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Old 29-09-2016, 07:30   #46
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by boatbod View Post
This thread brings up some interesting questions and exposes a whole bunch of misconceptions. Speaking the point of view of a currently certified marine electrician, I figured I'd try to hit a few of those points with (hopefully) simple explanations.

1. Any time you have *separate* sources of shore power, the current carrying conductors (i.e. both live and neutral) need to be separately protected with dual-pole circuit breakers, fitted with their own polarity indicators and bussed separately.

Interconnecting or attempting to "share" the neutrals is dangerous because you may - due to dockside wiring problems etc - inadvertently connect one shore cord to a reversed polarity outlet and the other to a regular outlet. Under such conditions you would effectively have a dead short from live to neutral occurring at the place where the neutrals were joined. Under the best case circumstances a breaker would pop and be a marginal inconvenience. Worse case your boat burns down or you die from electrocution.

2. If you have separate shore power inlets you need to protect both the grounding (green) wires from stray current using suitably sized galvanic isolators. It is permissible to share a single isolator and interconnect the grounding wires, but under such circumstances you must make sure the isolator is rated to carry the full fault current of both shore inlets simultaneously. i.e. you need a 60A isolator for twin 30A inlets.

3. A single 50A 120/240 shore connection has only one neutral wire. The reason this is allowable is because there is only a single dockside connector, and there is an implicit assumption that it is wired correctly and protected with a suitably sized triple-pole breaker. Equipment damage may occur if that assumption proves to be incorrect - a lost neutral is one of the most destructive failures you can experience with 120/240 service aboard a boat or ashore.

4. There should never be any connection made between the vessel's neutral bus(es) and the grounding system. This is a fundamental difference between shore based Natl Elec Code (NEC) and marine electrical systems. Bonds are required at sources of power (inverters, gensets) but these have to be switched in and out based on which equipment is the source of power at any given time. For this reason, shore-genset selector switches need to have sufficient poles to switch all live and neutral conductors at the same time. You'll need a 2 pole switch for single 30A service, 3 poles for a single 50A 120/240 service and 4 poles for twin 30A service. The neutral-ground bonds are made at the equipment providing the source of power, so when they are deselected the bond goes away too.

5. There should be a single interconnection between the vessel's shore power grounding (green) bus and the DC negative bus. Try to keep DC current out of the grounding bus as it can cause stray current corrosion. For that reason, always run both positive and negative wires from the breaker panel to DC appliances and resist the temptation to use the engine block as a convenient negative bus distribution point.

I could go on, but I think I've hit the essentials.
Excellent discussion and explanation boatbod! I have two 110v shorepower inlets put on by the PO. He had a competent electrician who put in two downstream systems with two separate main AC master breakers right near the inlets (although at a near impossible place to see the polarity lights). There are two isolation transformers. One side feeds the main AC panel and the other is dedicated to an onboard air conditioner/reverse cycle heater and another resistive heater in the cabin.

However the neutrals on both transformers are grounded to the ship's AC ground system so they are essentially tied together at that point. I had not thought of it until your post about the potential for a problem with reverse polarity on one inlet compared to the other. I don't think that is a problem though since the transformers establish a good neutral there which have to be the same and they don't care if the hots and neutrals are different coming in. If that is an issue please let me know.
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Old 29-09-2016, 07:43   #47
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Re: Question On Twin 30 Amp Shore Power Service

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My home's electrical panel has 220v service, which is typical. My furnace and clothes dryer are 220v so it's required in any case, but I think most all service panels in "modern" homes have 220 service. The panel splits the the two hots to the two sides of the panel. The 110v circuit breakers on one side are powered by one hot and the ones on the other side are powered by the other hot. The big 220v breakers for the furnace and dryer used both hots. So the 110v circuits are roughly required to share the two hots provided roughly the same number of breakers are on each side. It's important in a home as well as on a boat to balance the loads as well as possible.
Your thesis is correct, one should load each 'side' of the 240v service as evenly as possible.

A correction to your explanation. The panel isn't divided side by side, it's divided vertically, i.e. every other breaker position vertically is on the opposite line of the 240v. Take a look at your 240v breaker, they are double wide so it can pick up each line of the hot.

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Excellent discussion and explanation boatbod! I have two 110v shorepower inlets put on by the PO. He had a competent electrician who put in two downstream systems with two separate main AC master breakers right near the inlets (although at a near impossible place to see the polarity lights). There are two isolation transformers. One side feeds the main AC panel and the other is dedicated to an onboard air conditioner/reverse cycle heater and another resistive heater in the cabin.

