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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #11  
Old 09-26-2012
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Re: What size AC breaker?

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Originally Posted by DannyboyUpstate View Post
The maximum breaker size is bound by the type and size of the conductor going to the load.
This is the key. Many people think they can size the breaker based on the load they plan to put on the circuit. The breaker is there to protect the conductors in that circuit, not the load. In order to answer the question "what size breaker" you must know what size wire is being used in the branch circuit. You must NOT use a a breaker rated for higher amperage than the conductors in the circuit. The amperage of the shore power circuit coming into the boat is not relevant. The breaker on the dock is what protects that part of the circuit.
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Old 09-26-2012
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Re: What size AC breaker?

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Originally Posted by fairbank56 View Post
Only if the boat is equipped with a reverse polarity indicator or isolation transformer. If not, the branch circuit breakers must be douple pole.

Eric
100% correct! I too often assume that boats have RPI lights..... Thanks for catching that because on a boat already wired with Square D breakers they likely don't have an RPI either... Doh'.....
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Re: What size AC breaker?

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Originally Posted by SchockT View Post
You must NOT use a a breaker rated for higher amperage than the conductors in the circuit.
If there is not a standard current rating of a breaker equal to the maximum allowable current for the conductor, the next larger standard current rating may be used, provided it does not exceed 150 percent of the current allowed. It is also perfectly acceptable, and routinely done, to use a smaller breaker if you want to use one for the purpose of protecting the load.

Eric
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Old 09-26-2012
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Re: What size AC breaker?

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Originally Posted by fairbank56 View Post
It is protected by the breaker in the shore based distribution panel that feeds the shore power receptacle.

Eric
You are obviously knowledgeable in this area. I agree with what you wrote, but I would never trust the circuit breaker on the shore pedestal. Just me.
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Re: What size AC breaker?

Thanks for all the feedback.
When I bought the boat it had Blue Sea 30amp panel that apparently for some reason the double pole breaker blew and someone by-passed it by putting in a two circuit Square D box with 2 15 amp breakers. Hard wired from to the shore power. I want to put it right and get back to the Blue Sea system. Therefore I have ordered a new 30amp breaker and now am going to get a Blue Sea 3 circuit panel with 15amp breakers and hook these up to the 3 receptacles I have on board. I have one receptacle on one circuit and two receptacles on the other and both will have GFI's. I will rewire if need be. The boat is a Fisher 25 and has a wheelhouse. The Square D box is right next to where my feet are when sitting at the wheel, a perfect place to spill coffee! Of course if I am not at the dock it is not an issue!
Before I started this thread I was thinking I could just hook up the new 30amp and wire it directly to the receptacles but have seen the errors of my way and will then connect the 30amp to the 15amps and make sure all is well all the way down the line. Oh I believe that the shore power is a 20amp. I'll talk to the harbor people if it makes a difference.
Thanks again.
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Re: What size AC breaker?

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Originally Posted by svzephyr44 View Post
I would never trust the circuit breaker on the shore pedestal. Just me.
You have no other choice except to unplug as you said. That may or may not be feasable depending on your requirements. The main problem that Iv'e seen with shore power connections/cables is due to moisture/corrosion/loose connections. These cause the plugs/jacks/wiring to just sit there and slowly cook away and it has nothing at all to do with overcurrent. It's due to the heat caused by the resistance, caused by the bad connection. Iv'e come across shore power plugs that are turning black, melting and/or hot to the touch and the shore side breaker is never going to trip because it's not an overcurrent situation. If you make sure your cable plug connections are clean, tight and weatherproof, and your plugged in correctly, you shouldn't have to worry.

Eric
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Re: What size AC breaker?

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Originally Posted by fairbank56 View Post
If there is not a standard current rating of a breaker equal to the maximum allowable current for the conductor, the next larger standard current rating may be used, provided it does not exceed 150 percent of the current allowed. It is also perfectly acceptable, and routinely done, to use a smaller breaker if you want to use one for the purpose of protecting the load.

Eric
Of course you can undersize a breaker. Electrical code dictates a MAX breaker size for a given conductor size. As for being allowed to oversize a breaker by 150%, That is certainly not allowed in Canadian Electrical Code, perhaps it is different in the USA? By that standard you could put a 20a breaker on #14awg wire? Or worse yet, a 45a breaker on #10 wire?? I don't think so! Perhaps you are thinking about the sizing of over current devices to protect loads. Regardless, you will never have that problem because breaker sizes are available for all standard wire sizes.
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Re: What size AC breaker?

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Originally Posted by SchockT View Post
As for being allowed to oversize a breaker by 150%, That is certainly not allowed in Canadian Electrical Code, perhaps it is different in the USA?
This is as stated in ABYC rules which are recommended standards for boat wiring in the U.S.

Quote:
By that standard you could put a 20a breaker on #14awg wire? Or worse yet, a 45a breaker on #10 wire?? I don't think so!
The maximum ampacity rating of copper wiring is based on its physical size as well as the temperature rating of it's insulation and whether it's bundled, sheathed, in conduit..etc. Your obviously thinking of typical house wiring such as type NM (Romex), which has individual conductors rated at 90°C but is derated to 60°C which gives a maximum of 15 amps for 14 gauge wire as used in a home. Most wiring sold for marine use has a rating of 105°C and when bundled or sheathed with up to 3 conductors gives a maximum amperage rating (outside of engine spaces) of 24.5 amps for 14 gauge wire and 42 amps for 10 gauge.

There are many things to know in order to properly and safely wire a boat which is why the average DIY'er without a clue should not be doing it.

Eric
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Old 09-26-2012
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Re: What size AC breaker?

ABYC allows the main DC fusing to be up to 150% of the ampacity of the wire. This is strictly for short circuit protection, not overload.

They do not allow fusing for loads to be any higher than the wire ampacity.
Most AC wiring on a boat for 15 amp circuits is 14 awg, which has an ampacity of 30 outside of engine spaces - 90 C rated, unsheathed and not in bundle or conduit. The breaker on such a circuit should not exceed the 15 amp outlet rating.

If you have 30 amp inlet and supply there should be a double pole 30 amp breaker within 10' (wire length) of the inlet and that feeds the individual circuits which are usually 15 amp for outlets, water heater, etc.
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Last edited by mitiempo; 09-26-2012 at 04:53 PM.
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Re: What size AC breaker?

Interesting! Who would have thought that the standard for boat wiring would be so far out of step with the standards for dwellings on land!

I think if I was wiring my boat I would err on the side of caution and oversize my conductors. The thought of dumping 30a down a #14 wire just doesn't seem right! I've seen the stuff melt down with less than that. (I am not talking about Lumex...I don't do house wiring!)
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Last edited by SchockT; 09-26-2012 at 10:54 PM.
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