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Old 04-03-2015, 06:13 PM   #21
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Okay this has turned into a conversation way above my pay grade. Appreciate the quick input! I think the best outcome has just happened. Marina owner now has called again to say the plug has been switched to a standard 30 amp config, and the breaker has been changed to pop at 30 amp. No sheisty workaround necessary.
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Old 04-03-2015, 06:45 PM   #22
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Ski, do a little reading on Joule heating.
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Old 04-03-2015, 06:49 PM   #23
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Originally Posted by Ski in NC View Post
Also, in my investigations regarding shore cord fires, almost never was it an overload. The fault was a high resistance connection at the boat connection that was carrying below trip level current. This created enough heat to ignite something. Current draw likely never went up above normal. Never did the breakers trip until the fire caused conductors to short. At this point the fire was already started and the trip level of the breakers was irrelevant.
I think you will find the BoatUS Seaworthy article supports exactly what Ski states... it's the hi resistance - no overloading - that causes heat and heat is the culprit in boat fires.

I would bet there are many instances of 30 A cords plugged into 30A pedestals that have overheated and burned inlet connections or worse caused boat fires - I don't believe this is limited to using "Y" adapters - while I don't have the data I don't believe adapters alone have contributed significantly to boat fires - anyone have real data to prove me wrong??

Still not convinced... take a look at RC's/ Compass marine article on boat shore power cords... RC is an independent "ABYC Certified Marine Electrical Systems Specialist". If you are really concerned about overheating shore power cords I think you would be installing one of the new SMART PLUGS - No I have no interest in Smart Plugs but I believe what he has done a great job explaining the +/- in an unbiased fashion.

Just my opinion!
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Old 04-03-2015, 07:23 PM   #24
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Originally Posted by caltexflanc View Post
Ski, do a little reading on Joule heating.
I am very aware of the math surrounding resistive heating. Ex power plant systems engineer. Is there something besides that you suggest I read?
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Old 04-03-2015, 09:53 PM   #25
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That's the mistake people make, ignoring what can cause high amp draw even with the boat's demand minimal. And that's the kind of mistake that causes electrical fires on a few of those bazillion installations without good practices and very regular checkups.
What is the issue do you see arising between the boat and pole breakers that wouldn't trip a 50A CB but would trip a 30A?

Just about every issue reported is high resistance from corrosion...something that never seems to lead to tripping the breakers till there is already melted plastic and smoke.
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Old 04-04-2015, 01:11 AM   #26
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My boat uses 30-amp shore power, so I use an adapter to connect to 50-amp power at my berth. Here I'm removing the 50-amp adapter for docking at a "foreign" berth with 30 amps.


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Old 04-04-2015, 01:44 AM   #27
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My boat uses 30-amp shore power, so I use an adapter to connect to 50-amp power at my berth. Here I'm removing the 50-amp adapter for docking at a "foreign" berth with 30 amps.


Very foreign looking boat in the foreground.
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Old 04-08-2015, 11:20 PM   #28
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Remember that if your cord is a Marinco they will replace it for free if the cord/plug ends show signs of being burnt. I sent them a cord that had the boat side plug burnt. They sent me a new cord and a new plug for the boat. They said that I needed to replace both the cord set and the boat receptacle to insure the problem was fixed. Amazing!
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Old 04-09-2015, 02:22 AM   #29
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There is some fact and some misunderstanding here.

Ski's answer is the best so far...

A 50 amp receptacle what's ben called here 50/250 is really two hots each 120 volts to neutral. Those two hots are either 180 degrees out of phase resulting in 240 volts phase to phase, or are just as likely to be 2 legs of a three phase system resulting in 208 volts phase to phase and being 120 degrees out of phase.

Mathmatically it's 120 times the square root of three if anybody cares.

As far as using a spliter and two 30 amp cords on a 50 amp pedestal I have no issues with that, for the exact reason ski mentioned. Your boats panel should provide 30 amp overcurrent protection.

The reality of the situation is if you have a short circuit fault in your power cord(s) or inletsit's going to draw enough fault current and trip the 50 amp breaker. If you have a simple overload the boats 30 amp panel will trip out.
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Old 04-09-2015, 02:29 AM   #30
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Remember that if your cord is a Marinco they will replace it for free if the cord/plug ends show signs of being burnt. I sent them a cord that had the boat side plug burnt. They sent me a new cord and a new plug for the boat. They said that I needed to replace both the cord set and the boat receptacle to insure the problem was fixed. Amazing!

