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  #1  
Old 08-10-2008
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The green wire and inverters

I recently installed a Xantrex Pro-1000. I hard-wired it to all of my port & stbd AC outlets since everything I plug in I typically would like to use underway. Shore power currently only supplys a 10 amp battery charger and one outlet.

The inverter has a transfer relay to transfer loads from shorepower to inverter and vice versa automatically. The question is, if I hard wire the invert to shore power how many times is the AC grounding wire going to be connected to my boats ground? There is a chassis ground on the inverter separate from the inverter DC negative. There is also a grounding wire connected to the shore power inlet.... If I hard wire the inverter to shorepower it seems like I will have created two ground paths. Everything I have read has made it very clear that shore power grounding wire should be connected to ground on the boat only once. Would it be a good idea to cut the shore power inlet grounding wire? Hmmm....
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Old 08-11-2008
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Quote:
Originally Posted by sailboy21 View Post
I recently installed a Xantrex Pro-1000. I hard-wired it to all of my port & stbd AC outlets since everything I plug in I typically would like to use underway. Shore power currently only supplys a 10 amp battery charger and one outlet.

The inverter has a transfer relay to transfer loads from shorepower to inverter and vice versa automatically. The question is, if I hard wire the invert to shore power how many times is the AC grounding wire going to be connected to my boats ground? There is a chassis ground on the inverter separate from the inverter DC negative. There is also a grounding wire connected to the shore power inlet.... If I hard wire the inverter to shorepower it seems like I will have created two ground paths. Everything I have read has made it very clear that shore power grounding wire should be connected to ground on the boat only once. Would it be a good idea to cut the shore power inlet grounding wire? Hmmm....
First off, it would be a really bad idea to cut the shore power inlet grounding wire. If you do this, when connected to shore power, a phase-neutral fault (short) on the dock could electrocute anyone on board who happens to be using anything you have that's got a hard-wired earth (ie. not double-insulated).

If the inverter has a transfer switch, you connect the shore power to the Inverter AC input and the AC output directly to your AC distribution panel.

1. Connect the chassis ground on the Inverter to your boat's grounding point (engine mount, keel bolts, whatever). This is probably also your DC ground, but it is important that you have two separate connections (AC gnd & DC gnd) to the same point on the boat to avoid ground loops - it's called a "single-point ground".

2. Connect the Inverter AC output (phase, neutral & earth) to your AC panel.

3. Connect the shore power ground socket earth pin to your AC panel earth bar and nothing else.

Since the Inverter's AC ground pin should be connected to the chassis internally, the AC ground path is then from your equipment -> AC Ground bar -> Inverter chassis -> hull (and shore power ground if you're connected).

I hope I haven't confused you..
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Last edited by Classic30; 08-11-2008 at 03:34 AM.
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Old 08-11-2008
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I'd repeat what Hartley said: DO NOT CUT THE GROUND WIRE TO YOUR SHOREPOWER INLET

BTW, your inverter should probably be connected to the AC panel via a double breaker, with one for the hot and one for the neutral lines.

You might also want to consider having separate shorepower and inverter AC panels. The reason for this is you don't want to plug a battery charger into an inverter and charge the batteries on inverter power. Having separate panels means that is much less likely to happen.

For example, if you have the inverter connected to the Shorepower AC in, and the AC panel connected to it...and you're running the battery charger, if someone trips the breaker in the shorepower box or disconnects your shorepower cable... the charger is now running off the inverter... using up the batteries to try and charge the batteries and generating not much but heat.

A typical setup:

Main Shorepower AC panel
AC in (two breakers)
Inverter AC feed (one or two breakers depending on your installation)
Port side AC outlets
Starboard side AC outlets
Battery Charger
Refrigerator
Inverter AC Panel
Inverter output (two breaker setup like the main AC panel)
Port side inverter AC outlet
Starboard side inverter AC outlet

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Last edited by sailingdog; 08-11-2008 at 06:47 AM.
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Old 08-11-2008
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I am well away of the dangers of a floating ac system but that isn't exactly what I was saying. Take a look at my ac schematic with inverter hardwired to shore power. Look at the grounding wires and you will see two, one from the inverter chassis and one from the shore power inlet. I have followed every guideline and instruction I know of. The inverter emphatically says to ground the chassis but I can't help but thinking this has created some sort of ground loop which should be avoided. Maybe it isn't a problem but something just doesn't seem right to me... I don't have any high power ac devices so I see no need to install a second ac circuit separate from the outlets served from the inverter the automatic transfer relay seems like a nice feature but maybe more hassle than worth...
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Old 08-11-2008
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The chasis of the inverter really needs to be grounded to the DC system ground, which it will be attached to in any case, since the inverter has to be connected to the DC side batteries for the inverter to work properly.
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Old 08-11-2008
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SD, I have to disagree with you there.. sort of.

