DC and AC parallel circuits - can AC ground wire double as DC negative?

**bob-n** · 07-24-2021, 08:36 PM

Typically, ground and AC neutral are bonded at the distribution panel. I don't know any technical reason why you can't bond them at the ground rod instead. It must be a single point.

You have two ground rods. Run a wire between them. You don't want to trust the earth to keep them at the same voltage.

I don't know if it is permitted to use a metal container as a negative common bus, but it seems like a very bad idea. Are containers engineered to be electrically conductive for 30A? I would trust a fat copper wire, because I know how to make excellent connection to copper wire. I don't know how to make good electrical connection to a shipping container (aluminum? steel? painted? oxidized?) and don't know if that connection will remain good with time.

That said, automobiles do use the body as an electrical return connection. So there is a way to do it reliably and safely. If your container and your connection style meets those same standards, it could be OK, but I wouldn't risk it.

High-current DC is especially dangerous because long wires are inductive and that inductance will keep the current flowing even if the connection deteriorates. That sustained current on a bad connection will result in an arc that could make things worse (worse connection, heat, fire, etc.) For the same reason, for your DC circuits, make sure that any electrical switch is DC rated. A conventional 120VAC wall switch is not safe for DC switching.

If this is a commercial enterprise and there will be workers walking around, you need to get a licensed electrician to sign off on the plans and build. Otherwise, you could be held liable if anything goes wrong, even if it isn't your fault. The courts don't appreciate it when you tell them that you got advice on grounding from some yahoo like me on a discussion group.

If it's a home experimental cold-fusion reactor or something similar, I guess you can do anything you like. But whether you hire an electrician or do the work yourself, many towns require a town official to inspect the installation before you apply power. Again hypothetically, if there is a fire and you want to have insurance covering it, they will insist on you showing them that the project was approved by an official.

Many years ago, my father ran a business. The business was only open during daylight hours, but he was required to have lighted EXIT signs so that burglars could safely exit at night. There's a reason for everything. It may not be a good reason, but there is a reason.

Disclaimer: I am not a licensed electrician. This is comments from a yahoo on the internet, not professional advice.

**Mike90250** · 07-25-2021, 12:47 AM

I would keep the AC separate from the DC, if for nothing else, ease of troubleshooting. Check with the code inspector for your area, different cities/states have different codes and how they are interpreted.

**SunEagle** · 07-25-2021, 08:23 AM

IMO it would be wise to separate the AC and DC wiring. The saving in extra wiring could easily be eaten up by the amount of time and effort to trouble shoot each circuit.

**7GD** · 07-25-2021, 03:45 PM

Thanks for the replies thus far everyone,

Originally posted by bob-n

You have two ground rods. Run a wire between them. You don't want to trust the earth to keep them at the same voltage.

I was planning to run an 8awg ground wire, along with the hot and neutral wires, between the main panel and the subpanel at the shipping container. Is that sufficient?

Originally posted by bob-n

That said, automobiles do use the body as an electrical return connection. So there is a way to do it reliably and safely. If your container and your connection style meets those same standards, it could be OK, but I wouldn't risk it.

That was my thought, except that the shipping container has far more metal than an automobile. I figured that if a car chassis could handle that sort of amperage, a shipping container definitely could - if grounds were properly connected, of course? Your concern is noted!

Originally posted by bob-n

for your DC circuits, make sure that any electrical switch is DC rated. A conventional 120VAC wall switch is not safe for DC switching.

Noted!

Mike and SunEagle - would you mind sharing how this setup, of the AC ground wire between the AC panels also serving as the DC negative connection between the DC panels, would cause more of a troubleshooting headache? I'm not doubting you, just an amateur trying to understand and can't quite wrap my head around it!

I have one more question that I'm hoping you knowledgeable folks can help me wrap my head around. The inverter I'm using is the Magnum MS4448PAE... a 4400W 48VDC 120/240AC inverter (specs at link). The manual recommends a 30A dual-pole circuit breaker at the inverter output. If I'm understanding 120 vs 240 correctly, this means that each hot leg can still only supply, at most, 30A - correct? But if I run 240 to the shipping container subpanel, then I can supply 60amps total between the two legs to 120V circuits? Dividing 4400W by 120 = 36A though, so I don't really understand how this works. Basically, I'm trying to figure out if there any benefit to running another conductor from the inverter to the shipping container subpanel to provide 240 if I only plan on having 120V circuits in the container.

