Multiplus / Subpanel / Distribution panel / ground question

[FilterGuy]

Ok this is a multipart question.

I am planning on running a 120v subpanel which will then connect to a Nema outlet and plug that will be used to connect the Multiplus to the grid as needed. I elected to use 10/3 wire (30 amps) due to the cost of 8/3 ( 50 amps).

From the Multiplus, I will take the AC out and place in a distribution panel which will go to numerous locations in the house.

I am aware of the need to separate the neutral and ground on the subpanel in order to avoid what has been called objectionable current.

The Multiplus requires a ground. Would I be able to attach it to the ground in the outlet which houses the Nema plug since it ties back to the subpanel and then back to the main?

Does the AC output of the Multiplus feed back in any way. In other words, if there is a short in one of the lines from the distribution box from the Multiplus, does it feed back through the unit to the AC source, the metal case ground of the Multiplus, or is a separate ground feed needed?

Is this your proposed setup?




What wattage is the Multiplus? 3K?

I have two comments and one concern

Comments
1) The breaker in the sub panel should be 30A. (That is the max the 10AWG can handle)
2) Be sure to program the Multiplus to limit the input current to 30A or you are likely to pop the breaker in the sub-panel.

Concern:
When the NEMA plug is unplugged the Multiplus and it's output circuits are disconnected from earth ground.

The Multiplus requires a ground. Would I be able to attach it to the ground in the outlet which houses the Nema plug since it ties back to the subpanel and then back to the main?

Yes, you can ground through the plug, but as mentioned above, there is no ground when it is unplugged. Is there a reason you have the plug and receptacle? This appears to be a stationary install, why not hard-wire it.

Does the AC output of the Multiplus feed back in any way. In other words, if there is a short in one of the lines from the distribution box from the Multiplus, does it feed back through the unit to the AC source, the metal case ground of the Multiplus, or is a separate ground feed needed?

The multiplus outputs are well isolated from the inputs. However, I would feel a lot better if there was a permanent connection between the critical load panel ground and the ground of the rest of the house.

 

[JJJJ]

My primary question is two fold

Should I run the Multiplus case ground to the receptacle ground in case I unplug it?

Does the distribution panel need the ground fed to the receptacle or does it feed back through the Multiplus?

 

[JJJJ]

Your drawing looks much more professional. And it describes it correctly. Looks like I posted my question at the same time

 

[FilterGuy]

My primary question is two fold

Should I run the Multiplus case ground to the receptacle ground in case I unplug it?

Does the distribution panel need the ground fed to the receptacle or does it feed back through the Multiplus?

On the Multiplus (and all other inverters I have ever seen) the Case, Input ground and output ground are all tied together. In the diagram above, when the Multiplus is plugged into the receptacle, the critical load box will be grounded.

When the inverter is not plugged in, the outputs are all floating relative to earth. If the case of the multiplus and all points on the output ground circuit remain isolated from death ground this would not be too bad.... but I still consider it a poor set-up.

You could add a permanent ground as shown below, but that also creates a ground loop through the inverter (Purple dash).... and that is begging for broadcasting large amounts of RFI.

 

[JJJJ]

So in essence the plug creates more problems than it is worth. Having said that would it be necessary to ground the case since this has a grid input always connected?

On the other hand would an additional external ground be a hazard (eg 6 awg tie in to solar panel ground?)

 

[FilterGuy]

Having said that would it be necessary to ground the case since this has a grid input always connected?

If the grid input ground is always connected, there is no need to ground the case.

On the other hand would an additional external ground be a hazard (eg 6 awg tie in to solar panel ground?)

Whoop....... tell me more about your solar panel ground. Do you have a separate ground rod for your solar panels? Are they on your house?

 

[FilterGuy]

If you have not seen them, you may want to review the 4 part resources on grounding that is in the resource section.
This is the first part and has links to the other 3.

Grounding Made simpler - Part 1: AC/houshold grounding

To get the paper, click on the orange button at the top of the screen. The subject of grounding is a complex, multifaceted subject, that is often treated as an after-thought but needs to be considered from the beginning of the design and build...

diysolarforum.com

 

[JJJJ]

Actually I have not installed my solar panels yet. They will be ground mounted. Thank you for the update on grounding as this will simply things. Also it would appear that hard wiring will be the safest and simplest approach. Really, really appreciate the outstanding feedback!!!

 

[JJJJ]

I mentioned in another thread that I am planning on getting the Midnight Classic charge controller at this time. I appreciate the ground and arc fault features. Wish more vendors would get on board (e.g. Vict...)