Question about Parallel VFD Cable Runs

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The Great Sarducci

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Guys,

Quick question regarding using multiple parallel VFD cable runs to a large induction motor from an IGBT VFD drive system. To start, allow me to briefly describe the situation so that y'all can get a better overall picture of what I'm talking about;

I'm currently in the process of selecting some VFD cables to run thru a tray from the drive cabinet over to a large induction motor rated at 630A FLA. Long story short, I was originally planning for multiple parallel runs of #4/0 awg in order to cover the required ampacity. But now that we're thinking about going with an all new type of VFD cable rated at 110°C vs. the prior cable, which was rated at 90°C; the new combined ampacity ratings to feed the motor end up being about 180A more than we need with three 4/0s, and about 90A too low if we use only two 4/0 cables (quite obviously not an option here).

Now finally to my question;

While I know that its certainly acceptable to combine multiple identically sized cables in order to raise the overall circuit ampacity to meet required load demand for the motor; I'm now wondering, is it ever acceptable to combine cables of different sizes in order to arrive at a closer total ampacity to what we actually need? For example, is their any specific reason why I would not be able to consider perhaps two 4/0 cables & one 1/0 cable?

a Side-Note: Just to clarify; I realize that it never hurts to have the extra ampacity far and above what I need to drive the motor at full rated load. If cost weren't an issue, then I'd just use the three 4/0 cables and wouldn't even be asking this question. But unfortunately in my case, the powers (cheap-asses) that be, care an awful lot about the extra cost.
 
The Great Sarducci said:
Guys,

Quick question regarding using multiple parallel VFD cable runs to a large induction motor from an IGBT VFD drive system. To start, allow me to briefly describe the situation so that y'all can get a better overall picture of what I'm talking about;

I'm currently in the process of selecting some VFD cables to run thru a tray from the drive cabinet over to a large induction motor rated at 630A FLA. Long story short, I was originally planning for multiple parallel runs of #4/0 awg in order to cover the required ampacity. But now that we're thinking about going with an all new type of VFD cable rated at 110°C vs. the prior cable, which was rated at 90°C; the new combined ampacity ratings to feed the motor end up being about 180A more than we need with three 4/0s, and about 90A too low if we use only two 4/0 cables (quite obviously not an option here).

Now finally to my question;

While I know that its certainly acceptable to combine multiple identically sized cables in order to raise the overall circuit ampacity to meet required load demand for the motor; I'm now wondering, is it ever acceptable to combine cables of different sizes in order to arrive at a closer total ampacity to what we actually need? For example, is their any specific reason why I would not be able to consider perhaps two 4/0 cables & one 1/0 cable?

a Side-Note: Just to clarify; I realize that it never hurts to have the extra ampacity far and above what I need to drive the motor at full rated load. If cost weren't an issue, then I'd just use the three 4/0 cables and wouldn't even be asking this question. But unfortunately in my case, the powers (cheap-asses) that be, care an awful lot about the extra cost.
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They all MUST be the same size.
I don't think that the added ampacity at 110C will be of use anyway; there is no 110C column in the NEC.
 
Thanks for the quick reply Jraef. That's all I needed to know, and now have a stronger position for my design to necessitate three 4/0 cables despite the extra cost.

As for the 110°C cable ampacity ratings; these were figures that I obtained directly from the cable mfg. (Nexans/Amercable) cut sheet. But now that you mention the fact that the 110° ratings are not even provided for in the NEC, then perhaps I should just size everything based on the 90°C ampacities in the NEC table to be on the safe side.

Thanks again,
Ryan
 
Has the OP considered the fact that he will need the required ampacity @75 degree at the terminations at each end? In most cases the cost of transition to larger conductors at each end outweigh the cost of just sizing the whole run at 75 degree.
 
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