Fuse Sizing

[dhays]

I know nothing but....

First, pick the wire size to give you the voltage drop you want based on the length of the voltage, length of run and the amperage. There are tables/calculators which will give you the wire size necessary for 3% (critical systems) or 10% (non-critical) voltage drops.

Then you fuse the wire to protect the wire, not what the wire is feeding. The idea is to keep the wire from catching fire in the event of a dead short. Again, there are tables which will give you the fuse rating based on the AWG size of the wire. There are two standards, one for wire that runs through an engine space (presumed to be hotter) and for wire that doesn't run through the engine space. The fuse should be within 7" of the battery (or as close as you can get it).

That is the extend of my understanding.

[Wotname]

Originally Posted by masonc

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.

Well I guess it all depends on what terminations you are referring to and I accept your proposition the lowest rated component limits the design.

There remains however many mil spec terminations that are rated for 105C and higher. Additionally, other terminations are not required to be insulated and any covering of such terminations are for sealing, not insulation.

But all of this is getting away from the OP's questions

[john61ct]

Note that it is valid design to ensure **all** elements are rated for the max amps vs heat for safety, and then have the long stretch wiring in between higher rated against voltage drop.

The CP would be sized for the former, lower amps rating.

[longjonsilver]

Thanks, guys and gals, you have clarified a lot for me. Frankly and Wotname, you have both been very informative. Thanks again.

Originally Posted by barnakiel

I think this is the point of the drawing on the right - it exemplifies a fault condition. No?

There is no indication in Casey's book that the diagram on the right is an overload condition of the wires. It does say however that it would be an overload if both 30amp loads are applied at once.

Originally Posted by boatbod

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.

Do you have a link for these tables?

Originally Posted by dhays

I know nothing but....
The fuse should be within 7" of the battery (or as close as you can get it).

You mean something like this:

https://www.amazon.ca/Blue-Sea-Syste...+fuses+battery

My interest in this has to do with my fear of fire aboard boats. i will have no gasoline or propane aboard my boat, but over 55% of boat fires start with electrical problems. Since i am looking for an older boat, i am sure that i will have electrical problems, potential problems or most likely both. i have been studying Don Casey's book (as mentioned in the original post) and have ordered Nigel Calders, Boatowner's Mechanical and Electrical Manual, since these shortfalls (?) of Casey's book have raised these questions in my mind.
Blessings
jon

[masonc]

Originally Posted by dhays

I know nothing but....

First, pick the wire size to give you the voltage drop you want based on the length of the voltage, length of run and the amperage. There are tables/calculators which will give you the wire size necessary for 3% (critical systems) or 10% (non-critical) voltage drops.

Then you fuse the wire to protect the wire, not what the wire is feeding. The idea is to keep the wire from catching fire in the event of a dead short. Again, there are tables which will give you the fuse rating based on the AWG size of the wire. There are two standards, one for wire that runs through an engine space (presumed to be hotter) and for wire that doesn't run through the engine space. The fuse should be within 7" of the battery (or as close as you can get it).

That is the extend of my understanding.

If that's the extent of your understanding, you understand very well. You have captured the process pretty concisely.
For electrical systems that will be subject to inspection or for more professional applications, we also have to consider derating for:

• Insulation rating
• Conduit or free air
• Single cable or multiore
• Ambient temperature (engine room)
• Number of conductors in a raceway
• Terminal rating
• And the biggest one - continuous or intermittent use. Any load that will be continuous for three hours must be derated to 80%.

[Frankly]

On the NEC/UL side of the house most circuit breakers are rated/ tested for 75C conductor connections. Sizing can utilize higher temp insulation for purposes of overcoming derating requirements ( high temp environment similar to engine room) but the 75C table is used under normal conditions for conductor sizing. Not sure all this is applicable to ABYC and boats but probably a good rule of thumb to follow (don't think them electrons know where the wire is they are scampering through).

On the idea that if upsizing the conductor to overcome voltage drop I would argue that to select the protecting fuse IAW expected load current produces a safer design than pushing it to the actual limit of the conductor. While we talk about the conductor, I bet most overheating related failures in a marine environment are connector/ lug related. More margin more safety.

[john61ct]

Originally Posted by longjonsilver

Do you have a link for these tables?

http://www.pkys.com/Reference.htm#ABYC

The Blue Sea "Circuit Wizard" wire sizing app is IMO excellent, easier than any tables.

[masonc]

Originally Posted by Frankly

On the idea that if upsizing the conductor to overcome voltage drop I would argue that to select the protecting fuse IAW expected load current produces a safer design than pushing it to the actual limit of the conductor. While we talk about the conductor, I bet most overheating related failures in a marine environment are connector/ lug related. More margin more safety.

