[time-nuts] Line Voltage - USA

Gregory Maxwell gmaxwell at gmail.com
Mon Jan 2 03:36:58 EST 2017

On Mon, Jan 2, 2017 at 4:49 AM, Bill Byrom <time at radio.sent.com> wrote:
> Most US homes and small businesses are powered by what is commonly
> called a "split-phase" 240 V feed. The final distribution system
> transformer has a 240 V center-tapped secondary. The center tap is
> grounded, and three wires are fed to the building (actually it might be
> up to around 6 houses):
> (1) Leg L1 or phase A (red wire) -- This wire will measure 120 V to the
>     neutral or 240 V to Leg L2.
> (2) Neutral (white wire) -- This wire is grounded at the distribution
>     system and at the service entrance to the building.
> (3) Leg L2 phase B (black wire) -- This wire will measure 120 V to the
>     neutral or 240 V to Leg L1.

When someone here previously mentioned observing high voltage, one
possible cause for this in this common "split-phase" configuration  is
that if the neutral wire is overloaded, damaged, poorly connected, or
otherwise has high resistance,  the voltage on the two legs will swing
wildly and in opposite directions depending on load.

So, e.g. if you put a 1kw load on L1 while L2 is nearly unloaded then
perhaps L1s voltage drops to 108v while L2 rises to 132v.

The reason for this is that, e.g. imagine that the neutral were
removed completely you would effectively be connecting your appliances
in a parallel-series circuit (all on L1 in parallel, all on L2 in
parallel, the both in series) across the 240v feed.

I've had issues with neutrals several times in the past, and in one
instance, temporarily dealt with it by moving as much of the load to
240v as I could,  manually balancing the remaining loads, and then
using a digital multi-meter to dynamically control some additional
load to keep the voltage sane on each side.

I think the fact that you can end up with a much higher voltages at
the outlet if the neutral has problems is one of the more unfortunate
properties of the split-phase approach.

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