Wire sizing to Breaker Panel

[mitiempo]

Originally Posted by KP44

The panel comes with five 15 amp breakers and three empty slots.

The panel linked to comes loaded with eight 15 amp breakers.

https://www.bluesea.com/products/437...ay_8_Positions

[mitiempo]

Originally Posted by captstu

There are two separate issues at work here. Circuit breakers or fuses are intended to protect the wiring and downstream load from damage should there be a short of some kind.

14 gauge wire will generally handle 15 Amps without much heating or chance of damage unless the wire is coiled or run in a very tight space. Then the load capability must be derated because less heat can be lost.

But, the size ratings generally assume higher voltage circuits. The loss of a volt or two in a 120 volt circuit is in the range of 1%. The same size wire carrying the same current will lose approximate the same volt or two, but that is in the 10% range on a boat and is intolerable for most boat devices.

So, even though your load is within 15 amps and you could use 12 gauge wire, at about 50' (round trip length), you lose about 1/2 your voltage according to:

https://www.westmarine.com/WestAdvis...e-And-Ampacity

So, you have to calculate your round trip wire length, the expected load, the permissible voltage drop and choose the larger of the required size to produce an acceptable voltage drop or the size required by the protection you intend to install (14 gauge for 15 Amp breakers).

Fuses and breakers are designed to protect the wiring - nothing else.

Not sure what you mean by :....lose about 1/2 your voltage....."

In a 25' boat the panel will likely be 8 or 10' forward of the stern. The longest wiring run will probably be the bow red/green nav lights. If led as they should be a 30' run with 3% drop or less calls for 16 awg on my calculator. 14 awg works better. 3% drop should be easy to accomplish with 14 awg in most cases, possible heavier if there is a macerator installed.

The calculator I use: AWG by wire length/amps calculator

[Swift Drift]

You guys are all great and I'm comprehending your responses. A few salient points that have been confirmed in your gracious posts:

1) Wires leading from 15 amp breakers must be sized such that the 15 amp breaker will trip prior to the wire being compromised. It appears that with my run lengths, 14 awg should cover me. None of my device loads on the respective circuits will approach 15 amps, but nonetheless, wire must match breaker, not device lload within circuit.

2) The way to get by with smaller wire, is to downsize the breaker, but keep in mind that the total amp draw in the circuit from the associated devices must not exceed the amperage capacity of the wire nor the breaker....most importantly the wire (tripping breakers is annoying, fires more severe).

3. The Breaker protects the wire, not the device. Each device should be fused at the device to protect device circuitry from surges, etc.

Some follow up thoughts from my end:

A) All of my devices already have integral fusing for protection with the exception of my lighting. Why aren't lights fused? Is it assumed that light bulbs are expendable in the rare event of a surge where other more delicate/expensive items such as tiller pilots and VHF's are not?

B) I will likely keep my 15 amp breakers and go with 14 awg wire to keep things consistent regardless of the fact that its way over kill for the circuit loads. Its a relatively small project and therefore the cost difference is negligible. The only exception where i will downsize breaker will be my mast wiring since it will only have a couple of LED fixtures and it will be a long run (mast is aprox 30' and wire will run aprox 10' from panel to reach mast base which translates into an 80' round trip run). Installing wire to support a 15 amp breaker for an 80' run in this application would be nuts.

How'd I do coach?

[mitiempo]

Originally Posted by Swift Drift

B) I will likely keep my 15 amp breakers and go with 14 awg wire to keep things consistent regardless of the fact that its way over kill for the circuit loads. Its a relatively small project and therefore the cost difference is negligible. The only exception where i will downsize breaker will be my mast wiring since it will only have a couple of LED fixtures and it will be a long run (mast is aprox 30' and wire will run aprox 10' from panel to reach mast base which translates into an 80' round trip run). Installing wire to support a 15 amp breaker for an 80' run in this application would be nuts.

You are forgetting voltage drop over the length of the run. For your 80' round trip to the masthead with a load of 1.5 amps calls for 14 gauge to keep to a 3% voltage drop. If the load is 2 amps 12 gauge is needed. Nav lights need to be kept under 3% voltage drop - interior lights can be up to 10% drop.

