Since constructing the W8ZR StationPro II (SPII), I've been thinking about ways
to best manage AC power distribution for my desktop amps. The SPII is a
station accessory designed to easily route RF, audio, digital communications,
and control cabling between three transceivers and three amplifiers. Until
recently, I've been crawling under the operating table to switch my amplifier
AC power cables. I use one branch circuit consisting of a 30A OCPD (breaker)
and a #10/4 run to a 30A Twist-Lock receptacle. 4-wire cabling is used since
the older Alphas make use of a neutral for the blower.
To feed AC power into three amps, I had several choices: (1) add more branch
circuits; (2) add a J-box in the wall and spit one branch circuit that feeds
more than one receptacle; or (3) construct a relocatable power tap (i.e., a
240/120V version of a multiple outlet strip). I decided against options 1 & 2
since the house will one day go to a new owner and I didn't want to add even
more wall trauma. Further, the NEC limits option #2 to 20A circuits but as it
turned out, 20A service will be fine for my future needs.
Option #3 became the plan but what does UL have to say about 240/120V RPTs? UL
1363 sets the standard although another UL provision addresses portable power
distribution of the type generally found at construction sites. Even though I
will not mass-produce these RPTs , I wanted to know how close I could come to
making one that is 100% UL 1363 compliant. I got pretty close. In fact, it
complies but for the fact that the PVC J-Box has permanent mounting holes. UL
1363 states that any physical mounting must be effectuated without the use of
tools. Typically, we see home RPTs with mount blocks designed to slip over
screw heads so that the RPT is easily attached and removed without the use of
hand tools.
UL 1363 has a significant limitation: use of an RPT is limited to 20A service
and ALL device connectors must be of the same type and current rating. This
had a "trickle up" effect. Since the RPT is limited to 20A, then so are the
connectors. But now we move from UL to the NEC since the NEC is a premise
wiring standard and not a products wiring standard. As the plug is limited in
size to 20A, then the mating receptacle must be 20A. Finally, NEC 210.21(B) in
this application states that the serving OCPD must match the receptacle rating.
In essence, use of the 20A RPT forced a re-design of the branch circuit from
30A to 20A components although wiring is allowed to stay at #10/4.
130A-rated copper bus bars are used to distribute service into three outputs.
You'll see this in the photos. It was actually possible to forgo the bus bars
and use jumbo #10 winged wire nuts to join four #10 conductors together.
Kinda' scary, but they're used all the time. However, almost all of the wire
nuts are not rated for fine-stranded portable power cable. I really didn't
consider this a viable option.
A larger J-Box could have been used to install panel receptacles rather than
pig-tails but the box size gets pretty large when you consider space needed for
panel receptacles. Short pig-tails also allow the use of different styles of
20A plugs/receptacles, provided they remain the same type.
My amps are all legal-limit and 20A service is adequate. For many members on
this list, this RPT device would not work for obvious reasons. The #10 cabling
helps to reduce voltage drop losses over an 80 ft run. If MOV/TVSS devices are
considered for secondary surge protection, place only between Line 1 and Line
2, and one each from the line to neutral -- with no MOVs going to the grounding
conductor. Even then, I would not use MOV/TVSS unless "whole-house" protection
is first used at the service entrance as the primary surge protection device.
This is another reason why adding a neutral may make sense even if your present
amp only requires 3-wire service and no neutral. Adding the neutral also
ensures that you can properly use older Alpha and Henry amps in the future.
In a nutshell, here are some features of the RPT:
1) 1 x 3 device, designed for either 3 or 4-wire powering (240/120V);
2) #10 wiring and #10 hardware used;
3) Heyco compression connectors;
4) Nickel-plated copper bus bars (with protective covers) rated at 300VAC and
130 amps;
5) Dual fuseholders with 20A ABC ceramic fuses;
6) Dual neon light AC power indicators; and
7) 20A Hubbell Twist-Lock connectors used on all cable ends.
Some images in the links below. Photos taken before wiring the neon lamps:
http://tinyurl.com/oswgsfh
http://tinyurl.com/oqpkg2f
http://tinyurl.com/qeszuft
http://tinyurl.com/qjabggu
http://tinyurl.com/nlo9pk6
Paul, W9AC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|