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Auto Transformer Bonding

automatikdonn

automatikdonn

Becoming Offgrid
Joined
Oct 20, 2021
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331
Location
Texas
I know we have all beat the brakes off this discussion - however please stick with me. It will be entertaining at the very least. Also mods if this thread is in the wrong place, please move it to the right place. I didn't really see a section of the forum that covers my question / line of discussion.

I have a LISTED auto transformer that is designed to be connected to the grid and then automatically switch over to off-grid / genset / etc when the grid goes down. Now this AT was designed with the explicit intent of being connected to the grid and was listed as such.

The question is about neutral bonding. Should I be bonding the neutral / should I not be bonding the neutral. I have asked around locally and nobody really can give me a non-circular answer.

Auto Transformers are covered in a couple places in the NEC.

210.9 Circuits Derived From Autotransformers
Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

I read this as - if you have branch circuits coming off the AT - one must use a common ground. (SEE UPDATE BELOW)

215.11 Circuits Derived From Autotransformers
Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

I read this as - if you need to use the output of an AT as a feeder - it must use a common ground. (SEE UPDATE BELOW)

So this brings me to my question - should that common ground be bonded in my split phase panel or not? See the image below.

UPDATE:

The term grounded conductor means neutral. See https://diysolarforum.com/threads/auto-transformer-bonding.50955/page-2#post-647212

Here is the current chart for wiring that is in DEBATE. THIS IS NOT READY FOR PRODUCTION
Auto Transformer Connection from Service (4).png
 
Last edited:
automatikdonn said:
I have a LISTED auto transformer that is designed to be connected to the grid and then automatically switch over to off-grid / genset / etc when the grid goes down. Now this AT was designed with the explicit intent of being connected to the grid and was listed as such.
Click to expand...
Are you sure that you are not meaning to say "transfer switch" ?
Transformers don't switch between grid and generator.
Transfer switches do.
 
timselectric said:
Are you sure that you are not meaning to say "transfer switch" ?
Transformers don't switch between grid and generator.
Transfer switches do.
Click to expand...
Thank you for responding :)
That is a really great question and in this case - the ATS is built into the unit. It has two ABB contactors that will switch between on and off grid

Here is the manual for the unit


Here is the UL Listing

 
automatikdonn said:
Thank you for responding :)
That is a really great question and in this case - the ATS is built into the unit. It has two ABB contactors that will switch between on and off grid

Here is the manual for the unit


Here is the UL Listing

Click to expand...
That's a very odd setup.
I'm surprised that I haven't seen these before.
Signature Solar products are talked about here a lot.

To answer your question. Yes, you must bond the neutral and ground in the EPS loads panel. (Critical loads panel) because it's a newly formed neutral.
But in bypass mode, there can be objectionable current on the ground conductor, due to the parallel path.
My solution was to not bond a second time. And carry the original neutral through the system. And tie the newly formed neutral to it. This avoids all objectionable current.
 
As your diagram is drawn, ground wire will carry some neutral current from autotransformer which is a no-no.

The neutral-ground connection in main panel and the autotransformer neutral-ground connection means the autotransformer neutral is also connected to grid neutral.

Neutral current from autotransformer on ground wire will be caused by any imbalance in grid L1-N and L2-N as the result of autotransformer trying to correct any grid 120vac phase imbalance.,

Only legal wiring is to have ground connected to aux panel/autotransformer case only and autotransformer neutral and subpanel neutral is brought back to main panel and connected to neutral in main panel. Neutral in subpanel must not grounded at subpanel. Neutral is grounded at single point back at main panel,

You will have to take any autotransformer capacity eaten up by autotransformer trying to correct any grid imbalance.

In general, it is not a safe setup unless you at least have a 240vac disconnect on autotransformer based on transformer overtemp.
 
Last edited:
timselectric said:
That's a very odd setup.
I'm surprised that I haven't seen these before.
Signature Solar products are talked about here a lot.

To answer your question. Yes, you must bond the neutral and ground in the EPS loads panel. (Critical loads panel) because it's a newly formed neutral.
But in bypass mode, there can be objectionable current on the ground conductor, due to the parallel path.
My solution was to not bond a second time. And carry the original neutral through the system. And tie the newly formed neutral to it. This avoids all objectionable current.
Click to expand...
There is no neutral to carry through. With this setup there is no "bypass mode". It either makes a neutral leg from grid or from inverter/genset/etc. Hence why myself and my electrician are super confused on how this should be setup.
 
automatikdonn said:
There is no neutral to carry through. With this setup there is no "bypass mode". It either makes a neutral leg from grid or from inverter/genset/etc. Hence why myself and my electrician are super confused on how this should be setup.
Click to expand...
You don't have a grid neutral?
 
I have no idea why that just posted the same thing 4 times. I was writing from my phone. Apologies for that.
 
Then this makes me even more curious about how all of the LF inverters that use this sort of setup get away with it. For instance the Growatt 12k has only L1/L2/G inputs. Does it not use an autotransformer to make the neutral leg? Wouldn't the same problem exist in all of those units?
 
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