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Old 12-11-2010, 11:20   #16
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Originally Posted by sailvayu View Post
The reason for the larger cables is to eliminate voltage drop to the controller which is sensing voltage, I do not understand the #10 max though that sounds odd? I would re read that one and double check
The #10 maximum will relate to the controllers lug/pigtail size.
It's not uncommon for manufacturers to recommend cable sizes (due to length of conductors) larger than their equipment can accommodate.
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Old 12-11-2010, 16:05   #17
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There seems to be a lot of confusion over fuseing and cable size.

Firstly manufacturers rated fuses at a particular level for two reasons, one is to use a standard readily available fuse size ( known as the preferred value). Secondly the value chosen is higher then the normal surge/startup currents plus a bit. Remember in line fuses for electronic devices only protect the wiring inside the unit, not the electronics. You wll proably find that fuses from 10amp all the way to prehaps 5 amp will work, but 5amp is too close to the working current for a fuse.


Secondly the current carrying capacity of a PVC covered wire is a function of several things, including numbers in a bundle, voltage drop max working current. The requirement is to keep the PVC under normally the rule of thumbb for a single cable in free air is 40 degrees C over an ambient max of 30. But safety factors keep thi much lower.

Ultimately breaker size and wire size are not directly related only at the extremes. Even quite a small wire will carry short circuit loads that will trip a breaker without damage or excessive temp rise in the PVC. In most cases voltage drop over long distances is the determining factor. THis why you sometimes get changes from one wire size to another. NO system is designed so that the cables can carry short circuit currents for any length of time, thats why fuse or CB protection is there

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Old 12-11-2010, 16:21   #18
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Dave I hope you are not suggesting anyone use a smaller wire because it will handle a dead short load long enough to trip the OCPD. The OCPD is there to protect the wire not the device at the other end. An over current event just under the trip threshold is the most common cause of electrical fire. The wire heats and melts and the breaker never trips. It is ok to under size the OCPD for the wire but never oversize. Just because the conductor can handle a event short term does not mean it can long term.

This is simple protect the conductor first then the toy at the end.
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Old 12-11-2010, 17:17   #19
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Sailvayu,

What you've said is pretty clear.

In respect of the overall design, all my instruments draw 12A (maximum per specifications) with a combined fuse rating of 40A: Why can't the circuit be designed for 20A with an appropriate breaker? That is, a 20A breaker & cable up to the fuse panel which then feeds the various instruments protected by their respective inline fuses.

For example, my Raymarine depth/wind/tridata/multi draw a combined 0.26A have a recommended 5A fuse. They have no start up load.

Thus the main wire is protected from overload and all the devices are individually protected per the manufacturers recommendations. And the circuit can't overload.
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Old 12-11-2010, 17:19   #20
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Firstly OPCD are not merely wire protectors, That is just the common use for them particulary in boats where the builders cant be sure what youll do in the future. CB etc are quite effective as fuses for protecting devices ( in as much as any fuse / CB protects electronic devices.

What I was saying about shorts and CBs is that if you examine the trip times of CB's ( or the fuse blow time) against the rise in temperature of a wire carrying excessive current, you end up with a wide margin, Usually the decidsing factor is the voltage drop not the wire size to trip a breaker.

As an experiement, take say a 20 amp CB and connect smaller and smaller wires and do a simple short circuit test, You will get to suprisingly small sizes before it becomes a problem. Voltage drop , and or normal running current limits are the main driving factors in wire sizeing not short circuit ability.


I hate inline fuses, on my boat I have most major devices fed seperately from the panel, ie VHF, Autopilot, Radar, Chart Plotter and instruments. I size the CBs carefully and have removed all the inline fuses. its a better solution in my professional opinion.


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Old 15-11-2010, 09:44   #21
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Not sure what you are trying to say but it could be confusing to others. 12 volt wiring size is dependent on the distance to and from the power source and the amps that will be carried. This can be a 1/O or a 14 gage. Saying that most is 10 or 12 gage does not take these into consideration. Our "cables" from the main panel to the batteries are #1 because of the distance and maximum amps the wire might need to carry. Chuck
You are correct. The discussion is about what rating of wire to power a specific device.... and dragged on about wether to rate for 10 amps or 3.5 amps. Even though 30guage wire will carry 3.5 amps, I wouldn't use it to run from my battery bank to the fly bridge. At smaller wire sizes the fragility of the wire is more of a factor than current capacity. The main cables from the battery to the main panel could be asked to carry more than 40 amps. My inverter wires are 00 guage. 4 guage will carry the current, but with excessive voltage drop. For common boat sizes 30-40ft and common electrical device requirements 13v(nominal) battery powering a 12volt(minimum), chartplotter, radar, ... .5 volt drop is acceptable, few devices pull more than 40 amps. I wouldn't run less than 14 guage through a boat because of risk of chaffing.
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Old 15-11-2010, 11:54   #22
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1) Size wire to carry the device's peak current AND to keep voltage drop over the total length of run from the panel within the device's tolerance. This may result in a wire size that is capable of safely carrying more current than the device consumes.

THEN

2) Size the circuit breaker for the device's peak current.

