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Old 10-06-2017, 10:35   #1
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Fuse Sizing

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I have been studying Don Casey's book on sailboat maintenance and have a question based upon this diagram. Here we have two simple circuits. Both use number 12 wire rated for 45 amps. The first circuit has a load of two amps, the second circuit has two 30 amp loads in parallel. The first circuit is protected by a 5 amp fuse the second circuit is protected by a 50 amp fuse.

Question: How did they choose these sizes of fuses? Why did they put a 50 amp fuse in the second circuit when the wire can only handle 45 amps? Why is a potential 60 amp load put into a circuit with a 50 amp fuse and wire that can only handle 45 amps?

On the next page Don states that "The breakers in the panel are to protect the wiring from overload. Protecting an individual piece of equipment requires an additional fuse either built into the equipment or in series installed in the positive lead."

In light of the above quote how does this make sense with the diagram? What am i missing here? Again, how can a 5 amp fuse protect a 2 amp load? How can a 50 amp fuse protect a wire only rated for 45 amps?

Can anybody help me?
jon
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Old 10-06-2017, 11:32   #2
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Re: Fuse Sizing

Wire sizing is conservative, takes time for overloading to cause dangerous overheating, going a little over doesn't cause a fire immediately 8-)

CP measures need to account for short-term surges, avoid triggering unnecessarily. Note the "headroom" factor is pretty narrow.

But if you are sure of the actual max current a wire will carry you can size the CP smaller than the wire if you like.
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Old 10-06-2017, 12:52   #3
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Re: Fuse Sizing

I will be interested to see some responses from some electrical engineers but for mine I'd say the second drawing could pull 60Amps and blow the fuse before the wire fused. I would not have a 50Amp fuse protecting 45Amp wire. I know there is a safety factor in the wiring but it's not there so you can overload it.

The first drawing is safe but has unnecessary weight and cost. The 5Amp fuse isn't protecting the 2Amp load, it's protecting the wire. Not just the 45Amp wire but say the load is a fan, the wiring in a 2Amp fan maybe 4Amp, I would fit a 3Amp fuse because I don't think I could trust that the internal wiring and lead will be above 5Amp.

It's also with remembering that if your batteries are fully charged you may have 15% higher voltage and with it more current.

PS I can't read the notes on the drawing.
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Old 10-06-2017, 13:42   #4
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Re: Fuse Sizing

You generally install circuit protection rated at no more than 125% of wire ampacity. If protecting a motor (e.g. Bilge pump) fuse at manufacturers spec to protect from locked rotor condition.
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Old 10-06-2017, 18:24   #5
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Re: Fuse Sizing

Quote:
Originally Posted by longjonsilver View Post
Attachment 149675
I have been studying Don Casey's book on sailboat maintenance and have a question based upon this diagram. Here we have two simple circuits. Both use number 12 wire rated for 45 amps. The first circuit has a load of two amps, the second circuit has two 30 amp loads in parallel. The first circuit is protected by a 5 amp fuse the second circuit is protected by a 50 amp fuse.

Question: How did they choose these sizes of fuses? Why did they put a 50 amp fuse in the second circuit when the wire can only handle 45 amps? Why is a potential 60 amp load put into a circuit with a 50 amp fuse and wire that can only handle 45 amps?

On the next page Don states that "The breakers in the panel are to protect the wiring from overload. Protecting an individual piece of equipment requires an additional fuse either built into the equipment or in series installed in the positive lead."

In light of the above quote how does this make sense with the diagram? What am i missing here? Again, how can a 5 amp fuse protect a 2 amp load? How can a 50 amp fuse protect a wire only rated for 45 amps?

Can anybody help me?
jon
Well, Jon, there are several things to consider.

