Sizing conductors for loads with large startup current

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JoeNorm

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When doing voltage drop calculations should I use the starting current for the load current? Lets say i am sizing wire to run a well pump, which often have long runs associated with them. The running current and startup make a huge difference in the calculation. Or do you just use the sizing guides from the manufacture, seems like they usually give you a lot of info. Thanks
 
I use the starting current and article 695 Fire Pumps for seeing if a motor will start. 695 limits the startup VD to 15%.
For the 3 and 5% VD numbers I use running current.
 
I concur with mayanees. I believe that good design dictates that you consider the operational effect of voltage drop during startup separately concurrently with voltage drop during running.

IMHO it doesn't matter if you have a huge voltage drop during startup as long as you reliably start, but 'reliably start' is the key. So you calculate the voltage drop during startup and consider if the load characteristics will tolerate this. 15 or 20% is probably reasonable for starting a motor load if the starts are not too frequent.

Separately calculate the running voltage drop and check for reasonableness there. If either VD is too large, then increase the conductor size.

But while it may be reasonable to have a 3% drop for a running load, it is probably excessive to demand that for startup.

-Jon
 
One wants to be careful when sizing conductors to a submersible pump. Not only do you have the distance to the well but the depth as well. And the submersible manufacturers like Franklin and such are kind of picky about using their tables for sizing as submersibles have some unique characteristics.
 
Thanks. Thats good information. To complicate things further, lets say you are sizing cable to run a good distance to a sub panel that will have the well pump breaker. This sub panel may have other loads connected in the future, say an RV hookup perhaps. We deal with this kind of thing in rural areas where a few acres are being setup with infrastructure for future use. There are no plans to work from, there is a lot of guesswork. One approach is to oversize things greatly, but pulling large cables all over the place is nice to avoid unless it's actually needed. But I suppose it may be necessary to cover all possibilities.
 
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