the other is not.  It may be that to meet PLTC specs the rating has to be given at 'xxx' temperature.

So addressing your questions:
1. Because each individual conductor should stay below the individual conductor max temp of 105c when the cable is operated at xxx amps at yyy temp.  To see if this is the case, look up other cables in the same Belden family and compare the 2-conductor to a 12 (or more) conductor of the same AWG.  I bet the high-conductor count cable has a lower amp-per-conductor rating than the 2-wire.

2. Because they're different cables, meeting different specifications and intended for different use-cases.

3. I would treat  the number given is their max rating, but you should contact Belden for clarification.
 

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MJR7 said:

I was told that it is possible to replace an encoder cable which has braid shielded wires inside with one that uses no braided shielding but has wires twisted together in pairs for noise protection. Anyone familiar with this?

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In most cases this is not a good idea. The twisted pairs reduce cross talk between encoder channels. They do little to shield against outside noise.

The chances of having a problem depends on several factors. If you are using 5 volt single ended encoders then it is most certainly not a good idea. The presence of large contactors or a VFD are other reasons for shielded cables.

Shielded encoder cable is available in short lengths from McMaster Carr, I think in ten foot increments. The price is reasonable.

If you are really going for cheap then use good quality shielded cat5 patch cords. They are too flimsy for an industrial machine but at least they reject noise until they fail.

The argument over terminating which end or both ends of the shield is endless. Just do whatever was done with the original cable and you should be safe.

Doug

In most cases this is not a good idea. The twisted pairs reduce cross talk between encoder channels. They do little to shield against outside noise.The chances of having a problem depends on several factors. If you are using 5 volt single ended encoders then it is most certainly not a good idea. The presence of large contactors or a VFD are other reasons for shielded cables.Shielded encoder cable is available in short lengths from McMaster Carr, I think in ten foot increments. The price is reasonable.If you are really going for cheap then use good quality shielded cat5 patch cords. They are too flimsy for an industrial machine but at least they reject noise until they fail.The argument over terminating which end or both ends of the shield is endless. Just do whatever was done with the original cable and you should be safe.Doug

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