Adding another AC circuit

dlochner · Apr 2, 2019

John,

Yes you will need another GI for the new circuit.

The 30 Amp power cord connected to a 50 Amp circuit will not be adequately protected. If a short occurs before the main breakers (ECLI) then the cable can over heat.

jssailem · Apr 2, 2019

What Dave said.

jssailem · Apr 3, 2019

Not exactly John.
The pigtail is nothing more than a wire with two plug ends On fits a 50 amp connection and the other fits a 30 amp connection. On the 50 amp shore fitting the circuit breaker will be 50 amps. And you can draw 50 amps from it. The plug and wire you use to your boat fixture is designed for 30 amps. If you try to use 50 amps through it you can because the Shore source is designed to give it to you. What will happen is the 30 amp wire, plug, and boat wiring will heat up as it tries to meet your demand. Eventually something on the 30 amp side will breakdown. Hopefully it is the 30 amp fuse that screams enough. But if not it will be a smoking wire or plug that will break.

So running a 50 amp consuming appliance, like an electric stove, on a 30 amp service is bad.

If you limit your load in the boat to 30 amps, you can use the pigtail connection. Just be aware that the 50 amp breaker on the shore is not likely to trip.

Yes if you add a second shore power connection to your boat you will need to consider galvanic isolation on that separate shore power source. You are not combining them you are running two separate shore power systems.

dlochner · Apr 3, 2019

John Rand said:
Thanks for the replies.
OK, that's what I figured.
One more question: I'm guessing that the 50A to 30A pigtail adapter actually drops the amps down to 30A, meaning that I will need another 30A galvanic isolator. Is that correct?
Thanks again for your help.

No the pigtail adapter simply connects the 50 amp plug to a 30 amp plug. The Smart Plug cable is 10 gauge, for 50 amps you will need an 8 gauge cable.

If there is a problem between the 30 amp onboard breaker and the power pedestal the wires won't be protected.

One other thought, if the main breaker is tripping before the 25 amp A/C breaker, that means the cumulative load is above 30 amps for too long. What other devices do you have running when you turn the air conditioner on? You could manage this easily by turning off the other breakers or unplugging other AC devices.

Ron20324 · Apr 3, 2019

I'd say most boats that have air, have two, 30-amp, shore connections- one dedicated to the AC. Is there a chance the marina will convert the 50-amp back to a 30 for you?

jssailem · Apr 3, 2019

Ron20324 said:
Is there a chance the marina will convert the 50-amp back to a 30 for you?

That would be a fairly easy change for a longtime marina resident.

Ron20324 · Apr 3, 2019

John Rand said:
Just spoke to the marina folks. They will convert to a 30A outlet. I'l
Thanks for the guidance.

You're welcome!

Maine Sail · Apr 5, 2019

John Rand said:
A while ago, I upgraded the AC on my boat with a SmartPlug, a 30A galvanic isolator and a Bluesea panel (ELCI Main 30A Double Pole with two single pole 15A).

Excellent upgrade!

John Rand said:
A while later, I added a marine air conditioner and added 25A breaker to the panel for the air conditioner. This proved inadequate (keeps flipping the main breaker).

Not good at all and a a mistake many pro's and DIY's make. Despite having a SmartPlug you never want to load your 30A AC at more than about 80% of rated current. A water heater, air conditioner and a battery charger can easily overload your 30A service. Better yet try to keep it under 20A. The only proper way to add air conditioning to an existing 30A boat is to install a new AC inlet & AC panel.

John Rand said:
So I ordered another 50' 30A Smartplug, and then discovered that the second socket on the dock pedestal was 50A.

While adapting from 50A to 30A is done all the time it it is not a sound practice as your pedestal protection is now 50A on 10GA wire. The least expensive solution would have been to convert your entire vessel to 50A when you did your 30A upgrade. This would involve a new ELCI, new inlet, new power cord, new wire between inlet and AC panel and a new galvanic isolator.

John Rand said:
I guess what I should do is to get a 50A to 30A pigtail adapter and another 30A breaker, and connect the air conditioner to that.

To do this correctly you will need an new AC panel with AC Main breaker, reverse polarity indicator and a branch breaker for the AC system. You will also need to very carefully follow the ABYC standards when installing a second shore power system. AC Neutrals, from two shore power inlets, cannot share an on-board common bus, and both systems need to be separate from each other except for the Grounding/Earth circuit. If you understand ground fault protection (ELCI, GFCI etc.) it should be pretty easy to understand why this is so.

"11.5.3.6.2 Multiple Shore Power Inlets - Where more than one shore power inlet is used, the shore power neutrals shall not be connected together onboard the boat."

John Rand said:
Do I also need another galvanic isolator?

