3 phase distribution and neutral bonding


Old 04-23-19, 02:08 PM
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3 phase distribution and neutral bonding

It's been a while since I've posted on this forum, but I have a question that I just can't find an answer for. So here it is:

I do maintenance on a friend's old industrial/commercial buildings. They have 4 separate drops entering the buildings.

There are ( 2 ) 240VAC/120VAC meters, (1) 240VAC 3 Phase Delta meter (no 120VAC legs), and (1) 480VAC 3-Phase Delta meter.

In an effort to reduce his exorbitant electric charges, my friend recently installed a 50 Kw Solar Array on the 480VAC service. This required a new 400 Amp Service Panel (actually it's a cabinet).

The only loads on that circuit are a 240VAC single phase well pump and booster pump, a 3-Phase 480 VAC compressor that was operational but not in use, and a 120VAC panel for the water filtration system plus some light duty lighting and outlets.

​The 240 VAC/120 VAC loads are powered by a single phase step down isolation transformer.

​​​​​​The work was performed by a reputable contractor, but I always check work done on the building, just to make sure everything looks alright, and to familiarize myself with the system since I am usually the first person that gets called if there is a problem.

My question is about the ground to neutral bonding. The original service consisted of only the three phase wires, and a ground wire that connected to a ground rod. The neutral from the secondary of the step down transformer was also connected to the same ground rod.

The new system has the same drop from the power pole, but has added two inverters from the solar, each with its own neutral connected to the main panel ground bus, which of course, is also tied to the ground rod. This all seems normal, except I am not sure why the inverters need a neutral since they feed directly into the 480VAC delta, but I don't have any specs on them.

The thing that got me asking questions, is that the neutral from the secondary of the step down transformer is bonded back to the main service panel ground bus. I looked this up on line, and it said that the neutral from a derived system transformer should be bonded to ground at either the transformer or the disconnecting means for the derived system, but not in both places. It doesn't say anything about bonding it to ground at the main utility service panel. But then, when I thought about it, if you have several step down isolation transformers, which we might want to install someday to handle more light duty loads, then it might make sense to tie everything back to the utility panel? Also, the building that the PV panels are attached to is a metal building with a metal roof. The power to that building comes from an entirely different meter and service.

The buildings were built in the 1940's and 50's, but the Power Co. has upgraded all their lines and transformers in the last five years due to wildfire safety concerns.

So that's it. Any thoughts or insights by some of you electricians who have worked with 3 Phase Industrial systems would be appreciated. My experience has been mostly with single phase residential. This is a whole new and fascinating subject for me.


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Old 04-23-19, 02:14 PM
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What exactly is your question?
Old 04-23-19, 05:44 PM
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If A is bonded to B and B is bonded to C then A is bonded to C.

A given transformer powered by the aforementioned service and being the root of a separately derived system might have a neutral (coming from the transformer secondary) bonded to ground.

Meanwhile the service ground (incoming conductor) is also bonded to the same building ground (the latter being the grounding electrode system.

Presto or ergo the neutral of the aforementioned separately derived system will be bonded to the service ground.

All ground rods about any particular building are to be bonded; for a house, bonded using a #6 copper wire run exterior to the house as much as is practical.

Anything that needs to be grounded may be bonded to any point on the grounding electrode system although in some cases where this is done a grounding electtrode conductor might need to be fatter than #6.
Old 04-23-19, 07:04 PM
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All the grounds, regardless of where they come from, will be tied together one way or another. They are all part of the grounding system.

The neutral of the separately derived system is typically to be bonded at the first disconnecting means and inside the step-down transformer. Please post a link where it says not to connect them on both ends. I would like to read that.
Old 04-24-19, 12:32 AM
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CasualJoe, I didn't really specify my question very well, but I am asking if bonding the neutral ( grounded conductor) from the derived system at the utility service panel is correct.
Old 04-24-19, 12:46 AM
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AllanJ, it is my understanding that a neutral (grounded conductor) may only be bonded to ground at one place in the system. If it is bonded at multiple locations, then this creates parallel paths for normal current to return to the source over the equipment grounding conductors. This is considered a potentially dangerous situation by the NEC, and is generally prohibited. It is not as simple as A is bonded to B, B is bonded to C, therefore A is bonded to C
Old 04-24-19, 12:53 AM
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Tolyn Ironhand, here is the link that says the neutral from the derived system should only be bonded at one location. It does list an exception to this, but only for specific cases where multiple paths for the return current are not created.

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