Wiring a 120 Volt Mulitplus off-grid to 240 wired mobile home.

learaviator · August 9

My home is currently being powered by a Magnum MS4024PAE, 120/240 volt inverter charger. I am switching everything over to Victron equipment, this Multiplus II install is the last piece.
Nothing in the house is 240 volt, Everything is 120 volt. I am powering both line 1 and line 2 with the Magnum inverter.
I turned every possible thing on in the house at the same time and total system draw is 38 amps. Line 1 draw is 20 amps and line 2 draw is 18 amps so both legs are fairly well balanced.
I bought a Victron Multiplus II, 120 volt, 5000 va inverter as I was told I really only needed the Quattro if I was hooking up to the grid. I was also told I can just hook a jumper between Line 1 and Line 2 and run this Multiplus II 120-volt inverter this way.
Any thoughts on this? Also, I would like to keep the whole Magnum system as a backup, but trying to draw a schematic and figure devices needed is a later project.

Thoughts on tying line 1 and line 2 together by using the Multiplus?

Thank You!

littleharbor2 · August 10

I'm not sure if that's okay with the Electrical code but jumper between line one And Line 2 is the way to do it i probably use 8 or even six gauge wire

SumPower · August 10

learaviator said:

Thoughts on tying line 1 and line 2 together by using the Multiplus?

Thank You!

This is done quit often. Just use the correct wire size and corresponding breaker size to protect the wire. Any jumpered panel I worked on I labeled or wrote on the panel schedule "120 volts single phase only".

When someone follows the work and sees a single pole breaker as the main, they will realize there is a single (Edited) phase conductor feeding the panel and know that there is no 240 volt split phase available.

Dave Angelini · August 10

As said, label it to make it clear. If there is existing insurance, they probably do not care as long as the area looks safe and normal. Make sure the breaker/wire gage that Victron recommends is used. The main breakers can just be a disconnect if they are larger. Again, labels!

BB. · August 11

One thing to be careful about...

With 120/240 VAC split phase power (the normal North American setup), The Neutral (white) phase wires carry less current. For example if you have 10 amps on Red phase, and 8 amps on Black phase, the common white cable will carry 10-8=2 amps -- The "difference" between in current between the two phases.

When you setup for 120 Only power, the Red and Black current add together on the white phase... Example of 10+8=18 amps on common white phase.

So, potentially, the white phase wire needs to be sized for the total current flow.

This does depend on the wiring in your home... If you home has 2 wire + ground wiring from main panel to load, this is not a concern. The current flows "out" the black and back on the white.

If you home uses 3+ground Romex (black+Red+White+Ground), then the white wire carries the total current of black+red current... I..e, if you have 15 amp breakers on Black and Red, then the white return would need to be sized for 30 Amps return current. And you will not find B+R+W 3+1 Romex in that configuration. This is definitely a fire hazard.

You need to check/confirm your home is not wired with 3+1 Romex if you wire for 120 only single phase wiring--Or address the common neutral wiring (I.e., 2x current rating, reduce current flow such as 7.5+7.5=15 amp max white current flow, replace common neutral wiring with paired Hot+Neutral wiring, up the AWG size of such as 10 AWG 3+1 wiring, etc.).

If you have Code inspection or Fire Insurance, this could be a mess during inspection or if there is a fire insurance claim if not addressed.

https://usawire-cable.com/wp-content/uploads/nec-ampacities.pdf

Note that even 10 AWG is not a solution for common neutral if aluminium wiring is used (25 amp max current rating for topical NEC applications).

I have had this discussion decades ago on another forum where a knowledgeable person was doing this "120 only" "conversion" with a 120 VAC only backup Genset for his 120/240 VAC wired home. He did not even want to acknowledge that this was an issue.

Yes this is an "edge condition"... For normal house hold wiring, we normally do not load our wiring to 100% of rated current... But if you have, for example, a 15 amp/1800 Watt heater in two different rooms with a shared 3+1 wiring, you can easily over current the Neutral/White wire... And there is no 15 amp (or whatever) breaker on the Neutral to prevent over current conditions.

-Bill

Dave Angelini · August 11

Good point boss😉 OP will be fine with the source he has but if it changes? Best to check the neutral sizing.

littleharbor2 · August 11

BB. said:

One thing to be careful about...

