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Simple breaker size question...?

Old 11-18-2002, 07:54 PM
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Default Simple breaker size question...?

Ok, you have two peices of electronics. Both use 4 amp breakers (as per manufacturers).

So, if ya want to put them both on one breaker, what breaker do ya use? 4 amp? 8 amp(4 + 4)? 6 amp (middle of other two [img]/infopop/emoticons/icon_biggrin.gif[/img] )?

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Old 11-18-2002, 08:14 PM
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Default Simple breaker size question...?

Bird,
I am not an electrician on land or at sea. However I believe that you want a four amp for your needs. My idea is if one of the devices is on while the other is off, you definitely need a four amp. If both devices are on and there is a short, the breaker will trip before something gets cooked. Lastly, I THINK most electronics draw about a third of the fuse (breaker) rating under normal circumstances. I'm sure Thom and others will be able to help you more.

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Old 11-19-2002, 03:50 AM
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Default Simple breaker size question...?

You should have each piece of equipment on its own breaker/fuse. Each one should be 4 amps or as stated. If you go to 8 amp and you have only one piece on, it will draw 8 amps before the circuit breaker/fuse pops. Take a look at your house wiring...say 200 amp service, it is broken down into a panel with several breakers for each line. That's my opinion and I'm sticking to it...hmmmm, I'm not an electrician. Where's Thom? [img]/infopop/emoticons/icon_wink.gif[/img] [img]/infopop/emoticons/icon_rolleyes.gif[/img]
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Old 11-19-2002, 07:23 AM
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Default Simple breaker size question...?

Birdman,

Mumblerone is correct. If one of the devices shorts internally it can draw signifigantly more current than it is designed to handle. This could cause other components to fry that would otherwise have been protected. You also need to be carefull as to the type of breaker that is installed. Breakers have a trip curve built into them. This basically allows for different amounts of high inrush current before tripping. It's like a slow-blow fuse. You would want a fast acting or instantaneous breaker.

You could connect them off of one 8 amp breaker or even just a switch for on-off purposes and have a 4 amp in-line fuse to each device.
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Old 11-19-2002, 10:02 AM
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Default Simple breaker size question...?

Birdy;

The breaker is supposed to protect YOUR wires and not necessarily the devices attached to the circuit. However, the breaker should be able to handle the total current flow to all of the devices connected. So you need a 8 amp breaker and you should use a wire size on the load side of the breaker sufficient to handle the full 8 amps, which would be #16.
To protect the individual devices if you wish to do so, then in-line fuses (UGH!!) of 4 Amps should do the job. You may want to connect the 2 in-line fuses directly off of the load-side of the breaker and avoid using any #16 wire. The 4 Amp branch wires from the fuses to the devices should be #18.
If in doubt, always go with a larger wire size. In addition to the burn-up factor, the larger wire size reduces the voltage drop to the device.
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Old 11-19-2002, 01:15 PM
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Default Simple breaker size question...?

Birdman,

Let's make this easy, get a 10 amp breaker.

Now here's what else you need to consider. Each device requires 4 amps to run, there are two of them so they will need a supply of 8 amps to do their job. So you need at least an 8 amp breaker just to pass through the power necessary to operate the devices.

Now, consider that it is true that the breaker is there to protect the wire, not the device, but that doesn't mean that you choose a breaker to accomodate the wire in use. No way in hell. What you do is pick the breaker to accomodate the needed power and then you pick a wire of the appropriate size to meet that need. No one can just give a wire size and say that is what you need to be using. The reason is that there are two factors to be considered in choosing a wire size, the first is the power draw, which you already know, and the second is the length of the wire run. The length of the wire run. The length of the wire run. It is all important. So in you case you need 8 amps, size your breaker about a third above that and 10 amps comes close enough, then size your wire as if you were having to pass through even more amps, something on the order of 12-15 amps would be about right. If you do that and one of the devices spikes in power draw it will cause the breaker to do its job and open the circut. This will happen before the capacity of the wire has been reached so the wire won't light up you boat. That is the end desired result, to keep the boat from burning to the waterline.

To summerize Thom's wireing scheme. ((Load+Load+Load to the Nth) * 33% = Breaker capacity)*33% = wire current carrying capacity. Clearly the most important thing is the wire size. Keep that in mind. When using the charts pay absolutly no attention to the one you'll find for a 10% power drop, only use the chart for a 3% power drop.

Thom

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Old 11-19-2002, 01:45 PM
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Default Simple breaker size question...?

Thom...I gotta be missing something here. I fully understand breaker sized for wire gauge, wire length, and all that. Good to be reminded of that. [img]/infopop/emoticons/icon_redface.gif[/img] Here's the question. Let's say we're powering a VHF that requires 4 amps; should you not have a breaker/fuse to protect the unit after the MAIN 10 amp breaker? I always thought that little inline fuse was there to protect the piece of electronics from drawing too much current if it malfunctioned? This is what I was referring to. Thanks [img]/infopop/emoticons/icon_confused.gif[/img]
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Old 11-19-2002, 02:55 PM
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Default Simple breaker size question...?

