Shore Power Connections -- Again

[Dirk01]

Originally Posted by Mystic38

Sorry..but there is no 80% rule at all.

If there was an 80% rule then a 50A cable and connector would be rated at 40A.

perhaps "Rule of thumb" is the phrase you are looking for... an approximate guess based upon some random set of subjective bias.. which as you see is a ridiculous concept..

with a well fitting, clean set of connectors there is zero need for any rule of thumb regarding utilization ratio.. a 50A cable/connector set can be run at 50A continuous.. and if it could not..it would not be a 50A cable/connector set

Conversely, to support the OP first post... with a badly fitting corroded set of connectors even a 10% rule of thumb would be dangerous.

The bottom line here should not be a discussion of whether a cable should be derated based upon a knee jerk subjective number with no validation based upon actual data, or whether the shore power should be wired direct to the boat or not, but that ALL shore power connectors and cables should be inspected and serviced regularly.

Then just call it life experience. Doesn't matter if it's "a rule". It's safe. And it works.

[Auspicious]

Originally Posted by Mystic38

Sorry..but there is no 80% rule at all.

There is history here.



Story (you can skip this): When I was in college we had an assignment to design a bolt pattern and select bolts for a blanking plate at the end of offload pipes on an oil tanker. We all got pretty much the same answer but it didn't make sense. My school was big on practical experience so we had a pretty good idea of what the numbers should be. As a group we went to the professor who was very pleased with himself. We all trooped off to the classroom and he walked through the method. He came up with the same number of bolt holes that we did. Then he challenged us. Did any of us look at what actual happened when we went to sea? The deck crew would put every other bolt in and there might be a stripped thread or a dropped nut so one would be missing. Working backwards on that basis and with a reasonable safety factor we got the bolt sizes we expected. The moral is users don't use stuff right.



There is also history, especially in electrical systems, on duty cycle. For example you'll often see cigarette lighter power points fused for 30A. This is a real pet peeve of mine. Sure they can carry 30A for a minute for a cigarette lighter. The continuous duty rating for the connector is only 6A! I've seen three triplex adapters ganged and seven devices plugged in to charge. Open up the panel and see the charred wire insulation. Not good.


There are a lot of intermittent loads on boats. Aircon start-up is a big one. Charging flat batteries at the beginning of bulk is a big one. A boat plugging in with Aircon and battery charger and water heater all flipped on is huge. So the engineers size for the big load (30 or 50A in the US, 16 or 32A in the EU) for a while and continuous load is smaller. Here is what I would do: size for the maximum intermittent load, check the continuous load and make a subjective assessment of whether the margin is big enough. 20% isn't big from the design perspective but probably big enough for the market. You have to look at boundary conditions. In this case it is the specs on the shore power circuit breaker.

[Pelagic]

Good post Auspicious.
One of the most contentious issues I dealt with on New Build Superyacht projects, were my contractual requirements for a detailed "load analysis"

Primarily for deciding on balanced Generator Sizing

The electrical contractors were comfortable performing one, but the Builders resisted, claiming every boat is different in how they use power.

I never bought that line and insisted....., ....design for the worst case scenario and provide adequate non-preferential switching to keep things safe in a shore power scenario.

Same applies to our smaller yachts, that are having more and more AC PowerPoint's .... The original AC distribution board is too small and you often need to add a 2nd sub panel for smaller more appropriately sized breakers.

On the Super yachts with unlimited budgets, the only way to insure proper compliance was to specify UMS Certification, (Unmanned Machinery Space) as part of the required documentation..... Smaller boats should study those requirements

[Auspicious]

Originally Posted by Pelagic

The electrical contractors were comfortable performing one, but the Builders resisted, claiming every boat is different in how they use power.

I agree with you. Boats the size you're talking about generally have multiple identical generators with frequency synchronization. This is great for sparing and for operational efficiency.

Owners keep adding stuff and it all plugs in. You have to size for growth and multiple generators helps, especially if you leave space to drop in one more some day.

In my view personal electronics have helped, especially if you put USB charging ports on the 12VDC bus instead of the 117VAC/240VAC bus. Opinion on my part.

Got to get the galley loads right. Boat systems other than thrusters (often mechanical/hydraulic on superyachts) are less.

[Maine Sail]

Originally Posted by sailcrazy

followingI've never heard of "hard-wiring" the shore power cord in, and just pulling it out of the locker when you need it....eliminating the shore power plugin and receptacle all together. An interesting idea!

Very common on large power yachts and sport fishing boats but usually on a retractable reel. Glendinning makes a great shore power reel but you need some space for it. If hard-wiring just be sure the connections are in a j-box and you have plenty of strain relief. Not as uncommon as one would think but usually not found on production boats...

[Pelagic]

Originally Posted by Maine Sail

Very common on large power yachts and sport fishing boats but usually on a retractable reel. Glendinning makes a great shore power reel but you need some space for it. If hard-wiring just be sure the connections are in a j-box and you have plenty of strain relief. Not as uncommon as one would think but usually not found on production boats...

