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Old 05-05-2008, 11:51   #1
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ABYC Conundrum...

I have a question for all the ABYC experts here.

According to E-11 (2003 version), if a boat has two shore power inlets (in my case, two 30A 3-wire services), then the neutrals must not be connected together on the boat - they must be kept separate.

Not a problem - yet. I'm assuming that keeping them separate assures that there will still be only one point - back at the dock service point - that each safety ground is connected to its associated neutral only at that single point. I get that.

My plan was to feed the two AC services from shore and the genset's two outputs (L1 & L2) via a transfer switch to the main panel - which has all the high-load AC appliances - water heater, A/C, auxilary battery chargers, and the inverter. I don't have any 240V appliances, but I want that option for future use (maybe a dive compressor?).

The problem is my Magnum Energy MS2812 inverter. It has the nice capability of passing through not only one, but two, AC service circuits (L1 & L2) when in charging mode. Of course, when in invert mode, the neutral is internally tied to the ground - which is normal. The two hot outputs are internally bridged when inverting, so you have only 120V service (they do have a larger model 4024 which can support 120/240 output when inverting).

The inverter would in turn feed the low-load sub-panel - again with two AC services (L1 & L2).

However, note that I said "neutral" and not "neutrals". The unit supports only a single Neutral in, and a single Neutral out. Which is fine if I'm running a four-wire 120/240 50A split-phase system. But the marine electrician convinced me early in the project that I'd have load balancing problems, and recommended that we go with two 30A shore power inlets. Now I'm stuck... Both the main AC panel and the subpanel currently support a single hot side (although they could be modified somewhat)...

I just got off the phone with Magnum's tech support, and the guy I spoke with didn't think it would be a problem to tie the neutrals of the two shore power services together - as long as the marina's system was correctly wired. All bets are off if it's not.

My question is - what am I risking if I connect the neutrals together at the inverter? Or should I just have the inverter feed the subpanel with just one circuit - and possibly risk having the two service "unbalanced" - just what the electrician wanted to avoid in the beginning. If I went with a single 50A 4-wire shore power inlet, I wouldn't have this problem - only one neutral to deal with (just like the genset)...

The long term plan is for the genset to do most of the heavy lifting (supplemented by a wind generator and solar panels). Docks will be few and far between while cruising.

I could include a couple drawings if that would make it a bit clearer...
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Old 05-05-2008, 14:08   #2
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"what am I risking if I connect the neutrals together at the inverter? "

Offhand? depends on how creative the marina has been with their wiring. If they get the lines reversed on one loop, you could have a rather large spark and 120V short circuit in there. If they just have the lines unbalanced, you could have some current flowing all the time. I've found small (2-6V) imbalances between neutral and ground in building wiring that don't seem to set anything on fire, although they make for fine ground hums.

I don't think I'd want to chance what the marina might have in their wiring, though. Assume they MIGHT have one set reversed, are you willing to prevent or take that damage?
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Old 05-05-2008, 14:57   #3
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I don't think I'd want to chance what the marina might have in their wiring, though. Assume they MIGHT have one set reversed, are you willing to prevent or take that damage?
Well, the polarity indicators on the shore power disconnects and the galvanic isolators would show that before I even powered up the main AC panel, assuming I had it powered off or had the transfer switch set to the onboard genset.

Other than that, what transient problems could I expect? Someone on the same circuit plugging in a mis-wire shore power cord?

At any rate, it looks like I'll end up going with the first page of the attached drawing rather than the second. As long as I don't fire up the arc welder in the forward workshop, I should be ok
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File Type: pdf Beausoleil Drawings - AC Service.pdf (105.2 KB, 175 views)
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Old 05-05-2008, 17:46   #4
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The shore power neutrals are supposed to be disconnected

from your on-board neutrals. Your on-board neutrals are normally grounded back to your engine along with the negative grounds, but a marine ABYC rated inverter is supposed to automatically switch the shore power Neutral off (Disconnect it) from being connected to your boat grounds. If not disconnected, a short or an accidental miswiring on the shore circuit, could cause a diver or other person in the water around your boat, to be electrocuted.

That doesn't mean that I know why the two shore neutrals shouldn't be connected together, because I don't.
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Old 06-05-2008, 09:17   #5
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from your on-board neutrals. Your on-board neutrals are normally grounded back to your engine along with the negative grounds, but a marine ABYC rated inverter is supposed to automatically switch the shore power Neutral off (Disconnect it) from being connected to your boat grounds. If not disconnected, a short or an accidental miswiring on the shore circuit, could cause a diver or other person in the water around your boat, to be electrocuted.

That doesn't mean that I know why the two shore neutrals shouldn't be connected together, because I don't.
Yes -that's basically what I said. "Of course, when in invert mode, the neutral is internally tied to the ground - which is normal." I should have been more explicit and said "when in inverter mode, the boat's neutral is internally tied to ground".

If the source of AC power is on-board - say a genset or an inverter, then the neutral is to be tied to the AC safety ground at the source, which is in turn supposed to be connected to the boat's DC ground bus - normally at the engine negative terminal. As pointed out elsewhere, many knowledgeable people have a "problem" with that...

If the source of AC power is via a shore power connection, then the AC neutral is not to be connected to AC green safety ground anywhere on the boat - only back at the marina's electrical plant. The one exception is if you use an isolation transformer (which is essentially an on-board power source), then there's no metallic connection between the shore power system and your boat (except the transformer's shielded case is connected to the shore power ground - not the boat's) The AC transfer switch is supposed to switch your on-board AC neutral bus (as well as hots) between your genset (or inverter) and the shore power (or isolation transformer) neutrals. Any AC safety-grounded three-wire appliance on board which internally tied together its safety ground to its neutral is an accident waiting to happen...

We know that if the shore power system's hot and neutral are reversed, the polarity indicator on the shore power disconnect breaker will be lit when you plug in the shore power cable - even if the disconnect breaker is in the "Off" position. But what if the shore power's neutral and ground or reversed? Normal on-board (read that as ABYC/USCG) systems won't give an indication - but since ABYC allows the AC safety ground wire to be one size smaller than the hot and neutral - you have a fire hazard. And simple 3-wire outlet testers can't detect that - only hot/neutral or hot/ground reversal.

Back to the original question: why are you not supposed to connect the neutrals of two or more 30A three-wire (or 50A 3-wire or 4-wire) shore power connections together on the boat? Is it simply to reduce the chances that something bad can happen?

It should be easy to verify if the boat's AC system is wired correctly with respect to Neutral and Safety Ground - just make sure shore power is disconnected and any genset or inverter are powered off. With all your AC branch circuits off, use an ohmmeter and test for continuity between Neutral and Ground. Then switch on each branch breaker one at a time and read the meter. At any point if the meter reads less than infinity, you've found a problem. Unplug/disconnect any devices on the offending branch circuit - if you still see any continuity then you've found the bad circuit. If it reads infinity - you've got a bad appliance. Plug them back in one at a time until you find the bad one...

What can go wrong if your 2x30A inlets have their neutrals tied together somewhere on the boat - in my case, at the inverter if I connect the inverter's two AC inputs to both shore power circuits?

Gord, Rick? Any insight?
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