circuit breaker sizing

[drewpster]

I am using two main breakers to supply the "house" circuits on my boat. One breaker supplies the main fuse/ switch panel. And the other supplies the motor circuit for the head relay.

The panel has 6 branches supplying Interior lights, two spot lights, stereo ect. I want to allow for some expansion later. The head breaker supplies the head motor only. Both breaker wires are 10ga marine 105c and both are longer than ten feet.

How do I size the breakers for proper protection without nuisance tripping?

 

[bruceb58]

Re: circuit breaker sizing

Here is a good wire sizing chart. If you go by the guage wire given in the chart, you can go with the same size breaker as the load.

http://www.cmsquick.com/Tech_00.html

Here is another:
http://www.westmarine.com/pdf/MarineWire.pdf

 

[drewpster]

Re: circuit breaker sizing

Should I go through the trouble of finding out the total amp draw of everything then size the breaker accordingly? Or should I guesstimate?


Do you know an easy way to ballpark based on component specs, number of and total wattage of bulbs, voltage drop ect? Am I over thinking this?

As I understand it a breakers function is to protect the wire run and trip fast enough to save components. Does this mean that I should size the wiring to provide for the load....then size the breaker according to the wire size and run length?

 

[Silvertip]

Re: circuit breaker sizing

You better be doing things in the proper order. 1) determine the loads you will place on a circuit. 2) Size the wire for the amount of current the wire must carry, 3) Size the breaker/fuse for the load. Get the ampacity of the wire less than the breaker size and the wire becomes the fuse in that circuit.

 

[drewpster]

Re: circuit breaker sizing

Thanks for being so clear. I had not heard it put quite that way.

The part I am unclear about is accurately determining the loads. I have alot of information on getting the conductor size as long as I know the load. I am not so worried about conductor size as I have a good bit of experience working on DC. If anything I will probably end up overkilling conductor size just to be sure. (Mechanic)

The part that seems to confuse me is determining a nominal load and sizing the breaker accordingly.

 

[Silvertip]

Re: circuit breaker sizing

Virtually every electronic component has a maximum current rating and the manuals will specifiy that. Your radio, gps, locator, etc. all have current specfications. If you don't have those specs, go the Manufacturers web site or call their customer service. An individual bulb, depending on size, will draw less than an amp so you add up the bulbs (say you have six). Use about 1/2A for each so you have 3 amps worth of bulbs on that circuit. Now add the current from the other electronics. That total is what you need to size the wire for and the circuit breaker. If the load totals 7 amps for example, use a 10 A breaker. 16 ga. wire would be acceptable. The breaker is intended to protect the branch -- the 16 gauge wire in this case. If you connect a locater to that circuit through an in-line fuse, the 12V and ground leads to that device are usually not 16 ga but rather something less. If a short were to occur in those wires before the fuse, they will burn as the 10A breaker will not trip. However, if the locator is connected to a fuse or breaker panel and the appropriate size fuse for that device is used there will be no problem as a short cannot occur before the fuse or breaker.