Hi Maine Sail
One question on those shore Power Cable Reels.

American Engineer I worked with for many years on Super yachts, insisted that the large cable be unwound completely and flaked on the dock as opposed to leaving unused portion on the Reel.

Claimed that leaving this wound would create a magnetic coil.

Never argued, but always wondered if this was true?

Seems there would be a warning in the documentation, which I never saw.

[dfelsent]

I’m certainly not Maine sail but the coil would likely not be an issue assuming no unbalanced current, which would trip your RCD. The magnetic fields from the neutral and hot conductors will pretty much cancel each other out.
There is a valid argument that the tight coil of wire on the reel will run hotter than the loose wire on deck or dock. And that matters.
Coiled extension cords under a high continuous load ( space heater!) are a hazard. Don’t do that.

[GordMay]

Originally Posted by Pelagic

... American Engineer I worked with for many years on Super yachts, insisted that the large cable be unwound completely and flaked on the dock as opposed to leaving unused portion on the Reel ...

Cable is not stored on a reel, but in a bin beneath it.
Glendinning operating instructions do NOT require fully extending the cable out of the storage bin.
Ie:
Extend the power cord: Pull out sufficient cord length that will allow you to route shore power cord to the electrical distribution box and plug the molded end into the receptacle.
However:
Experience has shown that when only a short section of power cable is regularly used, the cable may be subject to “kinking”. To relieve this condition, routinely extend the power cable completely and stretch it on any smooth surface. Allow the Cablemaster to retract the cable onto the reel.

[Pelagic]

Hi Gord,
On the ones we had onboard, the shore cable was definitely stored on a large reel about 5-6 ft in diameter.
Ships demands were 3 phase 380v 350amps, so a substantial cable about 50m long was on that reel that powered in and out via a roller system thru hull openings (either side)

I remember that power fed out via the Axel of that reel to the step up/down isolation transformer, then to the Frequency Controller.

All huge equipment in the 90's

[Auspicious]

Originally Posted by Pelagic

American Engineer I worked with for many years on Super yachts, insisted that the large cable be unwound completely and flaked on the dock as opposed to leaving unused portion on the Reel.

Claimed that leaving this wound would create a magnetic coil.

Originally Posted by dfelsent

I’m certainly not Maine sail but the coil would likely not be an issue assuming no unbalanced current, which would trip your RCD.

There will definitely be a magnetic field. It won't be sufficient given the balanced current to have any effect.

Originally Posted by GordMay

Cable is not stored on a reel, but in a bin beneath it.
Glendinning operating instructions do NOT require fully extending the cable out of the storage bin.

Some reels store the cord on the reel and use slip rings. I'm not a fan of that sort but they are out there.

[GordMay]

My apologies, for my mistake. The Glendinning “CR” series are reel style, not bin storage.
➥ https://www.glendinningprods.com/pro...master-marine/

[Auspicious]

Originally Posted by GordMay

My apologies, for my mistake. The Glendinning “CR” series are reel style, not bin storage.

Correct. The CM series run from a bin, the CR series store on the reel. Bin storage takes a bit more room and alignment during installation is important to avoid the shore power cord equivalent of castling. The reel storage approach is easier to install but slip rings lead to the same sort of heat, corrosion, and resistance issues as a connector. In addition they are electrically noisy during operation. I'm not a fan.


I should point out that I'm not a fan of slip rings. Reel storage is fine. I've seen reels with pig tails--no slip rings--that are fine and pretty convenient. Big. Probably beyond the CF constituency.

[moseriw]

not only check them. One has to renew them every year by cutting the wire shorter.

[Bbolino]

A ground fault interrupter is not an over current device and must be paired with on on s circuit - ground fault breakers have both functions, ground fault receptacles do not.

A ground fault protector will usually trip when bad connections cause arcing

Current National Electrical Code (NEC) requires all shore power feeds to be ground fault protected - many old marinas are not.

The NEC mandates that over current protection and wiring systems be sized for 125% of the connected load. However receptacle loads are not subject to that and several 15 amp receptacles (the most common one in the US) can be connected to a single 15 amp circuit. While it is probably a good idea to limit loads to 80%, there is no requirement to do so and your wife's 1500 Watt hair drier draws about 13 amps (86%).

A circuit breaker will trip if subjected to rated amp loads over a long period of time, especially if its a thermal (vs magnetic) type breaker.

Finally, according to UL-489 the maximum temperature on electrical panels, switches, etc is 50 deg C (about 140 deg F) and the NEC defines the maximum temperature for wiring systems as 50 deg C above ambient. Both of those are pretty warm

[ArmyDaveNY]

Originally Posted by Bbolino

Current National Electrical Code (NEC) requires all shore power feeds to be ground fault protected - many old marinas are not.

While it is probably a good idea to limit loads to 80%, there is no requirement to do so and your wife's 1500 Watt hair drier draws about 13 amps (86%).

It is scary how bad some of the marina pedestals are. Thankfully I don't have to use shore power very often.

While her hair dryer draws 86%, I am guessing (hoping?) she doesn't need it more than three hours!!