First up is that fuses and circuit breakers are designed to protect wiring (and sometimes loads) from fault conditions. So one must consider what faults might cause problems (i.e. over current, over heating, fire etc)

Circuit 1 is for a light bulb. There is no practicable way to protect a lamp from over current and thus the fuse is there (as always) to protect the wiring from fault conditions. While the wiring looks rather large for the load (lamp), we don't know anything about it's length. It might be a 200' run and needs to large to achieve minimal voltage drop etc. As this is the only load on the wire, then the fuse can be (and should be) as small as suitable for the expected current draw. Thus 5 amp seems appropriate but so would say a 10 amp one.

Circuit 2 is for two unknown resistive load of 30 amps each in parallel with one switched on and one switched off. Clearly if both are on, the circuit is overloaded (which is noted on the drawing) and at some time the 50 Amp fuse will blow.

However, IMO, this is totally unsuitable as the fuse is rated above the wire capacity and will not protect the wiring from other unknown fault conditions. For example, assume the switched on load is working properly and thus is drawing 30 amps. Now add a fault condition somewhere in the circuit that draws 20 amps. Total load is now 50 amps and the fuse will not blow. A fuse should carry it's rated current indefinitely. Sooner or later the wiring will overheat and anything is possible after that and that anything is never good.

A word on ampacity of wire, many look at a figure and think the wire is always good for that current. This is false. The current rating for any wire is dependant on the ambient temperature of the wire . So wires in bundles are dependant on the currents carried in the other wires and wire in engine compartments are also effected by engine heat etc. Wire insulation as big effect on the ampacity of the wire also. Thus one 12 AWG wire might be good for 45 amps in one application, 60 amps in another and 30 in a third. The normally stated capacity is only an average.
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Old 10-06-2017, 19:26   #6
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Re: Fuse Sizing

The fuse/ circuit breaker is required to first protect the wiring against overload (30 amps on a 20 amp circuit) and faults/ short circuits (500 amps on a 20 amp circuit). Typically the fault condition (starter wiring shorting on motor mount) will be quickly cleared by a wide range of fusing sizes. Overload conditions are usually the more critical situation and there are tables that relate fuse sizes to wire size and also include a derating for higher ambient temp situations (wire running through the engine compartment).

As to protection of the load, sometimes you have it and sometimes you don't. Dedicated circuits (feeding a single load) can usually be sized to "kill two birds with one stone" (protect the wiring and protect the load most typically motor circuits).

A single circuit can feed multiple loads and if they do not operate simultaneously the sum of their ampere draw can exceed the rating of the feeding conductor. If they all can operate at the same time then the feeding circuit must be sized to protect the total.

I have a single 20 amp circuit feeding my steering pedestal with a 10 amp breaker feeding the AP, 10 amp feeding the radar, 5 amp cigarette lighter, 2.5 amps feeding GPS, and 2.5 amps feeding some remote wireless stuff. All these breakers are mounted in a small box near the pedestal. If I remember correctly there is an ABYC rule on the sum of these breakers but don't remember off the top of my head.

In 12 volt circuits often the wire is increased in size to reduce voltage drop if there is significant circuit length.

In the situation you asked about protecting the 2 amp load with a 5 amp fuse while not ideal better than protecting it with a 20 amp fuse. Protecting the 45 amp wire with a 50 amp fuse could be that for the fuse type in use it is not available in 45 amps. It is usually allowed to move up to the next larger standard size. In this case as 45 amps on a #12 is pushing it a little, think I would up the wire to #10.

On modern boats with all their electrics it is just not practical to protect every thing against every condition all the time (Hella Fan wires is one of my small wire irritants). There are some risks in all endeavors of life (assuming you get out of the bed in the morning). You try to meet the concepts of the rules, but there will always be a few areas where it just don't work out. Rules/ codes usually don't allow for judgement but good judgement is what makes the world work (judgement based on good technical info).

