Re: I Need Electrical Help!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>

This can potentially work, but has a number of pitfalls (aside from the
"non-code" part you already identified).

1) The current won't necessarily divide evenly.  It depends on the series
resistance between the distribution panel and the join point on each leg,
and also on how much load is on each leg already.

Example: Typical max resistance in one branch is around a tenth of an
ohm.(to meet the code requirement of max 2% drop at rated load)  Say one leg
is 0.05 ohms and one is 0.1 ohms. Simple Ohm's law calcs will show that one
leg will carry twice the current of the other (that is, the current will
divide 66/33 instead of 50/50).  Now add in the uncertainty of the
resistance at the plug/receptacle joint, your extension cords, etc.

If your receptacles are fed from subpanels, fed by bigger feeders (very
common in non-residential situations), then this applies in spades, with the
additional hassle that the voltage at the subpanel may vary depending on how
much load there is on the whole subpanel, which will then cause your
currents to distribute unevenly. In fact, if your load isn't connected, you
might even be feeding power from one circuit to the other.

Example: Same as above, branch circuit resistances are 0.05 and 0.1 ohms
(call them circuit A and B, respectively).  A 10 Ohm load (around 10-12
Amps) connected on each branch (not your load, something else already
there...). We'll make the numbers easy by assuming 100VAC.  The voltage on
Load A is going to be 99.5 volts.  The voltage on Load B is going to be
about 99 volts (i.e. more drop in the branch circuit).

Now, connect your lashup between circuit A and B, with no load.  Assume it
has a resistance of , say, 0.01 ohms on each leg, so you've effectively
connected 0.02 ohms between Load A and Load B.  A little calculation will
show that the voltage on B will rise a bit, and that on A will fall a bit,
because some current will now be flowing from A to B, through your cords.


2) The big safety reason.  When you do this, you are "backfeeding" one of
the circuits, so that if it is turned off (say, by the circuit breaker
tripping), the entire load will now be supplied by the remaining circuit,
feeding through your lashup.  Bad news indeed, especially when you think
that because of the uneven distribution that is inevitable, one circuit's
breaker will probably trip before the other. The saving grace is that if the
first breaker tripped, the second one is likely to trip right away, since it
probably is overloaded by a factor of 2.


A better way (but not great) to get more power from two outlets is to find
two outlets that are on opposite phases (i.e. there is 240V between them).
Run through a dual ganged breaker to your load. This still has the problem
that if only one branch circuit feeder trips, you are going to backfeed, but
at least it is through your load.


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, February 11, 2001 10:04 AM
Subject: I Need Electrical Help!


> Original poster: "Drew Murray by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<drewallmighty-at-hotmail-dot-com>
> My plan was to take two extension cords and clip off the female
connectors.
> i will then use a multimeter to test and see if the neutrals and the
grounds
> are on the right wires. Sometimes i have noticed that the hot and the
ground
> are mixed up on non professionally installed wall plugs. I plan on taking
> the hots and the grounds on the cords and connecting them together and
> attaching a length of 30a rated wire to them and using marettes to hold
them
> together. This i think will allow 15a to flow through each cord and 30a
> through the bigger cord to my 120-240v step up transformer and then to my
> pt. I will be sort of sharing two 15 amp breakers to get 30 amps.
> Is this possible or will i burn the school down? It may not be code but
will
> it work? Somebody let me know before i go ahead and try it anyway!