proper spacing of 208v 1ph feeder conduits

Status
Not open for further replies.

EHE

Member
Location
North Carolina
In a five story multifamily apartment building with several 208v 3ph 4w multimeter service gear assemblies, each with individual 102/208v 1ph 3w 125A branch mains and meters, there will be multiple residential unit panel feeders to individual dwelling unit main panels. Using building wire in conduit, what spacing is recommended for the parallel feeder conduit runs?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
You mean that the raceways are run in parallel not the conductors, correct? Personally I like a 1" or 1.5" space between conduits.
 

EHE

Member
Location
North Carolina
Thanks. Yes the feeder conduits would have parallel paths in free air till points of off-branching to panels, each conduit containing two hots, a neutral and a ground. Was curious about effects of spacing on inductive heating effects in adjacent conduits possibly requiring de-rating.
 
Ya know this is a interesting point. We cannot bundle NM wires without derating but we can make nice looking parallel runs of EMT or whatever with no hassles. I used to love to make parallel EMT runs in exposed areas.
I realize EMT being a ferrous metal would probably cancel inductive heating but PVC does not.

go figure.
 

rcwilson

Senior Member
Location
Redmond, WA
With properly wired 2 hots, neutral and an EGC, there is no magnetic field outside the conduit unless there is a fault. All current going out on either hot wire returns on the neutral or other hot. With the same current going opposite directions, any induced magnetic fields are cancelled. So there is no concern about induction. The 208V three-phase makes no difference.

There might be a spacing concern on the conduits, but not likely. Derating for grouping is not explicitly called out in the NEC. The Okonite cable handbook and other sources have derating tables for grouped conduits where spacing between conduits is < conduit diameter. Example: a horizontal rack of six 2" conduits with 1" space should be derated to 0.86 for one row, 0.79 for two rows, 0.74 for three rows. Three conduits vertically stacked are derated 0.85. (The table does not apply for spacing less than 1/4 diameter).

I would not worry about derating for your case. The table is based on derating cables from the manufacturer's stated free air ratings, which are usually higher than the NEC allowable amps. Table values also assume 100% load factor with all circuits operating at full ampacity 100% of the time. Other factors are used to account for load diversity and variable loading over time.

The NEC tables probably account for these factors already. If they didn't, there would be a lot of melted circuits out there.
 
Status
Not open for further replies.
Top