[BC] Electrical Code

[Lamar Owen]

> 120/240 three-phase (208 volts from the B phase to the neutral) is
> probably much more common than 120/208 three-phase.

> Certainly more new high-powered installations are 120/208 Wye or 277/480
> Wye, with 120/240 single-phase being derived locally, using a
> customer-owned "dry type" transformer.

What do you consider to be 'new', Peter?

WKJV in Asheville, NC, had an original 120/208 WYE run back in 1946 to run
an RCA BTA-5F; let's see, it was a 200A service IIRC (72KVA if I've done
my math right).

PARI's feed is much larger; we have four 1,000KVA services (two are hot at
this point), two 750KVA services (neither are hot), a 500KVA service (not
hot), and a small 22KVA service (sprinkler pumphouse, due to NEC
requirements has to be on separate primary).  Of the two 1,000KVA services
we are using, one is 277/480 and the other is 120/208.  The 120/208
services (two redundant 1,000KVA pad-mounted transformers owned by the
utility, with a two-to-three redundant main switching setup with tie
breakers and such) were put in in 1961, and the 277/480 was put in in
1991.  Other services around site were put in between those.  The 22KVA
service is also 120/208.  We run one of the 120/208 1,000KVA services, and
one of the 277/480 1,000KVA services, and route the rest of the switchgear
to power all the buildings from those two.

The 120/208 1,000KVA services have some impressive Westinghouse switchgear
associated; the busses are rated 6,000A, the two ties are 4,000A each, and
the three mains are 2,000 each.  Distrbution from the outside GE
switchgear (where the two-to-three conversion occurs) and to and from the
large distribution transformers is with 6,000A rated enclosed busbar,
except for two, which are fed with parallel multi-conductor 500MCM NM-type
cable.  One 500KVA transformer gets its feed with 5 parallel runs of the
500MCM/3 250MCM/2 NM cable.  Smallest branch circuit in the primary
switchgear is 400A.  Site distribution is at 2,400V delta; delta-wye
distribution transformers all over the place, with the smallest being
225KVA, and the rest ranging from 500KVA to 750KVA.  I'll take some
pictures of the various switchgear and transformer vaults if you'd
like....

I have a load sheet showing that when the site was in full operation as an
NSA facility it drew over 2.5MVA from the utility.  One of the
requirements at that power level is to have power-factor correction
capacitor banks; there are four banks of various capacities, hung off as
branch circuits with 400A and 1,200A breakers.  The outside switchgear has
the power factor metering necessary; 80% or less gets us a penalty on the
utility bill, which, although it's a small percentage, can add up to a
$1,000 to the monthly bill, depending upon demand.  We're running on a
shoestring right now, power-wise, only drawing a few hundred KVA (we're
not running the large underfloor 100-ton chillers; the site has a total of
750 tons of chiller capacity).  Cranking one 100-ton Trane would set us
back about $10,000 a month for 12 months if we ran it longer than 15
minutes due to demand.  Since the startup current is so high, we'd
probably at this point have to give 30 days written notification to the
utility before throwing power to it; locked-rotor current is over 1,800A
at 120/208.

A little smaller is WGCR's site, which has a 100A 120/208 service (36KVA).
 The Nautel 10K transmitter is so efficient that that's all it needed. 
That was the default installation for Duke at the time, which was 1989.

However, WJFJ in Tryon has a 120/240 delta installation, and has all the
required warnings.  It's a bigger install than the WGCR one due to the
much less efficient Continental 316F transmitter, and was built in 1984,
originally as an open delta, but now is closed.  That building has two
separate services, one at 120/240 single phase, and one at 120/240 delta. 
The delta service is, I think, 200A, and the single phase service is also
200A (heat pump is on that service).
-- 
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC  28772
(828)862-5554
www.pari.edu