However the neutrals on both transformers are grounded to the ship's AC ground system so they are essentially tied together at that point. I had not thought of it until your post about the potential for a problem with reverse polarity on one inlet compared to the other. I don't think that is a problem though since the transformers establish a good neutral there which have to be the same and they don't care if the hots and neutrals are different coming in. If that is an issue please let me know.
You don't have to worry about reverse polarity between L and N on the shorepower when using an isolation transformer. If the ground in the shorepower is hot, that can cause issues (hot transformer case).

Since the isolation transformer is a source of power, it is proper to connect the neutral and ground together on the secondary side of the transformer(s).
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Old 29-09-2016, 07:53   #48
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Re: Question On Twin 30 Amp Shore Power Service

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A correction to your explanation. The panel isn't divided side by side, it's divided vertically, i.e. every other breaker position vertically is on the opposite line of the 240v. Take a look at your 240v breaker, they are double wide so it can pick up each line of the hot.

I didn't describe it very well. You are correct.

You don't have to worry about reverse polarity between L and N on the shorepower when using an isolation transformer. If the ground in the shorepower is hot, that can cause issues (hot transformer case).

Since the isolation transformer is a source of power, it is proper to connect the neutral and ground together on the secondary side of the transformer(s).
That's what I expected but I appreciate the assurance just in case I overlooked something.
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Old 29-09-2016, 11:08   #49
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Re: Question On Twin 30 Amp Shore Power Service

Isolation transformers do have their own wiring standards, but in a nutshell you can share a suitably sized neutral between the outputs of two or more transformers providing the transforms also bond the neutrals and grounds. Basically an isolation transformer is considered a source of power and sets it's own polarity.
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Old 29-09-2016, 11:14   #50
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Re: Question On Twin 30 Amp Shore Power Service

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Isolation transformers do have their own wiring standards, but in a nutshell you can share a suitably sized neutral between the outputs of two or more transformers providing the transforms also bond the neutrals and grounds. Basically an isolation transformer is considered a source of power and sets it's own polarity.
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Old 29-09-2016, 15:06   #51
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Re: Question On Twin 30 Amp Shore Power Service

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Well if you unsubscribed I guess there is no hope for you to explain exactly you are complaining about. Please do - I'm always willing to listen to statements backed up by fact, none of which you provided so far.

Fact #1: isolation transformers do have to wired on the secondary side to set the polarity. This is where the neutral-ground bond comes in.

Fact #2: although unusual, there is technically no reason that you couldn't have two separate 30A inlets each feeding their own transformer. For cost reasons (transformers are expensive) most boats requiring this amount of power would usually have 50A 240V service and a single transformer providing 120/240 service aboard the boat..
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Old 29-09-2016, 17:31   #52
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Re: Question On Twin 30 Amp Shore Power Service

bod--good posts.
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Old 29-09-2016, 21:16   #53
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Re: Question On Twin 30 Amp Shore Power Service

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Excellent discussion and explanation boatbod! I have two 110v shorepower inlets put on by the PO. He had a competent electrician who put in two downstream systems with two separate main AC master breakers right near the inlets (although at a near impossible place to see the polarity lights). There are two isolation transformers. One side feeds the main AC panel and the other is dedicated to an onboard air conditioner/reverse cycle heater and another resistive heater in the cabin.

However the neutrals on both transformers are grounded to the ship's AC ground system so they are essentially tied together at that point. I had not thought of it until your post about the potential for a problem with reverse polarity on one inlet compared to the other. I don't think that is a problem though since the transformers establish a good neutral there which have to be the same and they don't care if the hots and neutrals are different coming in. If that is an issue please let me know.

on an iso transformer you do connect the N to G on board.
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Old 29-09-2016, 21:20   #54
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Re: Question On Twin 30 Amp Shore Power Service

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Well if you unsubscribed I guess there is no hope for you to explain exactly you are complaining about. Please do - I'm always willing to listen to statements backed up by fact, none of which you provided so far.

Fact #1: isolation transformers do have to wired on the secondary side to set the polarity. This is where the neutral-ground bond comes in.