You now have 60A of service coming on-board so a single 30A GI is not going to cut it protecting both AC grounding wires. The GI needs to be rated to handle the total "potential" on-board fault current and also needs to be of the "Fail Safe" type.

One alternative would be a "lock out slider" on the AC panel to disallow any other AC appliances being on at the same time as the air conditioner circuit. You now have an "either / or" situation that can keep your boat safe but you won't be able to run anything but the air conditioner when its breaker is flipped on..

jlp0217 · Apr 7, 2019

I don't understand those of you who worry about plugging a 30 amp cord into a 50 amp shore power receptacle. That would be like worrying about plugging a 10 amp toaster into a 15 amp wall socket protected by a 20 amp breaker. You all do that in your house. Every one of you.
You have a 30 amp breaker on your boat that will prevent you from carelessly turning on too many appliances and drawing more than 30 amps. Or from a short on you boat. What kind of problem between the pedestal and the on board breaker is going to draw significantly more current than 30 amps, but less than 50? And the 30 amp 10AWG cable is not going to burst into flames if you were to connect it to a 50 amp load, its just going to get hot. Any more load and you pop the 50AMP breaker.

dlochner · Apr 7, 2019

jlp0217 said:
I don't understand those of you who worry about plugging a 30 amp cord into a 50 amp shore power receptacle. That would be like worrying about plugging a 10 amp toaster into a 15 amp wall socket protected by a 20 amp breaker. You all do that in your house. Every one of you.
You have a 30 amp breaker on your boat that will prevent you from carelessly turning on too many appliances and drawing more than 30 amps. Or from a short on you boat. What kind of problem between the pedestal and the on board breaker is going to draw significantly more current than 30 amps, but less than 50? And the 30 amp 10AWG cable is not going to burst into flames if you were to connect it to a 50 amp load, its just going to get hot. Any more load and you pop the 50AMP breaker.

There are lots of things we do on boats and shore that stretch the limits. Back in the day, people used to put pennies in the fuse sockets in their homes to avoid blowing fuses and put 20 and 30 amp fuses in 15 amp circuits. But that doesn't mean there won't be problems some with disastrous consequences.

One problem that can come up is an overloaded circuit that gradually degrades the plug, leading to overheating and a fire. A little corrosion in the plug or in the receptacle will increase resistance and increase the heat generated. Take a look at some of the examples here: https://marinehowto.com/category/the-hall-of-fail/

It is not at all inconceivable that the corrosion in the plug could increase the current draw well above 30 amps but below the 50 amp threshold. Each time it over heats, the plug will degrade until there is catastrophic failure. Now if it is only your boat at risk, its your boat. However in a marina when you put your boat at risk, you put all the other boats at risk.

jlp0217 · Apr 7, 2019

Sorry, but corrosion will not decrease the resistance of the circuit, it will increase it. That can increase heat, but not current. Using a smart plug at the boat reduces that risk, its the stupid connectors at the dock pedestal and adapter that are at most risk. If they burn up, the boat looses power which could damage batteries, but is unlikely to do much more than that, except maybe in a boathouse.
Note that I follow Maine Sail's practice of not loading my 30 Amp circuits much more than 20 Amps, but UL, ABYC, our local power companies, municipal inpsectors, and our insurance companies all certify that it is safe to load them at 30 amps. Otherwise they would require larger wire sizes and better connectors or smaller circuit breakers.
I agree that it is possible for the connectors, especially the connectors connecting my 30 amp cords to the 50 to 2 * 30 amp pig tail could easily burn up without blowing a 30 amp breaker, let alone a 50 amp breaker, but you'll have a hard time convincing me that a 30 amp breaker would be significantly safer than a 50 amp breaker in the pedestal, as long as I have a good 30 amp breaker on the boat. There are just so few failure modes that a 30 amp breaker protects for and a 50 doesn't. Neither protects for corroded connections until they are so corroded that they fall apart and short out. Then both protect equally well.

dlochner · Apr 8, 2019

jlp0217 said:
Sorry, but corrosion will not decrease the resistance of the circuit, it will increase it.

Did someone write this? I don't recall anyone saying that corrosion will decrease resistance.

Maine Sail · Apr 8, 2019

jlp0217 said:
I don't understand those of you who worry about plugging a 30 amp cord into a 50 amp shore power receptacle. That would be like worrying about plugging a 10 amp toaster into a 15 amp wall socket protected by a 20 amp breaker.

A 15A outlet protected by a 20A breaker using 14GA wire would not meet NFPA/NEC code. A 15A outlet would normally be wired with 14GA wire and protected by a 15A breaker, a 20A outlet would be wired with 12GA wire and be protected by a 20A breaker. There are some caveats in the NEC/NFPA where a 15A outlet can be protected by a 20A breaker but the wire feeding it should be 12GA not 14GA..