With 120/240 VAC split phase power (the normal North American setup), The Neutral (white) phase wires carry less current. For example if you have 10 amps on Red phase, and 8 amps on Black phase, the common white cable will carry 10-8=2 amps -- The "difference" between in current between the two phases.

When you setup for 120 Only power, the Red and Black current add together on the white phase... Example of 10+8=18 amps on common white phase.

So, potentially, the white phase wire needs to be sized for the total current flow.

This does depend on the wiring in your home... If you home has 2 wire + ground wiring from main panel to load, this is not a concern. The current flows "out" the black and back on the white.

If you home uses 3+ground Romex (black+Red+White+Ground), then the white wire carries the total current of black+red current... I..e, if you have 15 amp breakers on Black and Red, then the white return would need to be sized for 30 Amps return current. And you will not find B+R+W 3+1 Romex in that configuration. This is definitely a fire hazard.

You need to check/confirm your home is not wired with 3+1 Romex if you wire for 120 only single phase wiring--Or address the common neutral wiring (I.e., 2x current rating, reduce current flow such as 7.5+7.5=15 amp max white current flow, replace common neutral wiring with paired Hot+Neutral wiring, up the AWG size of such as 10 AWG 3+1 wiring, etc.).

If you have Code inspection or Fire Insurance, this could be a mess during inspection or if there is a fire insurance claim if not addressed.

https://usawire-cable.com/wp-content/uploads/nec-ampacities.pdf

Note that even 10 AWG is not a solution for common neutral if aluminium wiring is used (25 amp max current rating for topical NEC applications).

I have had this discussion decades ago on another forum where a knowledgeable person was doing this "120 only" "conversion" with a 120 VAC only backup Genset for his 120/240 VAC wired home. He did not even want to acknowledge that this was an issue.

Yes this is an "edge condition"... For normal house hold wiring, we normally do not load our wiring to 100% of rated current... But if you have, for example, a 15 amp/1800 Watt heater in two different rooms with a shared 3+1 wiring, you can easily over current the Neutral/White wire... And there is no 15 amp (or whatever) breaker on the Neutral to prevent over current conditions.

-Bill

is any of this in the NEC code book? It makes sense to me but like you say when you buy, for example, 12/3 Romex the white wire is the same as the red or the black.

SumPower · August 12

littleharbor2 said:
is any of this in the NEC code book? It makes sense to me but like you say when you buy, for example, 12/3 Romex the white wire is the same as the red or the black.

NEC Art 210.4 addresses multiwire branch circuits.

But the physics (theory) that allows the neutral wire to be shared by the two phase conductors that are 180° out of phase with each other is taught during AC theory.

The basics is, the magnetic fields are opposing each other on the black and red conductor in the multiwire branch circuit. So the magnetic fields imposed on the neutral conductor cancel each other out if the amperage on the red and black conductor are the same. So no amperage is flowing on the neutral conductor when this condition exists and can be measured with an ampmeter as zero amps.

Like Bill posted above, if you have say 10 amps on the red conductor and 8 amps on the black conductor the amp reading would show 2 amps of current flow on the neutral conductor.

And if you have a three wire circuit that is not originating from a split phase source you wouldn't want to use the multiwire concept. As 16 amps flowing on the black wire and 16 amps flowing on the red wire would impose 32 amps on the neutral conductor, because the magnetic fields are additive when you only have 120 volt feed from a 120volt inverter.

The #12 wire is only rated for 20 amps. 32 amps would creat heat and a failure point.

littleharbor2 · August 12

Great explanation, That makes so much sense now that you explain it that way.

Dave Angelini · August 14

Very few people who care about code would even think about this.
Most of this issue is all offgrid and so less than 1% of the insurable properties would drop split phase to use 120vac only.

Another way to think about it is the NEC is from past mistakes and fires. How many go back to 120 only?

I thought Bill's thoughts were pretty good. This time and 15 years ago...

littleharbor2 · August 14

Bill is the rock that this Forum has been built on over the years. His answers to people's questions are always very concise. Not taking anything away from his answer above, I just was complimenting SumPower's answer.Thank you Bill for all you do

BB. · August 14

-Bill

Wiring a 120 Volt Mulitplus off-grid to 240 wired mobile home.

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