From the devices manufacturer point of view I think the in-line fuse the attach is a CYA device. At any rate protection anywhere in the line is just as good as protection anywhere else in the line so one protection device is all you want in there. Aside from that I really dislike in-line fuses. They have a habit of being in places where they are not accessable, they tend to be of different sorts so its not really reasonable to carry appropriate spares, they provide just one more break in the line to invite corrosion, they introduce additional resistance in the circuit, they don't smell good, attractive young ladys dislike them, and they may cause Grandmaw's dawg to be hairlipped. So don't use them.

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Old 11-19-2002, 03:49 PM
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Default Simple breaker size question...?

Thom

I hate disagree with you but with no inline protection after the breaker you risk frying wires between the larger feed wire and the smaller wire from the device. example say you have a 20 amp 12 volt feed for your electronics, going to the distribution terminal, this feed is protected by by a 20 amp breaker downstream towards the battery. When you connect power from that distribution block to your GPS with number 20 wire provided by the manuf. Now say that power wire is 6ft or the plug that connects the power to the GPS has a little resistance. If the GPS develops a internal short at the plug, that 20 gauge wire performs the classic meltdown without tripping the 20 amp breaker. Even though every thing you say about in-line fuses is true you have to protect the smallest wire.
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Old 11-19-2002, 03:55 PM
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Default Simple breaker size question...?

Thom...thanks again, I stand corrected. Hmmmmm...now I have a good reason to get ride of all those little fuses. [img]/infopop/emoticons/icon_cool.gif[/img]
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Old 11-20-2002, 05:19 AM
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Default Simple breaker size question...?

Finadic,

I wouldn't argue with your assessment for a moment. Its just your wireing scheme that I would not be using.

Let me start from the back side for a wireing setup and see if this makes more sense to you.

First there is the battery of course and it has a cable of whatever size is necessary leading directly to a Battery Switch. There is nothing else except the cable attached to the battery by the way. At the battery switch there is an output lug that supplies power directly to the engine but its also the point at which what you might call 'Ships Power' originates. This would be a fairly heavy wire leading to your distribution panel. On my small boat that wire happens to be 6 gauge, but it would vary on other boats. In that 6 Guage wire I have a single in line fuse of 30 amps. That fuse is located about 8" away from the battery switch. Exiting the fuse the line runs up to the console where it inters my current fuse block. The power comes into a single fuse, and that one is 25 amps, and then is distributed to individual fuses which are sized for the load they feed. Now, I'm going to be replacing the fuse block in my boat with a breaker panel this comming winter, but that's really not the point. In my fuse block I follow the advice I gave above. There's more of course though. The first thing is that I keep wires reasonably short and the way I do that is to cut off power supply wires that are provided with devices right at the always included in-line fuses. Then I run my own wire to the fuse block, and its almost always heavier than the wire supplied by the device manufacturers. I'm a bit partial to 12 and 14 guage myself for those relatively short (rarely over 3 feet) runs. So yes, you are right, there is that small bit of fine guage wire supplied by the manufacturer of the devices that will be the one to fry should the device itself go south, but I keep that short and at the same time within the fuse panel I tend to run a fuse that is actually of the same capacity that had been included with the origianal in-line that is now residing somewhere on the hill behind my house.

The point is that I don't use a direct feed from some high amperage fuse or breaker feeding directly to a device and to tell the truth I try not to stack up devices on a single breaker when that's avoidable as well. I also do this, and its something you might want to think about if you are planning on doing any rewireing, I stagger use expectations when deciding what loads to stack up on a single fuse or breaker. What I mean by that is something like this. I have my anchor lights and my trim tabs shareing the same fuse. I do that not only because the anchor lights are a minor power consumer whereas the trim tabs suck up some juice, but because if I have the anchor lights on I won't be using the tabs and if I'm using the tabs I won't be at anchor. See what I mean? In that case all I had to take into account when choosing a fuse size was the power draw of the tabs, the heavier of the two loads, because both would never be in use at the same time.

I hope I'm making a bit of sense here, I know I don't explain things all that well sometimes.

Oh, for whatever its worth I actually do have a breaker panel in the boat right now, but its separate from the fuse panel (and fed by the same 25 amp fuse that is the lead-in in the fuse panel). This one, which is small, just feeds the radios and the GPS units. I don't want them on fuses. My thinking on them is that in an emergency I will reset that breaker until smoke rolls up from under the dash if I have to, whereas if I had kept them protected by fuses I might run out of spares in an emergency. Make sense?

Thom

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Old 11-20-2002, 05:51 AM
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Default Simple breaker size question...?

