Small Busbar connections.

[coopec43]

Originally Posted by RaymondR

Or someone could just hook up an inverter welder, set the current output and see if the connection cooks??

Thank you for that. (I'm tired of people on these threads trying to impress with their theoretical knowledge)

[Wotname]

Originally Posted by coopec43

Hi Wotname

My logic tells me that a big cable requires a big flat surface area of the lug otherwise you'll end up with a tractor on bicycle wheels

Yep, a big flat surface on the lugs sounds good for a big cable but never forget the devil is in the detail.

E.G. one would use a big cable to carry 100A (at least 4AWG) yet a 100A fuse is typically around 14 AWG and the fuse wire does not need to have big lugs. Of course the fuse wire is designed to melt when the current is notably greater than 100A but the 4AWG lug should never melt when the current is >100A (assuming there is proper circuit protection in the circuit).

[Jammer]

Originally Posted by coopec43

Thank you for that. (I'm tired of people on these threads trying to impress with their theoretical knowledge)

Yeah, because the fact that it worked once means it's safe, right?

[coopec43]

Originally Posted by Jammer

The terminal isn't a wire. The ampacity of a wire is based on the temperature the insulation can withstand and the heat loss properties of the wire, insulation, jacket, and enclosure system as installed. Wire that carries large currents gives off heat, quite a lot when the larger sizes of wire are considered. The insulation around the wire, the jacket, any conduit, nearby wires, and so on all contribute to the maximum temperature of the insulation. This is why there are different ABYC ampacities for engine room use (higher temperature) than in other places in the hull.

So the thermal situation is totally different, and if you're going to try to model it and figure out how hot it's going to get, you'll need the expensive thermal modelling plug-in for an expensive CAD system, and you'll still have to test it in the lab to see if you got the results you expected before you can believe them.

When I read the above you absolutely lost me!

As far as the average boaty is concerned what the hell has temperature got to do with electrical conductivity?

Why not just go a bit oversize on the cabling and fuse everything? (But keeping AYBC requirements in mind)

[Jammer]

Originally Posted by coopec43

When I read the above you absolutely lost me!

Sorry. I'm not that good of a writer. Used to do it for a living and decided I shouldn't.

As far as the average boaty is concerned what the hell has temperature got to do with electrical conductivity?

The whole point of proper wiring is to keep it from getting hot. If it's already warm, it's harder to keep it from getting hot.

Why not just go a bit oversize on the cabling and fuse everything? (But keeping AYBC requirements in mind)

You'll do fine, but don't oversize the wires, that creates its own problems and sufficient safety margins are already baked into the aybc stuff.

[RaymondR]

Empiricaly yep, it's just a small sample size.

[Wotname]

Originally Posted by coopec43

........
As far as the average boaty is concerned what the hell has temperature got to do with electrical conductivity?
..........

To expand on what Jammer noted and without getting too theoretical, the fuller answer is - almost everything.

Conductors, connectors, terminals, busbars, switches, insulation, fuses etc should never get hot. If they do get hot, then that is the first sign of a problem developing. Thus the design (and the standards) are all predicated on keeping the circuits cool. If we decided to go to a DIY design, we need to keep in mind that heat is the enemy.

We all know the electrical stuff stops working when the smoke gets out. Smoke is the result of heat...no heat, no smoke!

[coopec43]

Originally Posted by Jammer

Sorry. I'm not that good of a writer. Used to do it for a living and decided I shouldn't.

The whole point of proper wiring is to keep it from getting hot. If it's already warm, it's harder to keep it from getting hot.

You'll do fine, but don't oversize the wires, that creates its own problems and sufficient safety margins are already baked into the aybc stuff.

Thanks Jammer! Your points are a whole lot clearer now.

Now, is there a disadvantage (apart from cost) in using very much oversize cables?

I use the analogy of a garden hose spurting water (2 GPM/ 20PSI) into a 6 inch pipe to fill a tank. It would eventually fill the tank but there would be a delay. (But only the first time it was "hooked up" After that you wouldn't notice? Maybe that is the answer?)

Does that analogy hold true in a 12V DC electrical world?

[coopec43]

Originally Posted by Wotname

To expand on what Jammer noted and without getting too theoretical, the fuller answer is - almost everything.

Conductors, connectors, terminals, busbars, switches, insulation, fuses etc should never get hot. If they do get hot, then that is the first sign of a problem developing. Thus the design (and the standards) are all predicated on keeping the circuits cool. If we decided to go to a DIY design, we need to keep in mind that heat is the enemy.

We all know the electrical stuff stops working when the smoke gets out. Smoke is the result of heat...no heat, no smoke!

I don't have a problem with that. I'd be very careful to allow circulating air around high amperage gear.(I don't have battery boxes for example)

[RaymondR]

The simplest answer to a lot of these problems is to move to higher voltage systems.

[coopec43]

Originally Posted by RaymondR

The simplest answer to a lot of these problems is to move to higher voltage systems.

Yes, I have done that. My yacht is now 24V. I have to replace the 12V oscillating fan and the inline blower in the toilet (it sounds like a jet aircraft about to take off)

But they only make 12V radios so I'll have to wire that up specially

[Wotname]

^^ Plenty of 24V radios if your pockets are deep enough; well maybe not plenty but certainly enough. Probably won't find any at recreational retail outlets but professional radio guys should be able to supply.

Certainly the choice is way way better in the 12V world.

[RaymondR]

The maths is easy, for instance a 1,200 watt 12v anchor winch motor requires a 100 amp cable but a 48v motor only requires 25 amp cabling.

Where we might want to power low power items like radios 48v to 12v dc/dc converters are cheap.

There is also a large weight advantage in higher voltage machines.

They'll never make them until we demand them.

[Jammer]

Originally Posted by coopec43

Now, is there a disadvantage (apart from cost) in using very much oversize cables?

Weight, larger bend radius, must be held in place with more secure clamps or strapping to portions of the boat that can accept higher forces, incompatibility with/potential physical damage to connected equipment due to the higher bending force, difficulty of service, more space devoted to electrical equipment.


Windlasses and bow thrusters are examples of where this comes up. The terminals on the contactor or control box or whatever are only so big and so strong, and if you're going to run 4/0 wire to a windlass or 1000 mcm wire to a bow thruster you run the risk of breaking a stud. The usual way around it is that you put in a splice of some kind and run smaller wire for the last foot or two. Otherwise you can make really solid brackets that hold the wire immobile about 6" away from the control box so that the wire doesn't put any sort of twisting or bending force on the studs.


Sometimes it's really necessary but more often it's overkill that doesn't add any safety or reliability to the system.

[Shaneesprit]

What is the load you are using?
If it’s a 45A constant Load then I would suggest 6mm wire is not a good choice, likewise for a heating circuit, you can buy it then depends on the installation method of the cable and how long it is.
Stop worrying about the screw size, but do check the rating of the crimp you are using. and if it’s a Chinese crimp give it your own derate factor