Being a little more anal than most I have a number of 5 and 2.5 amp breakers scattered around my boat, but bet that Garmin Chartplotter wire and load are well below 2.5 amps. Also have several 1 and 2 amp fuses feeding instruments.
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Old 10-06-2017, 23:50   #7
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Re: Fuse Sizing

I wound never fuse 12awg more then 20a
Nobody runs wire single stranded in free air. Which is maybe how you get to 45.
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Old 11-06-2017, 02:24   #8
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Re: Fuse Sizing

Frankly, I gotta agree with the bulk of your post but I disagree (slightly) with this bit
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...... Protecting the 45 amp wire with a 50 amp fuse could be that for the fuse type in use it is not available in 45 amps. It is usually allowed to move up to the next larger standard size. In this case as 45 amps on a #12 is pushing it a little, think I would up the wire to #10. ........
If a 45 amp fuse was unavailable, then in the circuit in the OP would be better served with an 40 amp one or as you suggest, increase wire size to #10.
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Old 11-06-2017, 02:53   #9
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Re: Fuse Sizing

A 50a circuit requires #6cu cable.
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Old 11-06-2017, 05:18   #10
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Re: Fuse Sizing

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A 50a circuit requires #6cu cable.
While I'm sure that in some (many?) circumstances this is accurate, it doesn't account for all circumstances.

As an example, a #10 Cu conductor in free air with an ambient temperature of 70C and an insulation rating of 200C can carry 78 amps while a insulation rating of 105C is only good for 41 amps. When in a bundle of 33 or more wires with 20% or less harness current, the same 105C insulation rating now allows for only 17 amps.

Reference is SAE AS 50881
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Old 11-06-2017, 06:43   #11
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Re: Fuse Sizing

Matching load, wire size, circuit protection, and voltage drop is not for the faint of heart. Those of us that were raised on a NEC diet that #12 AWG is to be protected by a 20 Amp circuit breaker tend to look at some of the ABYC ampacity tables with suspicion. Both ABYC and Anchor Marine rate 105 degree insulated #12 at 45 Amps continuous (ABYC-single conductor in free air, Anchor Marine duplex cable not in engine space). I have never seen any reason to doubt any of this technical info, but as said before in 12 Volt systems voltage drop is often the short pole in the tent.

If you are standing in front of the Anchor wire rack in WM then bumping up one wire size is probably a good approach. If the problem is the wire is already in place and how much can it stand then the wire tables are a safe guide (this is the getting out of bed part).
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Old 11-06-2017, 07:40   #12
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Re: Fuse Sizing

ABYC publishes ampacity, bundling and voltage drop tables. The standard says you pick the wire size that meets all the specs. For 12vdc this means the voltage drop spec is typically the limiting factor. For 120vac it's usually bundling.
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Old 11-06-2017, 08:57   #13
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Re: Fuse Sizing

Quote:
Originally Posted by longjonsilver View Post
Attachment 149675
...
In light of the above quote how does this make sense with the diagram? What am i missing here? Again, how can a 5 amp fuse protect a 2 amp load? How can a 50 amp fuse protect a wire only rated for 45 amps?
...
Pretty sure you aren't missing anything and the diagram is a mistake (whether a lapse in judgement or just a simple typo). Should either be a 40 amp fuse or #10 awg wire.


With electricity there are a bajillion standards and rules of thumb, but a little bit of logic and some respect for what can go wrong goes a long way. Noticing that mistake is an example of using a little logic.
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Old 11-06-2017, 09:01   #14
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Re: Fuse Sizing

Quote:
Originally Posted by Wotname View Post
an insulation rating of 200C
While your insulation value may be 200C, the termination is not rated more than 75C. You need to limit your calculations for realistic temperature. I see electricians here change the insulation temperature setting to give them the result they want when they don't want to put in the correct cable, but they forget it's the termination that restricts the temperature value. THe higher temperatures are for specialized machine applications, such as inside alternators in generators. The rest of use 60VC or 75C.
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Old 11-06-2017, 09:34   #15
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Re: Fuse Sizing

I think this is the point of the drawing on the right - it exemplifies a fault condition.

No?

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