Thanks again everyone, for your immensely valuable help.

**SunEagle** · 07-25-2021, 10:01 PM

Originally posted by 7GD

Thanks for the replies thus far everyone,

I was planning to run an 8awg ground wire, along with the hot and neutral wires, between the main panel and the subpanel at the shipping container. Is that sufficient?

That was my thought, except that the shipping container has far more metal than an automobile. I figured that if a car chassis could handle that sort of amperage, a shipping container definitely could - if grounds were properly connected, of course? Your concern is noted!

Noted!

Mike and SunEagle - would you mind sharing how this setup, of the AC ground wire between the AC panels also serving as the DC negative connection between the DC panels, would cause more of a troubleshooting headache? I'm not doubting you, just an amateur trying to understand and can't quite wrap my head around it!

I have one more question that I'm hoping you knowledgeable folks can help me wrap my head around. The inverter I'm using is the Magnum MS4448PAE... a 4400W 48VDC 120/240AC inverter (specs at link). The manual recommends a 30A dual-pole circuit breaker at the inverter output. If I'm understanding 120 vs 240 correctly, this means that each hot leg can still only supply, at most, 30A - correct? But if I run 240 to the shipping container subpanel, then I can supply 60amps total between the two legs to 120V circuits? Dividing 4400W by 120 = 36A though, so I don't really understand how this works. Basically, I'm trying to figure out if there any benefit to running another conductor from the inverter to the shipping container subpanel to provide 240 if I only plan on having 120V circuits in the container.

Thanks again everyone, for your immensely valuable help.

Based on my history anytime I was in a troubleshooting mode I found it was easier to determine the issue if I kept the circuit simple. Using the same ground wire for both AC and DC circuits IMO complicates any troubleshooting. But I understand your desire to save some money by using the same ground wire for both systems.

**Mike90250** · 07-26-2021, 02:51 AM

To trouble shoot, you need to isolate. Sharing wires stops that. Los Angeles code in 2002 required DC wire in metal conduit.

**checkthisout** · 08-18-2021, 12:48 PM

Originally posted by 7GD

-Do I bond the shipping container 12VDC buck converter output negative to ground, in addition to the 48VDC negative? or instead of?

Your choice. You can tie them together but sometimes stuff fails internally. It increases the possibility (though slim) of putting 48V in your 12V stuff.

Originally posted by 7GD

-Can the AC ground wire from the inverter to the shipping container also serve as the 48VDC negative wire between the battery bank DC circuit breaker and the shipping container buck converter? If so, can I run 4 conductors in a single conduit to the shipping container - one AC hot, one AC neutral, one 48VDC pos, and one AC gnd/48VDC neg? I assume, if the AC gnd wire can in fact double as DC neg, that one would need to be of a much larger size... 4awg?

Treat them as separate systems and run both an insulated DC negative wire and an insulated or bare A/C equipment ground wire. Run your ac and dc wire in different conduits between the structures.

Originally posted by 7GD

-For the branch circuits in the shipping container - can the shipping container itself serve as the DC negative return path, and AC ground? Meaning, I just run a single conductor from 12VDC (+) to each DC end-use appliance/outlet (+) input, and then bond the appliance (-) to shipping container, and run AC hot and neutral to each AC appliance/outlet, and bond the appliance/outlet gnd to shipping container?
l!

No, keep them separate and don't bother trying to use the shipping container as an equipment or DC ground. The hassle of using paint-cutting screws, crimping ends etc will offset any little cost savings you might net in wiring.

In your case, why not just run an a/c refrigerator as well? They are often free on craigslist and you get way more choices in size. I would still run some DC to the container as you are doing because there is a lot of RV lighting you can use.

Your results will be fine but do us and yourself a favor and draw up a diagram instead of trying to think the whole thing through while maintaining a mental picture. It also give us something we can see and give feedback on.

**bcroe** · 08-18-2021, 01:46 PM

Just to add, most buck DC-DC converters have the negative input and output leads
already tied together for operation. Isolated converters do not. Bruce Roe

DC and AC parallel circuits - can AC ground wire double as DC negative?

DC and AC parallel circuits - can AC ground wire double as DC negative?

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