Wiring Musing, Dual 30A shore power and Bonding concerns

Last of the Wiring Musings - here is one in response to a T&T question about how best to configure the neutral (white) wire when a boat has two independent 30A shore power cords.

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Morning, and thank you - though be careful taking too much stock in the ramblings of yet another dunder-head...

In talking specifically about two 30A shore power cords and the issue of tying the white neutral wires together at the boat.  There are perhaps two large groups where 'issues' can occur:

1) Issues around how the power box is wired.
2) Issues around the two shore power cords.

There are likely other issues, but those two come to my mind readily.


1) Power Box wiring.

If, as you say, the two independent 30A sockets are in fact wired to the same network/feed (Say a massive 120v/200a wire set that is brought into the power box).  In this situation the two 30A sockets white wires are connected to the same neutral wire inside the power box.  As the power plugs are in parallel all voltages start at the same potential, and any imbalance that has to be 'corrected' by the boats bonding will be limited to differences between the two shore power cables/socks (See below).

If perhaps the power box is served by a 240v service, be is 3-phase or split-phase, we have the same situation: Again, the two 30A shore power cords will plug into a socket where the two white wires are connected together at the power box.  And again, our only concern is any imbalance in the two shore power cords/sockets.

But:  What If the power box is served by two independent feeds and the two sockets' white wires are NOT bonded at the shore power box?  In this case the common point would be back at the main power distribution point for the whole marina. For the life of me I cannot think why this would be done, but who knows what all is out there.   Now, in this case if we have connected the two shore power white wires together on our boat we not only need to bring into balance any differences in our shore power cords/sockets, but also the marina's wiring all the way back to the main power distribution point!   If on that return there are say other boat on the feed line we will be helping to balance any load differences from those boats as well.  Given enough boats, and enough marina wire back to their power distribution point, we could see a several amps of extra 'balance' current being sent through our boats white neutral wires.  And this is IN ADDITION to any current our boat is drawing.   IF we get 5A balance current, plus say 27A of our own consumption we have exceeded the 30A rating. (Side note:  a 30A shore power cord is not really designed for 30A continuous; they are de-rated to around 27A continuous.  This is perhaps one of the reasons we see so many burned shore power sockets, trying to pull 30A all the time - right at the breaker trip limit!  It is one reason I use a 25A shore-power inlet breaker on Viking Star.  The other being if I trip a breaker, I would rather it be the one inside MY boat, not the one outside on the power box.  In the rain . . . )

So, in THIS case we have a potential problem in that our boat is working to bring into balance a major portion of the marina.  And if our installation uses only a single pole breaker (e.g. does NOT have a breaker in the white wire) for the shore power cable this will cause problems very quickly.  And if we do have a dual-pole breaker we will get a bunch of nuisance trips due to the boat having to carry not only its load, but also any balance load the marina's wiring sends our way.




2) Issues around shore power cords.

OK, so the above is rather unlikely - finding a marina wired up in a very odd way that brings two independent feeds down for each 30A socket (as opposed to one large feed, or sharing a split-phase or 3-phase feed) are we out of the woods?  Not really.  Even if you are plugged into a power box that uses a 3-phase feed (rather common I think) and hence the two sockets neutral lines are wired to the same point in the power box we need to consider what CAN happen on our shore power cords.

In this example we have the two white wires bonded at each end; at the shore power box and our boat.  So, these two white wires (in the two shore power cables) are in parallel.  Ohms Law says and any current transferred will be allocated in direct inverse relationship to the TOTAL resistance of the two wires -AND- their sockets/plugs.   (Ohms Law is also what creates what I am calling 'balance current' in the example above, only in that case it is Ohms Law against the marina's wiring)  So, the resistance of the wire set, the shore box sockets, our boats sockets, the plugs on the shore power cord.  All come into play.

Ohms Law tells us that if the resistance of the two shore power cord/socket sets are exactly the same, current will be equally split between the two.   If we are pulling 15A over the black wire on one shore power cord, and 25A on the other, that gives us 40A being delivered to the boat.  Equally split we will send back 20A on each white wire.   Do note that what is sent back on each cord set is not the same as what is brought over!  And here is the 1st potential issue: if the shore power box has a GFI type device it will quickly trip!!! (That is their basic function: if what is sent out does not match what is brought back the GFI type devices figure current must be going somewhere - perhaps through you - so they trip)

Going further, let's say the two shore power cords are different ages and perhaps one has a little more resistance in it.  Or perhaps one of the socket/plugs has a little more corrosion on it then the other.  Combining to make a slight bit difference in total resistance between the two.  Now, instead of the return amps being equally balanced we start to see a shift in the balance.  Maybe it goes from 20A/20A to 19A/21A.   Or maybe the boater before you had dipped their cord in the sea before plugging into the shore box, and that one socket has a bit of corrosion on it.  So our balance becomes 14A/26A.   Or maybe it was you who did the dipping and we get 9A/31A.  Or .....    Bottom line is, the two will NEVER be the same.  And as a result one cord's white wire will always be carrying more current then the other.  Over time, more current = more heat = more stress, and perhaps we start to see that ONE shore power sock showing signs of heat stress, which will increase its resistance - sending current over to the other guy - who then starts getting burned.

Now we are back to the boats breaker question:   Once this imbalance crosses over the line do we a) start nuisance tripping the breakers, or I burn out the cords (aka, fire) if there is only a single pole breaker installed...



And you last question " Are not the two "independent neutral wires - one for each shore power cord" as independent as two neutrals from two normal branch circuits?"    AKA, how houses might be wired?   Yes they are, but in 'two normal branch circuits' one would not re-bond the two natural wires at the terminus of those two branch circuits.  All natural wires, be the individual circuits, or sub-panel / branch circuits are brought back to the main house panel and a common natural bonding bar inside that main panel.


So, bottom line:  There are a bunch of failure modes that can occur when tying the two white wires of independent 30A shore power feeds together on your boat.  Just not a good idea....


Hope this helps, and keep in mind this is about two INDEPENDENT 30A shore power feeds, using two separate cords and two separate socks sets and hence two separate white wires.  All the above does not apply when talking about a single shore power cord.  Be it a single 120v/30A feed, or a 240v/50A  shore power feed.  In these cases there is only one cord and only one white wire inside that cord - and no opportunity to 'rebond' anything.