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Power Cords, and the like



With the recent series of posts talking about power cords, I'd like to
mention that I have a web page with the NEC ampacities for various flexible
cords at http://home.earthlink-dot-net/~jimlux/hv/cordamp.htm

My experience has been that the cord itself usually isn't the problem, but
rather the connectors, and specifically, the connection of the wire to the
prongs or pins in the connector.  For instance, when a stress is applied to
the cord, it pulls the wires out of the screw clamps.  Or, the blades of the
plug are inserted into a couple of springs in the receptacle.  Everytime the
cord gets yanked, the springs get bent a little more.  Most of us have
encountered 115VAC receptacles where the springs are weak and the plugs
don't stay in or make good contact.

Thermal fluctuations also cause problems.  As the connector changes
temperature, the various components expand and contract, often at different
rates, and connections can loosen, etc.


How do you make good cables and connectors?

Using good quality connectors.  When you are designing for cost, you want to
save all you can on manufacturing costs and materials, so you make the
springs out of the least expensive alloys, or, you use the thinnest wire
that will carry the current and/or meet code.  More expensive connectors
tend to have better quality components.

Using locking connectors (like the Twist lock series from Hubbell and
others).  The mechanical design of these tends to reduce the side forces on
the blades and pins when the plug gets bumped or pulled.  The really nice
connectors are the big plastic housing (or aluminum housing) multi pin ones
that have round pins, are weatherproof, and are quite durable. They're also
$100 each or more.  For high currents, the "cam lok" type are really nice,
except for the fact that they are only single conductor, so connection
sequence is important for safety.

Using good strain relief on the connectors. The cable is usually fairly
strong, as is the connector, so you want the loads from one or the other to
be transmitted cleanly, without being coupled into the conductors, or worst
of all the connection from pin/blade to wire.  A decent strain relief helps
a lot.  Companies like Kellems and Woodbury(?) make stuff that works real
well.  Just using a physically large connector helps, because there is more
slack wire inside the connector, and the contact area between cable and
connector is larger.