[R-390] R390A power cord specifications

[Jim Whartenby]

JohnThe line cord for the R-390 is CX-1358/U which is an 8 foot long with two 18 AWG conductors for hot and neutral.  There was no ground wire as part of the power cord in the original R-390.  See: https://aerospace-supplies.com/wire-assemblies/electrical-power-cable+assembly/cx1358u
I would expect that this specification was carried through with the later R-390A but with an added ground wire.

Power consumption for the R-390 is 270 watts with ovens on and 170 watts with ovens off, per the manual.Power consumption for the R-390A is 250 watts with ovens on and 140 watts with ovens off, per the manual.
>From previous posts on line voltage:115vac,117vac and 120vac line voltages are just voltage standards, used todifferentiate between the two Edison inspired AC voltages availableto the home (originally 115 & 230), not the actual line voltage present at the outlet.  One cannot expectthe AC line voltage to remain at a steady state voltage throughoutthe day under the varying loads presented to the AC distribution system.
Theactual line voltages for the three standards are:115vac +/- 10% or 103vac to 127vac117vac +/- 7.5% or 108vac to 126vac120vac +/- 5% or 114vac to 126vac
These three line voltage standards are the result of surveys done in the 1920's,1940's and 1960's by either NEMA (National Electrical ManufacturersAssociation) or other groups that also included NEMA or it's laterincarnations. The purpose of these standards was to set a range ofline voltages for equipment manufacturers that would be availableacross the country. . As you can see,the maximum line voltage was around 126vac from the very beginning around 1927.  The lower line voltagelimit was the only voltage raised in the subsequent standards.
I would think that maximum power consumption would be at the high line voltage limit (126 vac), not at the average line voltage. 

I personally have not seen installers modify line cord lengths when equipment is installed in racks.  That would handicap operation on the workbench during repairs or the installation of the R-390A at another rack position.
The plug you mention is the UP-131M.  Besides having a rotatable ground pin, the neutral prong was wider then the hot so that it was polarized and would not allow a reverse insertion into a socket. There are at least two other models made by the same manufacturer, the APM company.  The original UP-120 with no ground pin and non polarized with both prongs the same width and a second with an external fixed ground pin which could be installed facing away from the socket so that it would mate with either a two or three conductor socket.RegardsJim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: John Wendler <wendlerjrv at gmail.com>
To: r-390 at mailman.qth.net
Sent: Thu, Dec 29, 2022 9:56 am
Subject: [R-390] R390A power cord specifications

Does anyone have an authoritative specification on the power cord for the
as-shipped R390A?  I do not see a length requirement in MIL-R-13947B and
have not yet spotted a part number in the manuals that I looked at.

The power requirement (275 Watts, max) in conjunction with the lower
voltage (115 VAC) says the maximum current that cord will carry is about
2.4 Amps.  The brackets that capture the power cord on the rear panel set a
maximum outer insulation diameter of approximately 0.36 inches. 18/3 SOOW
should work, but I was wondering about the historical lore.  I expect that
installers would have chopped or replaced as necessary for a neat
installation.

As an aside, one of my units came with a plug that had a rotatable ground
pin for use with either 2-wire or 3-wire power outlets.  Point the pin
forward for 3-wire outlets; rotate it 90 degrees out of the way for the
2-wire outlets.  The ground wire is secured by a lug to the screw that the
pin rotates around.
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