Every once in a while, we need to tell the experienced sound guys to take a break and re-introduce a basic topic that does not get discussed much. Power distribution (distro) connectors are such a topic. It’s as good a time as any to now go over the common types and discuss their usage.
NEMA 5-15
The National Electrical Manufacturers Association (NEMA) is a non-government organization that helps set standards for the industry, and has evolved from many separate associations into one. About a century ago, Thomas Edison invented the light bulb, but before it could light anything, the whole electricity infrastructure — from DC generators to wiring, switches, light sockets and receptacles — also needed to be invented. While not every item Edison’s engineers designed survived the conversion to AC power, instigated by the Nikola Tesla/Westinghouse/General Electric lobby, but many standard items did. One item was the two-pronged electrical plug and mating receptacle, now called an “Edison” plug and receptacle. In the 1950s, increased safety concerns brought the third or U-ground prong into usage, which now is described as the 5-15P plug and 5-15R receptacle. Figure 1 shows the typical examples for a heavy duty 5-15P plug and duplex wall-mounted 5-15R receptacle.
Also in the middle of the last century, the need for slightly more ampacity for kitchen electric gadgets and in business locations brought forth the NEMA 5-20 plug and receptacle. While the 5-20P plug is very similar to the 5-15P, the 90-degree rotated neutral prong prevents the 20-amp rated plug from mating with a 15-15R receptacle. Figure 2 shows the 5-20P plug and 5-20R wall receptacle. In sound systems, it fairly common to see 5-20P plugs on 20-Amp rated power conditioners and high power audio amplifiers, especially used on subwoofers.
NEMA L14-30
The NEMA L14-30 series is a higher power plug and receptacle commonly used in moderate sized power amp racks, and contains four prongs/contacts. The “L” in the L14-30 designation, means it is a twist-loc design with the prongs and receptacle slots curved for the twist feature. The 14 stands for the “14” series of designs, and the 30 is the maximum AC current rating of the prongs. Of the four contacts, the neutral is across from the safety ground (prong with bent tab), with the two 120 volt hot contacts in between. This is described as a 120/240VAC design, as it is has the neutral as a center tap between the 120VAC contacts or 240VAC across the hots when used in single phase power distribution systems. Figure 3 shows the L14-30P plug and L14-30R receptacle.
In many modest sound company B and C rigs, it is common for the portable power distribution unit (PPDU) to have a few L14-30R receptacles to bring out power to amplifier racks located behind stacks of mains speakers at downstage left and downstage right. This makes sense as locating the amp racks close to the speakers makes the speaker cable losses low, and the audio fidelity from the speakers improved. Obviously smaller sound systems will use only one mains power amplifier rack, and run longer speaker cables across the stage instead. A good example of NEMA L14-30 usage is on some Motion Labs Rac-Pacs as a means of chaining PPDU power to the Rac-Pacs.
NEMA 14-50
For us club rig sound companies, the 14-50P “range plug” is very much a standard disconnect for small PPDUs. The 14-50P plug and 14-50R receptacle are the current standard interconnect for household electric ranges and contain 4-prongs for neutral, safety ground, and the 120/240VAC hots. While most electric ranges use the 240 VAC across the hots for the oven heat elements and stovetop heat elements, the addition of the neutral allows appliance manufacturers to use 120VAC light bulbs, clocks and control circuitry. Previously the NEMA 10-50 plugs and receptacles were used, but they did not have the neutral contact. Figure 4 shows the 14-50P plug and 14-50R receptacle.
Although informally used a couple decades ago, the NEMA 10-50 interconnect is forbidden to be used as a 120VAC power distribution connection because the safety ground and neutral wires would have to be shared on one contact. Forbidden or not, I still see 10-50R receptacles at older clubs, some shady sound companies with 10-50P distro plugs and even an occasional extra green safety ground spring clip. If that neutral/safety ground contact should ever open circuit, a lot of 120 to 240VAC hot shocking current would be found onstage looking for something or someone to complete the path to ground.
A Reminder
If you have not done it recently, make sure all your AC distribution cabling gets the connector contacts cleaned periodically, or at least during the build of the cables. For me, contact cleaning is a bunch of Q-tips and small bottles of Caig Labs red DeoxIT and blue PreservIT fluids. The process is to use the red bottle brush and liberally apply the fluid on all the metal prong surfaces and in the cable receptacle slots (unpowered of course); and then use the Q-tips to absorb the extra DeoxIT while leaving a uniform coating. The blue PreservIT fluid is applied and removed the same way, with the second Q-tip collecting both red and blue fluids in excess. The red fluid removes the metal oxidation and carbon build-up, while the blue fluid protects against future oxidation. As a handy preventer of warm AC connections by having coated contacts, some of that fluid will even help wipe and clean the venue’s receptacles.