In "most" circumstances you are correct that the cable won't suffer much undue harm. However, there are a lot of bad electricians or DIY's out there doing things like "splitting" a 30A service once inside the boat with two 30A main breakers, one for an air conditioner and one for the other AC circuits. I once worked on a large Bertram that used Square D panels and one 100A main breaker and then two sub panels each with 50A breakers. This was all on a single 50A shore cord.. This is why pedestals also have breakers in them. While this also does not meet ABYC standards, at least on land you have inspectors inspecting the work to try and ensure we're not protecting a 15A outlet & 14GA wire with a 20A breaker.

jlp0217 said:
You all do that in your house. Every one of you.

Not at our house. All of our 15A rated outlets are protected by 15A breakers. Our 20A outlets are protected by 20A breakers and use 12GA wire.

jlp0217 said:
What kind of problem between the pedestal and the on board breaker is going to draw significantly more current than 30 amps, but less than 50?

The scenario I described above, which is not as rare as it may seem.

jlp0217 · Apr 8, 2019

dlochner - "It is not at all inconceivable that the corrosion in the plug could increase the current draw well above 30 amps"
The only way to increase current is to decrease resistance, since voltage is consstant - Ohms law.
Ok, people do stupid things. Splitting the input on the boat and adding another 30 amp breaker means the boat is no longer a 30 amp boat, but a 60amp boat with undersized wire.
Around here, houses are wired with 12ga wire & 20 amp breakers with 1 breaker protecting many outlets. The outlets are all 15 amp outlets, I've only ever seen 20 amp outlets in garages & shops, never in a house, and they are usually wired 1 outlet per breaker. 14 gauge wire & 15 amp breakers are only used in lighting circuits, never outlets. That is until of course the home owner starts doing his own work.
Neverless, you don't insist your 60 Watt lamp have 12 ga wire or be protected by a 1/2 amp breaker. You plug the 18 gauge or smaller wire into an outlet rated at 15 or 20 amp with a 20 amp breaker. I plug my 2 30 amp shore power cables into an 50 Amp outlet protected by 2 50 amp breakers. I don't believe it would be significantly safer if I could replace the breakers with 30 Amp breakers. The 50 amp breaker is not going to shove 50 amps down the cord whether I need it or not. A short or ground fault is either going to be significantly less than 30 Amp (water intrusion) or significantly more than 50 Amp.
The main reason I posted here was there was a thread a while back (it may have been on another forum) where a Beneteau owner found that his brand new boat was set up with 50 Amp inlet connectors and a custom cord with a 50 amp female end and a 30 amp male end. The majority of posters said "OMG, you should get that fixed immediately", "what if someone actually plugs it into a 50 amp cord or outlet?" Of course his dealer declined to replace the inlets and cords. My thought was - Good for Beneteau, there's nothing worse than those 30 amp connectors. Smartplugs may have been better, but 50 amp connectors are way better than 30s. And if someone did plug in an $800 50 amp cord or use an 50/30 adapter on a 50 amp pedestal - so what? Its still a 30 amp boat, it wont draw more than 30 amps for more than 30 seconds.
Its not at all like plumbing a 60-90 psi municipal water supply into a fresh water system designed for a 30-40 psi onboard pump.

dlochner · Apr 8, 2019

jlp0217 said:
dlochner - "It is not at all inconceivable that the corrosion in the plug could increase the current draw well above 30 amps"

I stand corrected on Ohm's law.

Davidasailor26 · Apr 8, 2019

dlochner said:
I stand corrected on Ohm's law.

Well, it's not Ohm's law that would lead to the condition you originally cautioned about - corrosion increasing current - but you're still right that it can happen. Some devices will draw a constant power regardless of input voltage (within range). For example, switching power supplies and some motors will just draw more current as voltage begins to drop, to provide the same output power. If you have a corroded connection in line with these devices the voltage drop will cause them to draw more than they were spec'd to. Having said that there is still the same 30A breaker on the boat if the total current draw gets too high.

As I see it limiting the dockside connection to 30A stops three potential failure modes:
- Failure of the boatside receptacle or cord that allows it to draw more than 30A without being a dead short.
- Failure of the 30A breaker on the boat to trip if genuinely overloaded.
- Strange previous owner or OEM configuration of on board circuits as Maine Sail describes.

None of the above strike me as being frighteningly common, but if you can take an easy step to elimintate that already low risk, why not take it? In the OP's case, his marina was willing to change him down to a 30A outlet, so he gets a free belt to go with his suspenders. I don't see the harm in that.