My boat has two 120 volt circuits with a common neutral wire similar
to most homes. Normally they are supplied with 220 volts from either
a generator or shore power (120-0-120). There are no 220 volt
appliances or circuits out of the breaker panel.

I would like to be able to feed both 120 volt legs with a single 120
volt inverter through a rotary selector switch (both legs in phase,
instead of 180 degrees out of phase). There would be no possibility
of backfeed or cross connection with shore/generator power.

Does anyone know of any issues with doing that?

I can't think of any problems that might arise, as long as there is no
possibility at all, ever, of connecting to 240 volt shore power with the
two 120 volt circuits connected in parallel! Doing so would probably
trip the pedestal breaker instantly before any real damage, but
nonetheless something to be avoided.

Go for it.

Chuck









Hoo wrote:
My boat has two 120 volt circuits with a common neutral wire similar
to most homes. Normally they are supplied with 220 volts from either
a generator or shore power (120-0-120). There are no 220 volt
appliances or circuits out of the breaker panel.

I would like to be able to feed both 120 volt legs with a single 120
volt inverter through a rotary selector switch (both legs in phase,
instead of 180 degrees out of phase). There would be no possibility
of backfeed or cross connection with shore/generator power.

Does anyone know of any issues with doing that?

On Mon, 14 Nov 2005 10:14:18 -0500, Hoo
wrote:

My boat has two 120 volt circuits with a common neutral wire similar
to most homes. Normally they are supplied with 220 volts from either
a generator or shore power (120-0-120). There are no 220 volt
appliances or circuits out of the breaker panel.

I would like to be able to feed both 120 volt legs with a single 120
volt inverter through a rotary selector switch (both legs in phase,
instead of 180 degrees out of phase). There would be no possibility
of backfeed or cross connection with shore/generator power.

Does anyone know of any issues with doing that?

My only concern would be the size of the neutral wire.

When fed 120-0-120, the neutral current can never be more than the
current in either hot wire (it will actually be the difference between
the two hot wires), but if the two hot wires are in phase, the current
in the neutral will be the sum of the two hot currents.

If your inverter is 2000 watts or less, and the wiring is #12, you
will be OK, as the inverter will limit the total current to about 20
amps, and #12 wire is rated for that. If you have a larger inverter,
you may have to use larger wire.


--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq

In article ,
Peter Bennett wrote:

On Mon, 14 Nov 2005 10:14:18 -0500, Hoo
wrote:

My boat has two 120 volt circuits with a common neutral wire similar
to most homes. Normally they are supplied with 220 volts from either
a generator or shore power (120-0-120). There are no 220 volt
appliances or circuits out of the breaker panel.

I would like to be able to feed both 120 volt legs with a single 120
volt inverter through a rotary selector switch (both legs in phase,
instead of 180 degrees out of phase). There would be no possibility
of backfeed or cross connection with shore/generator power.

Does anyone know of any issues with doing that?

My only concern would be the size of the neutral wire.

When fed 120-0-120, the neutral current can never be more than the
current in either hot wire (it will actually be the difference between
the two hot wires), but if the two hot wires are in phase, the current
in the neutral will be the sum of the two hot currents.

If your inverter is 2000 watts or less, and the wiring is #12, you
will be OK, as the inverter will limit the total current to about 20
amps, and #12 wire is rated for that. If you have a larger inverter,
you may have to use larger wire.

The neutral question that Peter is concerned about would be dealt with,
IF you drive your 120-0-120 panel with wire that would support the total
current of all the loads at one time. I use this exact system for
feeding the panel in my cabin from my Trace 4024. As the cabin was
originally designed and fed as a 240/120Vac Single Phase, when I moved
in and introduced the Trace inverter, rather than replace the panel,
I choose to bring both legs together at the Transfer Switch that feeds
the cabin. In Winter Mode, It is driven by 120Vac directly from the
output of the 4024. In Summer Mode, it is feed by 208/120Vac, Two out of
Three Phase, from the 2.5 Megawatt Powerhouse Distribution System.
The feedwires to the cabin from the Winter Powerhouse are "2", so
that there is plenty of copper for the maximum load of te cabin, and I
have NO 240 loads on the cabin subpanel.