[Swift Drift]

Originally Posted by mitiempo

You are forgetting voltage drop over the length of the run. For your 80' round trip to the masthead with a load of 1.5 amps calls for 14 gauge to keep to a 3% voltage drop. If the load is 2 amps 12 gauge is needed. Nav lights need to be kept under 3% voltage drop - interior lights can be up to 10% drop.

Noted! I confirmed that with the voltage calculator you provided earlier in the thread.

So another question, does the run length of my wire alter the relationship with my breaker at panel? In other words, if 14 awg wire is appropriate for a 15 amp breaker with a 10 foot round trip, would it also be appropriate for an 80 foot round trip.
*this question obviously doesn't address voltage drop or device loads on the circuit, just concerned with the breaker tripping when it needs to before wire fails due to short, etc.

[mitiempo]

Originally Posted by Swift Drift

Noted! I confirmed that with the voltage calculator you provided earlier in the thread.

So another question, does the run length of my wire alter the relationship with my breaker at panel? In other words, if 14 awg wire is appropriate for a 15 amp breaker with a 10 foot round trip, would it also be appropriate for an 80 foot round trip.
*this question obviously doesn't address voltage drop or device loads on the circuit, just concerned with the breaker tripping when it needs to before wire fails due to short, etc.

Yes a 15 amp breaker is appropriate for an 80' round trip. Breaker sizing relates to wire gauge, not length.

[AKA-None]

Slight clarification, breaker size relates to current draw. Wire sizing relates to voltage drop at a particular current draw.

[mitiempo]

Originally Posted by AKA-None

Slight clarification, breaker size relates to current draw. Wire sizing relates to voltage drop at a particular current draw.

Disagree.

Breaker (or fuse) protects the wire. It should be sized between the load and the wire ampacity. Typical panel breaker is 15 amps, such as all pre-loaded breakers (and fuses) on Blue Seas panels. This applies whether the load is close to 15 amps or a string of Led's totaling 1 amp. Wire size is 14 awg to larger depending on wire run there and back.

Electronics typically has an inline fuse close to the load to protect the item.

[MarkSF]

Please note that ABYC standard is for minimum wire gauge of 16. This is for physical strength. I agree that you might as well go for 14 as a minimum, given the minimal savings going from 14 to 16 gauge.

Really like the two core duplex cable. In fact I always keep some 14 gauge duplex cable handy.

Have you looked at Genuinedealz.com for the wire?

Blue Sea Systems make nice fuse boxes. They are a great solution for fusing low current things. For example, I have an instruments breaker, 15A. This powers up a fuse box, via 14 gauge duplex cable. The individual instruments have their own smaller fuses in, and are wired to, the fuse box. A happy consequence is that one bad egg doesn't take out all the instruments.

This one :

https://www.bluesea.com/products/502..._Bus_and_Cover

[Swift Drift]

Quick update on the project, it isn't finished yet but is well on the way. I ended up incorporating main + and - bus bars directly after the battery which allowed me to have a clean integration of my panel, solar charge controller, and ac charger. Ended up using a blue sea 6 switch fused panel rather than the breaker panel referenced earlier, simply because I already owned it making it essentially free.

[AKA-None]

Originally Posted by mitiempo

Disagree.

Breaker (or fuse) protects the wire. It should be sized between the load and the wire ampacity. Typical panel breaker is 15 amps, such as all pre-loaded breakers (and fuses) on Blue Seas panels. This applies whether the load is close to 15 amps or a string of Led's totaling 1 amp. Wire size is 14 awg to larger depending on wire run there and back.

Electronics typically has an inline fuse close to the load to protect the item.

Yes a breaker or fuse protects the wire but the size (resistance) determines voltage drop. And with too much voltage drop the wire heats up and melts. Hence why you don't try to drive 15 amps through a 20 gauge wire.

[mitiempo]

Originally Posted by AKA-None

Yes a breaker or fuse protects the wire but the size (resistance) determines voltage drop. And with too much voltage drop the wire heats up and melts. Hence why you don't try to drive 15 amps through a 20 gauge wire.

Yes wire is sized by voltage drop - less than 3% for virtually every item except interior lights per ABYC. Load and distance determine wire size at whatever percentage is required. Smallest wire allowed is 16 awg also per ABYC.

[john61ct]

Originally Posted by AKA-None

with too much voltage drop the wire heats up and melts.

You'll find the voltage drop issue drives the heavier gauge long past any overheating issues.