Wire is protected (perhaps over-protected) and the device is protected.
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Old 15-11-2010, 18:47   #23
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1) Size wire to carry the device's peak current AND to keep voltage drop over the total length of run from the panel within the device's tolerance. This may result in a wire size that is capable of safely carrying more current than the device consumes.

THEN

2) Size the circuit breaker for the device's peak current.

Wire is protected (perhaps over-protected) and the device is protected.
Hi Erict,

That is basically my question - the combined instruments draw 11.9A per manufacturers specifications. They are also individually fused with a total of all the fuses being 40A.

I've asked whether peak current is calculated from the figure of 11.9A or the combined fuse ratings of 40A?
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Old 15-11-2010, 18:55   #24
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Originally Posted by capn_billl View Post
You are correct. The discussion is about what rating of wire to power a specific device.... and dragged on about wether to rate for 10 amps or 3.5 amps.
My original question was about how to calculate peak current not wire sizes. I wanted to know whether a group of instruments drawing a combined 11.9A but with individual fuses totaling 40A would be OK on a 20A circuit.

Manufacturers spec sheet or combined total of all the devices fuses: How would you calculate peak load?

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Old 15-11-2010, 19:06   #25
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It seems that your club know-it-all really does; whereas your electrician probably isn't.
The wire must be sized to a minimum of 10 Amps, if protected by a 10A fuse. The actual load doesn't matter.

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Huh? The wire size should be the greater of (1) the size specified for the voltage drop expected, and (2) the size that can handle an continuous overcurrent of 10A (the fuse rating) without heating beyond its insulation rating. Most selections that provide a suitable voltage drop at 3.5A can be safely protected with a 10A fuse. But again, it depends on length, the short runs being more vulnerable to overheating because the voltage drop calculation will give a small wire size.

Sizing wire based on the sum of the fuses for the instruments is silly.
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Old 15-11-2010, 19:11   #26
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Ok I finally understand the question. Then total of the individual fuses means nothing. If your total combined AMP draw is 11.9 AMP I would use a 15 AMP OCPD but yes it would be ok to use the 20 AMP Use the spec sheet, each fuse is going to be a bit higher than it needs to be so adding them up will give you a higher total. Hope that is what you were really after

And anyone that suggests using a smaller wire because it will trip a breaker in a dead short situation is looking for a fire. Most fires start with a high amp overload that create heat without tripping the breaker. The breaker is there to protect the conductor period! The inline fuses are at the instrument to protect the instrument this makes perfect sense
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Old 15-11-2010, 19:15   #27
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Originally Posted by daddle View Post

Sizing wire based on the sum of the fuses for the instruments is silly.


That is basically my question - the combined instruments draw 11.9A per manufacturers specifications. They are also individually fused with a total of all the fuses being 40A.

I've asked whether peak current is calculated from the figure of 11.9A or the combined fuse ratings of 40A?
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Old 15-11-2010, 19:30   #28
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Ok I finally understand the question. Then total of the individual fuses means nothing. If your total combined AMP draw is 11.9 AMP I would use a 15 AMP OCPD but yes it would be ok to use the 20 AMP Use the spec sheet, each fuse is going to be a bit higher than it needs to be so adding them up will give you a higher total. Hope that is what you were really after

And anyone that suggests using a smaller wire because it will trip a breaker in a dead short situation is looking for a fire. Most fires start with a high amp overload that create heat without tripping the breaker. The breaker is there to protect the conductor period! The inline fuses are at the instrument to protect the instrument this makes perfect sense
Thanks Sailvayu,

To recap: My original question was prompted by the club 'know-it-all' insisting that my Sparky was an idiot for suggesting a 20A circuit to a 11.9A load. Know-it-all insisted that the wiring needed to be sized according to the total fuses (40A) protecting all the instruments.

Per your post, it looks like my sparky is correct: Sparky suggests sizing the wire and circuit for 20A to allow future equipment. Thus, the circuit will be sized & fused for a 20A load with a 3% voltage drop to the panel using 8 AWG tinned wire. It will be carrying an 11.9A instrument load.

At the panel each instrument will be individually fused and connected with appropriately sized wires. The total instrument fuses will be 40 A.

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Old 15-11-2010, 20:18   #29
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That sounds right, Muskoka. The most confusing part of this thread has been the "40 amp" number. It's a red herring.

It's still not clear exactly what components will be in the circuit, although, you implied they are all resistive and will not have an inrush current or peak on startup. That being case, the plan for a 20 amp circuit is appropriate.

Are you planning to use multiple wire sizes within this circuit? It sounds like there will be a main cable or bus from the panel that is expected to carry 11.9 amps continuously and the bus will branch to components that are idividually fused. I believe the branch wires should always be sized to match the individual branch fuses. Of course from a safety perspective, there is nothing wrong with using larger wire throughout, say the 8 AWG you calculated to carry the full 11.9 amps.
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Old 15-11-2010, 21:25   #30
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Erict,

The panel will be fed with the 8 AWG and all the instruments will have wiring sized per their fuse size and run.

The components are chart plotter, depth/wind/tridata, sounder, ais, autopilot. The autopilot is the reason why I want extra capacity in the circuit. I can imagine situations where it may demand more from the circuit momentarily.

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