Lowering the fuse rating does not allow for more current. If the application needs more current (lower voltage drop), then it needs it. Changing the fuse will not solve that. Fuses do not limit current, they open the circuit.
The standard response to a fuse blowing is to increase the size of the fuse. I cannot tell you the number of times maintenance people have told me about replacing a breaker because it was bad. "Look, it's not tripping any more" Very few try to find the real cause of the problem. When the wiring burns out, it was bad wiring.

[longjonsilver]

Originally Posted by john61ct

The Blue Sea "Circuit Wizard" wire sizing app is IMO excellent, easier than any tables.

i downloaded the app for my phone.

jon

[redsky49]

There are clearly issues with the illustration on the right. Are you certain the author doesn't reference it in the text?!

If not, shame on him for creating this confusion.

[longjonsilver]

Originally Posted by redsky49

There are clearly issues with the illustration on the right. Are you certain the author doesn't reference it in the text?!

This is what he writes on that very page "The size of the fuse or breaker is determined by the sum of all the loads on the circuit OR by the current-carrying capacity of the smallest wire in the circuit, whichever is smaller. For example, 12 AWG boat cable to the masthead tricolor is adequately protected by a 50-amp breaker - marine standards allow the breaker threshold to be up to 150% of the wire rating (45 amp for 12 AWG). But since the normal load on this circuit should never exceed 2 amps, a 5-amp breaker will detect a problem sooner."

This raises more questions than it answers. The last two sentences appear to be referring to the left diagram, but the first sentence appears to imply that the right diagram is not in conformity because of "the sum of all the loads on the circuit".

[boatbod]

Originally Posted by longjonsilver

Thanks, guys and gals, you have clarified a lot for me. Frankly and Wotname, you have both been very informative. Thanks again.



There is no indication in Casey's book that the diagram on the right is an overload condition of the wires. It does say however that it would be an overload if both 30amp loads are applied at once.



Do you have a link for these tables?



You mean something like this:

https://www.amazon.ca/Blue-Sea-Syste...+fuses+battery

My interest in this has to do with my fear of fire aboard boats. i will have no gasoline or propane aboard my boat, but over 55% of boat fires start with electrical problems. Since i am looking for an older boat, i am sure that i will have electrical problems, potential problems or most likely both. i have been studying Don Casey's book (as mentioned in the original post) and have ordered Nigel Calders, Boatowner's Mechanical and Electrical Manual, since these shortfalls (?) of Casey's book have raised these questions in my mind.
Blessings
jon

ABYC E.11 is the standard in the US. I can't post a link because it is a paid service, but I'm sure you can find abstracts online.

Alternatively, Blue Sea Systems has a nice smartphone app that has the tables built in.

[Stu Jackson]

Wire Sizing Charts from West Marine http://www.westmarine.com/WestAdviso...e-And-Ampacity www.bluesea.com also has good material on this and fusing, but it's a PDF download and doesn't link. Here's the link to the page, scroll down for the PDF download of the Fuse & Wire Sizing Tables: https://www.bluesea.com/support/reference

[redsky49]

First off, here is the applicable section from AYBC:

11.10.1.5 Branch Circuits

11.10.1.5.1 Each ungrounded conductor of a branch circuit shall be provided with overcurrent protection at the point of connection to the main switchboard unless the main circuit breaker or fuse provides such protection.

11.10.1.5.2 Each fuse or trip-free circuit breaker shall be rated in accordance with E-11.10.1.3 and E-11.10.1.4 and shall not exceed 150 percent of the conductor ampacity in TABLE IV. (See FIGURE 15.)

[masonc]

Originally Posted by longjonsilver

This is what he writes on that very page "The size of the fuse or breaker is determined by the sum of all the loads on the circuit OR by the current-carrying capacity of the smallest wire in the circuit, whichever is smaller. For example, 12 AWG boat cable to the masthead tricolor is adequately protected by a 50-amp breaker - marine standards allow the breaker threshold to be up to 150% of the wire rating (45 amp for 12 AWG). But since the normal load on this circuit should never exceed 2 amps, a 5-amp breaker will detect a problem sooner."

This raises more questions than it answers. The last two sentences appear to be referring to the left diagram, but the first sentence appears to imply that the right diagram is not in conformity because of "the sum of all the loads on the circuit".

The breaker size is determined by the cable, not the load. The cable is determined by the load.
My other problem with this is the masthead light is a continuous load, so the cable should be derated for that. Of course, LEDs don't take any power so the load is irrelevant. The cable size for continuous load of 50A, 25ft, is AWG 4, not AWG 12. I think Don Casey has been smoking wacky tobacky.