Fact #2: although unusual, there is technically no reason that you couldn't have two separate 30A inlets each feeding their own transformer. For cost reasons (transformers are expensive) most boats requiring this amount of power would usually have 50A 240V service and a single transformer providing 120/240 service aboard the boat..

yes I have put 2 iso's on a boat before. but you certainlly wouldn't connect the 2 N's together as you posted earlier. they are treated totally separate. with separate double pole breakers and separate N buses.
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Old 30-09-2016, 11:44   #55
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Re: Question On Twin 30 Amp Shore Power Service

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My home's electrical panel has 220v service, which is typical. My furnace and clothes dryer are 220v so it's required in any case, but I think most all service panels in "modern" homes have 220 service. The panel splits the the two hots to the two sides of the panel. The 110v circuit breakers on one side are powered by one hot and the ones on the other side are powered by the other hot. The big 220v breakers for the furnace and dryer used both hots. So the 110v circuits are roughly required to share the two hots provided roughly the same number of breakers are on each side. It's important in a home as well as on a boat to balance the loads as well as possible.
One thing which is important to remember is that some docks are two phases of a three phase service (generally called 208), most homes are "split phase" where each 110 leg is 180 degrees of phase with respect to the other (generally called 220).

For 220V or 208V equipment the neutral may not be used at all (my boat works this way, I use an isolation transformer which only uses 220/208 as the input with no neutral connection at all.

With balanced loads on 220v Split phase the neutral current is just the difference in the two loads (assuming that the loads are purely resistive and not reactive) is pretty minimal.

On the other hand, for 208V docks (two phases out of three) I think (would have to go think about it more to be sure) that there are significant amounts of current flowing through the neutral line when both phases are evenly loaded with 110V equipment. Obviously for purely 208V equipment there should be no neutral current.

This is another place where connecting neutral to ground becomes a really bad idea.


In the case
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Old 30-09-2016, 11:50   #56
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Re: Question On Twin 30 Amp Shore Power Service

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yes I have put 2 iso's on a boat before. but you certainlly wouldn't connect the 2 N's together as you posted earlier. they are treated totally separate. with separate double pole breakers and separate N buses.
Exactly right, one should not connect the OUTPUTS of isolation transformers to each other. I have discussed this with several vendors and all have said not to do it, with the exception of some switch mode versions which are designed for it.

In my case, if I need more power I will use one isolation transformer for feeding primary power. If I need to supplement I use a second isolation transformer and a battery charger to support the DC bus, the inverter goes into "support" mode and provides an additional 3kva if needed.

Thus 7kva on the main transformer, 3.6kva on second transformer through the battery charger to the DC bus providing about 3kva. Thus I can source 10kva without using any battery power. Inverter will support a maximum of 7kva continuous (14kva total available) and 16kva short term (23kva total available).
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Old 30-09-2016, 12:42   #57
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Re: Question On Twin 30 Amp Shore Power Service

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Exactly right, one should not connect the OUTPUTS of isolation transformers to each other. I have discussed this with several vendors and all have said not to do it, with the exception of some switch mode versions which are designed for it.
I wasn't suggesting you could parallel the output (hot and neutral) of two or more transformers, but you CAN combine the neutrals. In fact, assuming you have correctly polarized the transformer outputs (tied neutral to ground on each unit) then you have joined the neutrals together anyway.
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Old 30-09-2016, 13:03   #58
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Re: Question On Twin 30 Amp Shore Power Service

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I wasn't suggesting you could parallel the output (hot and neutral) of two or more transformers, but you CAN combine the neutrals. In fact, assuming you have correctly polarized the transformer outputs (tied neutral to ground on each unit) then you have joined the neutrals together anyway.
Thank you for pointing out my not reading your description correctly.

You are absolutely right about the ship side neutrals being connected to ground (and thus to each other).

The hots from each of the transformers may or may not be in phase and so might be 0v relative to each other or 208/220v relative to each other based upon the relative phase of the two independent 30A inlets.

With a GFI on each side trying to pull any power between the two hots will immediately trip the GFIs because return path is now not back via the neutral.
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Old 30-09-2016, 15:41   #59
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Re: Question On Twin 30 Amp Shore Power Service

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yes I have put 2 iso's on a boat before. but you certainlly wouldn't connect the 2 N's together as you posted earlier. they are treated totally separate. with separate double pole breakers and separate N buses.
But the N(s) would be connected together at the ground bus bar.
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Old 30-09-2016, 15:51   #60
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Re: Question On Twin 30 Amp Shore Power Service

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Thank you for pointing out my not reading your description correctly.

You are absolutely right about the ship side neutrals being connected to ground (and thus to each other).

The hots from each of the transformers may or may not be in phase and so might be 0v relative to each other or 208/220v relative to each other based upon the relative phase of the two independent 30A inlets.

With a GFI on each side trying to pull any power between the two hots will immediately trip the GFIs because return path is now not back via the neutral.
This would be easily detectable. If the neutrals measure 120v, reverse them on one transformer. You should be able to combine the two and have 60A of power on the same line.
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