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Marine Shore Power Cords

Proper Use and Maintenance Will Increase your boat’s safety

aftermath of boat fire

Let’s face it–we all love the convenience of shore power–those ubiquitous yellow power cords allow us to connect marina utilities to our boat’s AC power distribution systems, providing us with all the comforts of home. But there can be a serious downside to shore power when power systems are overloaded or when equipment is not properly maintained. I recently witnessed two on-board fires in neighbouring marinas, where, fortunately no was injured but where the consequences of the fire were none-the-less devastating for the owners. I had the opportunity to view the rather sad aftermath of one of the fires and it was obvious the fire originated at the shore power inlet.

In fact, fires that occur at the dock are often caused by shore power connections; the marine environment can cause problems with even the best designed devices. If problems can be detected when they are small, it can save the boat owner time and expense later on. The most common problem with electrical connections is corrosion resulting in overheating. Fortunately, overheating can be easily detected and quickly remedied.

What you should know about shore power connectors

Basically, there are five receptacle and connector systems approved by the National Electrical Manufacturers Association (NEMA) for marine use. The three most common systems are the 20 ampere (amp) 125 volt; 30 ampere 125 volt; and the 50 ampere 125/250 volt. Each system has a terminal arrangement that varies in radius and blade shape so as to avoid mix ups be-tween different types of plugs. The 20 amp, 30 amp and 50 amp plugs/receptacles for each of these systems have different, non-interchangeable configurations. This is a safety feature de-signed to prevent a plug from one system from being used with a different system so never force a plug into a receptacle. As long as the L-shape terminals of a marine shore power plug remains unaltered, it is hard to plug your boat into the wrong type of shore power. Forcing a modified 30 ampere 125 volt plug into a 50 ampere 125/250 volt receptacle can produce disasterous results.

Some important safety considerations when using a shore power cord

  1. Shore power plugs and receptacles have L-shaped or locking terminals. Be sure to twist the plug into the receptacle until it is fully locked into place–this is the first step when connecting a power cord. The second step is to thread the locking ring on to the inlet and securely lock the cord to the inlet. Movement of the cord plug can compromise the waterproof seal of the con-nection allowing moisture to enter causing corrosion to occur, increasing resistance, and gen-erating heat and possibly fire. This is why you should always thread on the locking ring and ensure it is tightened up. In general, If a cord is missing its locking ring, then it shouldn’t be used.
  2. Electrical inlets or receptacles should be ‘out of the way’ and not subject to crew accidentally kicking or stepping on (or tripping over) the cord/connection. If you cannot easily relocate the inlet, rig up some way of securing the cord by tying it down to the deck or to a suitable rail or fitting to prevent stress or damage to the connection.
  3. A shore power cord should not be run directly from the boat to the dock without being secured to the vessel in some manner. If they are, the plug may be stressed every time the vessel moves, resulting in a poor connection. Failure to secure the connection leaves it vulnerable as the cord and plug move with each tide, or passing wave, or boarding of the boat. As the unsecured plug works its way out of the receptacle, the contact area of the blades is reduced. If the loads are high—water heater, air-conditioner, high-output charger, and the like—the sur-face area of the connection becomes inadequate to meet power demands, possibly leading to overheating.
  4. Attach a string or other support to the cord near the connector to provide strain relief.
  5. Like many other components, shore power cables have a limited life and should be replaced well before they give out entirely. Exposure to the elements, age and corrosion can lead to catastrophic failure.
  6. Try to avoid the use of adapters — although this is almost impossible while staying at different marinas. If you need to use an adapter, connect it on the end of your dock-side outlet. It is also good practice to provide some support to the cord/adapter connection (e.g. using a light line).
  7. Use only approved cords that are UL-Marine listed, that have no splices and can carry a minimum of 15 amperes (14 AWG size wire). When it comes to wire gauge sizing do not use undersized wire. When in doubt, use the next heavier gauge. Lighter wire is not only risky, but it makes appliances sluggish. Here is a guide for insulated conductors and their ampere capacity:
    Wire Size (AWG) Amperes
    14 15
    12 20
    10 30
    8 40
    6 55
  8. Understand the load your electrical appliances draw. For example the Caframo heater I use to keep my boat warm has a rating of 1500 watts (all appliances must have these specs displayed on them somewhere). To determine the amps used by the heater, I use this formula: watts / volts = amps. So for example, my 1500 watt heater uses 12.5 amps because 1500w/120v = 12.5 amps.
    My wife’s hair dryer uses 1200 watts which converts to 10 amps. My battery charger uses about 6 amps (this can vary) the total demand is now 28.5 amps. Now factor in some resistance in the on board wiring and at all the connections in the system and we probably have more than the 30 amp limit of my shore power cord. This demonstrates how quickly and easily we can ‘max out’ our electrical systems.
burned out plug

Overloading your boat’s circuits can cause damage, overheating or fire hazards. It is important to design and install or upgrade an AC system that can accommodate the electrical load you need.

