Breaker AMP rating

[South FLA]

What size circuit breaker are you guys running on your boat's house battery? 

My breaker was corroded and I would lose power intermittently so I ordered one. When speaking to the Ranger boat part rep. she said that the breaker size was a 50 AMP model, but the actual size of breaker in the starboard breaker switch was a 30 AMP Carling Technologies breaker.  I asked if she was sure and that I though it was weird that they would spec a 50 AMP on the house battery side since the original was a 30 AMP. Anyway, I ordered two and now I am thinking I should have pressed the issue more since 50 AMP seems too high for my house battery breaker, by the way the trolling motor breaker is 50 AMPs, so I am thinking she might have been confused. Should I go ahead and send one back and keeping  the other for my trolling motor side replacement or should I just put the new 50 AMP breaker in the housing?

[Wayne P.]

A circuit breaker is sized to the wire that it is hooked to, It is to protect the wire. A circuit breaker or fuse that is too large just makes the wire the failure point (aka FIRE hazzard)

[Way2slow]

The load determines the size of the breaker and the wire.  If your running a 50 amp load on 30 amp breaker, it going to trip.  If you're running a 50 amp load on a 60 amp breaker and the wire is too small, it's going to over heat, to the point it can catch on fire.

[Fishing Rhino]

Unless I'm mistaken, the large amp breakers are for the main wires which go to all the electrical/electronic systems of a boat.  All secondary circuits need to be protected with the appropriate amp fuses or breakers for their purpose.

For example, my Humminbird installation instructions say to use a three amp fuse in its circuit.  Anything larger will void the warranty.

[Team_Dougherty]

Circuit breakers and fuses are made to protect the wire and the wire only. it has nothing to do with the load. If you have 14awg wire it needs to be fused at 15 amps 12awg 20 amps etc...  you can go with a lower fuse but not bigger.

[Ann-Marie]

Thanks Team_Dougherty, couldn't have said it better.

If a load is relying on a cirucuit breaker or fuse for protection it is already dead by the time the breaker trips.

Keep in mind the rating applies to the lightest gauge anywhere in the circuit.  If you have a heavy gauge main line that branches to a light gauge feed to an instrument, then that light gauge needs its own protection since the main one could supply enough power to melt the small wire without tripping.

[South FLA]

Thanks everyone, went to the Palm Beach boat show this weekend and picked up a catalog from Boat Owner's Warehouse, flipped through it and found the exact 30 Amp. breaker that corroded on my boat and was causing issues.  Cool thing is that in the catalog the had a $10 boat show purchase coupon that I used to by that and some weather strip adhesive for my latches  ;D  Problem is now I am stuck with (2) 50 Amp breakers that I don't need. I will keep one for my trolling motor breaker which is same size as OEM.

By the way after I found this cool web page after reading your posts and surfing some more

http://www.powerstream.com/Wire_Size.htm

[Way2slow]

Yes, you are 100% right, the protection circuit is should not be larger than what's specified for a given wire guage. However, as I said, the LOAD determines what guage wire and circuit breaker must be used.

Ann-Marie,

That's the second time I've seen you post that the load was not relying on the circuit breaker or fuse for protection and yet to figure out where you get that idea from. Over the years I've seen hundreds of motors and pieces of equipment save by a fuse/circuit breaker. Motors and most equipment have a certain amount of over current protection designed in so that if you stall or overload it, when properly fused or the proper size breaker is installed, they blow/trip before it damages the motor. You run a large enough wire to handle the load, and you run the proper size protection circuit to protect the load.

In a dedicated circuit like for a trolling motor, the wire size is usually much larger than the actual breaker size because you have to allow for internal resistance of the wire that will cause a reduction in performance. I you used your logic, and used the size breaker specified for the larger wire and you stalled the motor, you would smoke your motor instead of tripping the breaker.

Yes, in a branch circuit you fuse to the wire size on the main line, but still, the main wire has to be large enough to handle the total load Then you fuse to each individual load requirement off that. Just because you are running your depth finder off a 14 guage wire coming off a 10 guage main line, doesn't mean you should use a 15 amp fuse as you would suggest. After all, if the fuse is not to protect the load, you wouldn't have to worry about all those individaul fuses for each piece of equipment as long as you had the proper size fuse for the branch line feeding them.

[Fishing Rhino]

I'm sure we've all seen reset buttons on electric motors, vacuum cleaners, and other appliances.

They exist to protect the unit, not to prevent meltdown.

While fuses were initially used to prevent house fires. Now, automatic circuit breakers which will reset when they cool down are to protect devices from damage.

Windshield wipers on cars for example. You turn them on a winter's day and ice prevents them from working. The circuit will shut down, and reset when it cools, thus saving the wipers from self destructing.

Sometimes they may blow a fuse. Replace the fuse and they will work.

It's common to read in the trouble shooting instructions, when a unit doesn't work, to check the fuse that can usually be found on the back of the unit. Remove the fuse cap, replace the fuse, and the unit will often work.

The assertion that fuses aren't intended to protect devices is wrong. Some are, some aren't.

Ground fault interrupters are a unique type of circuit breaker/resettable fuse, the purpose of which is to prevent injury, or even death. 

[Ann-Marie]

Agree, you CAN provide precise controls matched to the load characteristics but since those specifications are so load dependent protection is usually provided by the manufacturer IN the load, not as a separate external fuse or circuit breaker.

If it is a "motor control" with only a single specific motor on the output side you are correct, they have thermal and magnetic sensors matched to the load time constants and ratings.  These can detect things like locked rotor and remove power befor the target self destructs.

It is rare in small boats that an individual load would have a dedicated fuse or circuit breaker that would be precise enough to protect the device from overload without considerable research and expense.

In general the discussions here typically refer to a distribution bus carrying multiple loads rather than trying to match individual loads.

[Team_Dougherty]

If it is a "motor control" with only a single specific motor on the output side you are correct, they have thermal and magnetic sensors matched to the load time constants and ratings. These can detect things like locked rotor and remove power befor the target self destructs.

http://www.camiscomponents.com/acdc-motor-thermal-protection-relay-p-28493.html

[Way2slow]

I was the factory tech rep for almost all major brands of stereo equipment sold in northern Italy to military and civilians markets for four years. I would not have a clue at how many hundreds of pieces of equipment I repaired that the only fix was to replace the fuse.

I deal with electric, industrial material handling equipment on a daily bases. A lot of it running 500 amp fuses and again, it's very common for it to blow the fuse before it damages anything else. The ones that get expensive is the idiot that installed a 500 amp fuse in a 400 amp circuit, he usually buys a very expensive motor.

I've seen outboard motor, voltage regulators go bad and cause large spikes that blew electronics fuses. When properly fused, I just replaced the fuse and they were back in service. How would you like to send your HDS-10 in for repair and send you a major size bill because it was over fused?

All I can say is, I will never over fuse anything of mine, using your idea of "it's not going to protect it". I have waaayyyyyy too much experience to the contrary.