Thom...stay with me on this, just trying to learn. I understand the CYA attitude of the manufactures...point well taken. Now, lets say a given piece of equipment need 1.0 amp of current to operate. I was under the impression that the fuse (breaker) would blow/break if the current exceded that (because of a problem in the equipment), thereby protecting the equipment? It seems your way would allow, lets say, 4 or 8 amps, whatever, to flow...frying the equipment. Granted the wire would be intact; but the equipment would be toast? I thank you for your explainations and patience, but I still must be missing something! [img]/infopop/emoticons/icon_confused.gif[/img] [img]/infopop/emoticons/icon_wink.gif[/img]
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Old 11-20-2002, 06:30 AM
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Default Simple breaker size question...?

Yep, it might well fry the equipment. That's one of the potential problems you face when stacking up items on a single protection device. In a better world you would have a separate fuse or breaker for each device, in which case you could easily fuse to save the device and simply run wire so heavy that you couldn't possibly get it to burn. The problem with that is that breaker panels with more than about 5 holes start to get pretty expensive. A lot of guys can get away with an 8-holer, but one of them cost upwards of $170 and that only typically includes 5 breakers, with you having to buy three more to fill it up. If you are equipment rich on a small boat you may find yourself having to go all the way to one of those 13 holer breaker panels that Blue Sea Systems makes (Paneltronics probably has a similar one) but they run just under $400 and come preequipped with nothing but 15 amp breakers. Of course you can buy breakers to suit but they run close to $15 a clip and that can get into some cash in a short time.

So, in an ideal world you would have one breaker per device, and it would be sized with both the longeivity of the device and fire-prevention in mind. In the real world cash dictates that you stack them up but with some forethought you can minimize the impact on device longevity and still maintain that fire-safety. And then there's the case where some driver or another keeps you down to a minimum number of fuses or breakers and if that is the case forget about trying to protect the device and concentrate on safety (fire prevention) and let the chips fall where they may.

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Old 11-20-2002, 06:36 AM
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Default Simple breaker size question...?

Thanks! [img]/infopop/emoticons/icon_cool.gif[/img]
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Old 11-20-2002, 07:57 AM
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Default Simple breaker size question...?

Hey Birdy, why don't you ask a "complicated" question on breaker size the next time and you may get a simple answer.

Regardless of Thom's feelings regarding in-line fuses, use one on each of your 4 amp devices. I hate them too, but pouring up to 10 amps into a device rated at 4 amps max is plain stupid. Personally, I have found the bulky, Yellow- rubber in-line holders work pretty well. I have 2 of them off of my battery switches, one with a 20A and the other 30A fuse and they have performed perfectly for over 10 years. I smear a little teflon grease on the ends of the fuses and on the male end of the yellow rubber before assembly. Also put some grease on each end where the wire comes out of the holder.

Finally Birdy, why don't you post the lengths of your wire runs from the breaker to your 2-4Amp devices. Maybe Thom will pick up the post and tell us the wire size to use based upon a 3% voltage drop. Personally, I would like to know also since I ain't got no tables.
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Old 11-20-2002, 08:17 AM
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Default Simple breaker size question...?

Gil,

The tables are pretty easy to find. One place they are printed is in the West Marine catalog and I have seen them on line as recently as last week. I think they may be at the West Marine site under their Westadvisor or whatever they call it. I think they are also posted on Ancor's site as well. They're pretty easy to find at any rate. Also, you need to understand that the tables refer to a 2-way-run so keep the ground wires in mind when you are figuring out your lengths as well. If I run across a set of the tables I'll come back and edit this and add links to them.

Here you go: http://www.ancorproducts.com/technical.html

Thom

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Old 11-20-2002, 08:43 AM
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Default Simple breaker size question...?

I will check your two suggestions on the Tables. Perhaps a lot more THT users should download the Tables and some redundant questions would disappear. Also, I am fully aware that the ground return is just as important as the 12v feed.
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Old 11-20-2002, 09:25 AM
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Default Simple breaker size question...?

I was helping out a buddy who didn't beleive me.

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Old 11-20-2002, 10:37 AM
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Default Simple breaker size question...?

As long as we are on this topic, I would suggest running at least 2 or more separate feeds for electronics. When I wired our boat I ran a 3 seperate feeds of number 12 with each circuit having its own breaker. each feed then goes to a master switch at the helm, from that switch it feeds a distribution block with built in fuse holders. Each distribution block can feed up to 4 devices. the fuses can be sized for the proper application. Addtionally I run a seperate circuit from the battery for each depth sounder. this helps to eliminate any electrical noise that the sounder may generate. Also protects each device over current protection.
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Old 11-20-2002, 11:00 AM
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Default Simple breaker size question...?

So Birdman, what did you tell your buddy? [img]/infopop/emoticons/icon_razz.gif[/img] I hope you told him there are no simple questions... [img]/infopop/emoticons/icon_biggrin.gif[/img]
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