Here is another equation to help you determine requirements: amperes x volts = watts. A 15 ampere, 120 volt system has 1,800 available watts; a 20 ampere, 120 volt system has 2400 available watts; a 30 ampere, 120 volt system has 3600 available watts. By comparison, a 50 ampere, 250-volt system has 12,500 available watts. For reference, a common appliance such as a space heater uses about 1,500 watts; a toaster uses 750-1200 watts; a vacuum cleaner uses 800 watts; a television uses 300-750 watts

What to look for

Lightly touching the shore power plug at the inlet to feel for excessive heat is a good start. After turning off the power, disconnect the shore power cord and closely examine both ends. Look for discolouration or melting around the blades of the plug (male end) and around the slots on the connector (female end). Examine the face of the inlet on your boat and look for discolouration or melting around the blades and the inlet. Examine the outlet or receptacle on the dock and look for discoloration or deterioration around the slots. Problems that may go unnoticed in the summer (light load) can become a big issue in the win-ter with the additional use of heaters!

aftermath of boat fire

What can cause overheating

Severely corroded blades or contacts are a result of exposure to a corrosive environment, most commonly salt water. If the ends of the cord set are dropped into salt water and not properly cleaned and dried, the contacts will eventually corrode. Corroded contacts do not make a good electrical connection and overheating results.

High resistance at the connector, coupled with a steady load over a long enough period of time can create significant heat. This increases the resistance even further. Eventually, a high enough temperature is reached to melt the wire insulation and/or the plastic core of the shore power connector. This can lead to a short circuit and possibly ignite causing a fire. The reason is that circuit breakers are designed to trip when there is too much current flow (short circuit) but may not trip when there is high resistance somewhere in the circuit.

In addition to corrosion, not turning off any loads within the boat (such as a running electric heater, water heater or refrigerator) prior to plugging in or unplugging the cord can cause elec-trical ‘arcing’ to occur, resulting in pitting, corrosion (because the contact’s plating is damaged), and carbon deposits on the plug blades. The AC shore power is connected and disconnected hundreds times over the years and if this arcing ‘n’ sparking happens enough times a carbon film may form, increasing a resistance that can generate more heat.

Failing to properly connect the shore power plug into the inlet can cause poor connections and heat generation (a no-brainer). The shore power cord uses a combination of twist-lock plug blades and a locking-ring to achieve a secure mechanical and electrical connection to the inlet.

Sometimes one of the connecting screws inside the inlet can cause increased resistance through vibration, seasonal changes (freeze/thaw), condensation, salty environment or any combination. A bad connection in an inlet can also cause the mating connector to overheat.

Here are some suggestions to reduce the risk of fire and extend the life of your shore power cord and AC electrical connectors:

Turn off all the boat’s loads (or the main AC breaker) and the dockside breaker before plugging in or disconnecting the shore power cord. It is good practice to plug the female end of the shore power cord into your boat first and then the dockside power box. Reverse the sequence when disconnecting prior to shoving off.

Never leave a shore power cord plugged into the dockside power box with the boat end disconnected laying on the dock. The free end can be accidentally kicked into the water creating a hazardous situation.

Take a moment to inspect the inlet and the plug end every time it is connected or discon-nected. Regularly check your shore power cord for cuts, exposed wire, breaks, discoloured plug, inlet or receptacle terminals, bent, broken or corroded plug blades, deterioration around the slots and corrosion. Also important is to inspect the inboard side connections of your shore power inlet checking for discolouration and corrosion. It’s better to err on the side of caution, and if anything appears to be failing, replace the connector and the cord at the same time.

Always leave the dockside breaker off when shore-power cables are not connected to the boat.

If you accidentally immerse your power cord in salt water, shut off power to the cord immedi-ately (if necessary). Rinse cord thoroughly with fresh water and allow to dry thoroughly. When dry, spray the cord with an electrical contact cleaner. This will displace any remaining water.

Do not let your power cord ‘hang’ so that there is strain on either end (boat inlet or dockside box). Always provide some support or strain-relief by using line or velcro straps.

Be aware that problems that may go unnoticed in the summer (light load) become a big issue in the winter with the additional use of heaters so be extra cautious when preparing your boat for the winter.

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