Bruce in alaska
--
add a 2 before @

Agreed.

Bruce in Alaska wrote:


The neutral question that Peter is concerned about would be dealt with,
IF you drive your 120-0-120 panel with wire that would support the total

On Mon, 14 Nov 2005 16:14:10 GMT, chuck wrote:
I can't think of any problems that might arise, as long as there is no
possibility at all, ever, of connecting to 240 volt shore power with the
two 120 volt circuits connected in parallel! Doing so would probably
trip the pedestal breaker instantly before any real damage, but
nonetheless something to be avoided.

Go for it.

Chuck
================
Chuck, thanks to you and everyone else for all of the good
information. The rotary switch will select between generator, shore
power or inverter so there is absolutely no chance of a power
collision. The inverter is rated at 2KW but will normally be running
at much less, so capacity of the neutral wire should not be an issue.
We will have to carefully manage the breaker panel in order to not
power up too many things at once but that should be relatively easy.

One mistake that I once made on my old boat was to accidently turn on
the battery charger which caused the inverter and charger to chase
each other and pull down the batteries pretty quickly but that's easy
enough to spot if you are watching things.

Hoo wrote in
:

The rotary switch

Make damned sure this switch is a BREAK before MAKE affair or it will soon
be welded shut!

I like the contactor approach, myself. You plug in shore power, the
contactor closes hooking shore power to the panel until you unplug shore
power, when it connects your inverter to the panel.....automatically....

No runs, no drips, no errors, noone left on 3rd base....goofproof.

--
Larry

["Followup-To:" header set to rec.boats.electronics.]
On 2005-11-14, Hoo wrote:
My boat has two 120 volt circuits with a common neutral wire similar
to most homes. Normally they are supplied with 220 volts from either
a generator or shore power (120-0-120). There are no 220 volt
appliances or circuits out of the breaker panel.

I would like to be able to feed both 120 volt legs with a single 120
volt inverter through a rotary selector switch (both legs in phase,
instead of 180 degrees out of phase). There would be no possibility
of backfeed or cross connection with shore/generator power.

Does anyone know of any issues with doing that?

make sure your neutral wire is thick eough (or fuse your supply's
live output apropriate to its thickness) as the neutral wire will be
acrrying upto twice as much current as it would have with the two
180 degree phases.

--

Bye.
Jasen

Just to clarify this issue:

The only place the neutral could be carrying twice the design current
would be in the shore power cable. Since that cable surely would not be
used to connect the onboard inverter to the boat's AC system, we can
forget about it.

From the distribution box, each of the two 120 volt branches would
already have properly sized neutrals. How could they not?

There should be no question that the single-phase inverter's neutral is
the same size as its hot wire.

So there is simply no part of the proposed system in which a neutral
wire will be carrying more than its intended current.

The only other way an undersized neutral could be an issue is if the OP
wanted to convert his shore power cable to a 120 volt cable by simply
switching the plug and paralleling the two hot conductors. Since this is
probably the farthest thing from his proposed plan, there is no need for
concern.

Hope this helps.

Chuck

Jasen Betts wrote:
["Followup-To:" header set to rec.boats.electronics.]
On 2005-11-14, Hoo wrote:

My boat has two 120 volt circuits with a common neutral wire similar
to most homes. Normally they are supplied with 220 volts from either
a generator or shore power (120-0-120). There are no 220 volt
appliances or circuits out of the breaker panel.

I would like to be able to feed both 120 volt legs with a single 120
volt inverter through a rotary selector switch (both legs in phase,
instead of 180 degrees out of phase). There would be no possibility
of backfeed or cross connection with shore/generator power.

Does anyone know of any issues with doing that?


make sure your neutral wire is thick eough (or fuse your supply's
live output apropriate to its thickness) as the neutral wire will be
acrrying upto twice as much current as it would have with the two
180 degree phases.