Good to know. Thank you! Did you get in touch with them through their website?
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Old 04-09-2015, 07:38 AM   #31
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Originally Posted by Bacchus View Post
I think you will find the BoatUS Seaworthy article supports exactly what Ski states... it's the hi resistance - no overloading - that causes heat and heat is the culprit in boat fires.

I would bet there are many instances of 30 A cords plugged into 30A pedestals that have overheated and burned inlet connections or worse caused boat fires - I don't believe this is limited to using "Y" adapters - while I don't have the data I don't believe adapters alone have contributed significantly to boat fires - anyone have real data to prove me wrong??

Still not convinced... take a look at RC's/ Compass marine article on boat shore power cords... RC is an independent "ABYC Certified Marine Electrical Systems Specialist". If you are really concerned about overheating shore power cords I think you would be installing one of the new SMART PLUGS - No I have no interest in Smart Plugs but I believe what he has done a great job explaining the +/- in an unbiased fashion.

Just my opinion!
Has not Smartplug been debunked? A very active thread on this about two years ago.

Also, breakers at dock trip on time and voltage. A good test is to intentionally load the dock breaker above 30 amp and monitor voltage and time.
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Old 04-09-2015, 09:55 AM   #32
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As often is the case, there is a fair amount of confusing several issues into one. Kevin summed it up best. There is a reason there is a breaker at the dock pedestal, and that it is sized a certain way. And why there should be an appropriately sized fuse or breaker right after any connection point leading to a smaller sized cable or lower ampacity circuit.

There are a variety of ways fires happen. The RC (who I am generally a big fan of) article discusses an all too common one, nicely and in depth. Though he is discussing why he loves Smart Plugs, and hates the old standard, his cautions about proper use of traditional cords and connectors is essential reading for any boater.
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Old 04-09-2015, 02:18 PM   #33
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I just got a call from the marina owner (it isn't your typical shiny new big marina, it's just one long old wooden floating dock) and he said he would change the breaker so that it will trip at 30 amps. So I'll be all set with a plug adapter.

No real need .

Most boats with a 30A power cord are wired so if the total on board load goes over 30A a breaker on the boat will pop.

Your power cord is protected because your boat is limited in its load.
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Old 04-09-2015, 03:23 PM   #34
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Most boats with a 30A power cord are wired so if the total on board load goes over 30A a breaker on the boat will pop.

Your power cord is protected because your boat is limited in its load.
This thread has kind of gone full circle. What FF is saying is true. Most boats have an onboard breaker and those that do not should because it is an electrical requirement.

NFPA 302
9.9.2* Overcurrent Protection Location. Conductors other
than cranking motor conductors shall be provided with overcurrent
protection at the source of power unless permitted by
9.9.2.1, 9.9.2.2, or 9.9.2.3.
9.9.2.1 Where not practical to locate overcurrent protection at
the source of power, conductors other than cranking motor conductors
shall be provided with overcurrent protection no more
than 7 in. (18 cm) from the point at which the conductor is connected
to the source of power measured along the conductor.
9.9.2.2 If the conductor is contained in a sheath or enclosure,
such as a junction box, control box, or enclosed panel,
throughout the required distance, and where not practical to
locate overcurrent protection at the source of power, conductors
other than cranking motor conductors shall be provided
with overcurrent protection no more than 40 in. (102 cm)
from the point at which the conductor is connected to the
source of power.

With an onboard breaker, the argument that the 50 amp shore breaker does not protect the 30 amp shore cable is almost mute. To make it more insignificant,.... look at the electrical standard. All 30 amp shore power cords using #10 wire with 140 degree F insulation is good for 40 amps. With 176 degree F insulation and #10 wires it is good for 50 amps. Outdoors in open air anybody that questions the ability of a 10 gauge 30 amp shore cable carrying up to 50 amps is blowing air in a paper bag.

NFPA 302
Table 10.13.6(a) Ampacities of Insulated Conductors
Temperature rating of insulation
AWG 140F 167F 176F 194F 221F
10 ...40A .. 40A.. 50A . 55A . 60A
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