Grouping cables under raised floor
- Thread starter PowerdT
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Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
According to 390.17 you have to follow 310.15(B)(3)
Then for 5*3=15 active conductors the adjustment factor is 50%.
If the insulation withstands 90oC [XLPE or EPR ] then you may start from 40 A. Then 40*0.5=20 A [still less than 30 A permitted in 240.4D]
Then for 5*3=15 active conductors the adjustment factor is 50%.
If the insulation withstands 90oC [XLPE or EPR ] then you may start from 40 A. Then 40*0.5=20 A [still less than 30 A permitted in 240.4D]
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Not sure what you mean by a power whip. Is it a SO type cord or MC cable Or individual conductors?
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
Some are liquid-tight flex. If they're individual assemblies, I wouldn't be concerned about "grouping".
You could maybe put two circuits in each flex conduit run (this would be 8 current carrying conductors and you'd probably be able to still use them a 30A each because of the low ambient temperature of most server rooms under the floor). But why combine them? Racks tend to move around. Now if a rack typically needed two L21-30's then I'd consider running two circuits in each conduit and putting two L21-30's at the end of the conduit.
Assuming you have flex whips under the floor, or EMT with short flex whips. Is that what you have or something else? Any by grouping, did you mean grouping flex conduits together, or running multiple circuits in a raceway?
Assuming you have flex whips under the floor, or EMT with short flex whips. Is that what you have or something else? Any by grouping, did you mean grouping flex conduits together, or running multiple circuits in a raceway?
Don't group them. The only issue I've seen is when you do the cable mining, removing the old cables from the floor. If all the cables takes the same route from the PDU, turn 90 deg at the same point, run and turn together, it makes the future cable pulling out into an unnecessarily difficult job.
I would want to run them radially from the PDU to each load. The cables that are not packed together into group structures pull in and out much easier.
I would want to run them radially from the PDU to each load. The cables that are not packed together into group structures pull in and out much easier.
Instead of pulling individual conductor from the PDU to the rack. I was thinking to group them withEMT and terminate into pullbox then from there to use flex whip to receptacle
You could do that, but you will have to upsize the conductors (probably to #6) due to bundled derating. You would then transition back to #10 in the whips so you need a splice box.
Usually, i think it is better to use more smaller conduits than fewer larger ones with larger conductors. No splices. No conductor confusion. Easier to relocate the whip.
Usually, i think it is better to use more smaller conduits than fewer larger ones with larger conductors. No splices. No conductor confusion. Easier to relocate the whip.
(3) 3/4”C EMT each with 2 circuit or even upsize the conduit size to 1”, just in case they add few more equipment later on
With 3 phase computer loads, you're about done once you've done 2 circuits. That's 8 current carrying conductors. Putting in more than 1 more current carrying conductor drives you to a derate that requires conductor size increase. So no point having larger conduits than needed unless they are considering going to a 50A or 60A plug. This isn't common but possible. The L21-30 was my favorite server room receptacle as it was easy to turn into six 5-15's with a PDU, a 6-30, or three L5-30's with plug adapters. We worked with other companies a lot so things would show up that we had to power up and it would take our Facilities group months to change an outlet from one flavor to another.
That is true, I will keep only 2 circuits. thank you very much on the advice
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