SB, the way you've shown it, the Inverter is wired up more like an uninterruptible power supply (UPS) - just like any other bit of AC gear - but I don't think that's quite what the manufacturer intended. I suspect the unit is designed to be installed upstream of the AC Panel..

In any case, if you're going to do this, you need to delete the "Xantrex Ground" connection between the Inverter chassis and the boat and either not connect it at all (if it's internally connected to the AC Input - which it should be) or connect it directly to the AC Input ground screw instead. That will get rid of your earth loop issue.
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Last edited by Classic30; 08-11-2008 at 11:38 PM.
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Old 08-11-2008
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Quote:
Originally Posted by Hartley18 View Post
SD, I have to disagree with you there.. sort of.

SB, the way you've shown it, the Inverter is wired up more like an uninterruptible power supply (UPS) - just like any other bit of AC gear - but I don't think that's quite what the manufacturer intended. I suspect the unit is designed to be installed upstream of the AC Panel..
Actually that is exactly what the manufacturer intended "transfers seamlessly from shorepower to battery power" as it says on the box. Downstream of the AC main gives the option of leaving just the battery charger energized, or the inverter with all of its outlets energized. Pretty neat system really.

Quote:
In any case, if you're going to do this, you need to delete the "Xantrex Ground" connection between the Inverter chassis and the boat and either not connect it at all (if it's internally connected to the AC Input - which it should be) or connect it directly to the AC Input ground screw instead. That will get rid of your earth loop issue.
I'll probably end up calling them. The installtion instructions advise using 8 gauge wire for the chassis "xantrex ground" which is larger than the 10 AWG shorepower ground wire. I haven't had shorepower since I ripped out the old breaker box last spring so I'm not really sure what all the new stuff will do when it gets finally utility juice I've been doing fine w/o it.. and the $20/month I have been saving has almost paid for all the fancy new wires I have put in! But alas winter is looming and i'll have to plug her in again.
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Old 08-12-2008
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Quote:
Originally Posted by sailboy21 View Post
I'll probably end up calling them.
A good idea. Can't hurt at all.. I'm surprised they don't give you clear installation instructions with the unit.

Quote:
Originally Posted by sailboy21 View Post
The installtion instructions advise using 8 gauge wire for the chassis "xantrex ground" which is larger than the 10 AWG shorepower ground wire. I haven't had shorepower since I ripped out the old breaker box last spring so I'm not really sure what all the new stuff will do when it gets finally utility juice I've been doing fine w/o it.. and the $20/month I have been saving has almost paid for all the fancy new wires I have put in! But alas winter is looming and i'll have to plug her in again.
I'm not familiar with the Xantrex unit, but the larger gauge wire would be intended to carry higher currents than the shorepower circuit could handle if a fault developed in the unit. Your AC ground wire (AC Panel to vessel ground) should always be pretty heavy gauge stuff and certainly heavier gauge than the shorepower connection.

The way it needs to be connected will depend entirely on the internal wiring of the unit. I've been assuming that the chassis ground is connected internally to the AC input ground. If it's connected to the DC -ve instead (possible - but I would have thought unlikely), or not connected to anything at all (more likely), then you'll need to connect it to the vessel earth point... but then you don't have an earth loop either.

It's all good fun.
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Old 04-14-2009
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Sailboy21,

Inverter manufacturer's assume that you do not already have the single point connection between AC green ground bus and DC negative bus, therefore to cover their butts, they tell you to do it.

Without it, if AC from the inverter (or any any device that has both AC and DC), leaks to DC, then the boats entire DC negative has 110 volt potential. Not good. With it, that potential has a path to earth ground on shore through that single point connection.

Off the dock, when inverters are inverting, they bond the green and white, so any leakage will trip the breaker since you've lost that path to earth ground on shore.

Bottom line if you already have it you don't need to do it again, but you still need to ground the inverter chassis to the AC green bus, although internally, in the inverter, the chassis should already be connected to the green terminal.

Everyone should install a galvanic isolator to isolate their boat from everyone else in the marina. The AC green wire connects you with all the "hot boats" in the marina. That is why boatowners were foolishly cutting the green wire instead of installing a galvanic isolator.

Wirewiz
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