Electrical cable size confusion

[Blue Seas]

Hi Folks,

Unless I'm mis-reading something (no surprise) I seem to be getting some considerable variation in tables for recommended cable sizes (24 volt system).

Oceanflex and several of their distributors simply give an amperage rating next to their cables e.g. 1.5mm2 = 21A, 2.5 = 29A, 4.0 = 39A but without any further info (though they do have a voltage drop calculator). That is undersized compared to existing fit.

Victron on the other hand quote 4.5A / 7.5A and 12A for the same cable sizes at max 5 meter length. They also quote a rough rule of thumb of Current divided by 3 = cable in mm2.

I want to play it safe and get it right. I have 5, 10, 15 and 20A C/B's so I was thinking of 2.5mm for the 5 and 10's and 4.0 for the 15's (20 is not in use at moment). No overly long runs.

That seems fine by the Oceanflex tables but not by Victron - am I misreading something? Any quick rule of thumb or reliable table based on experience?
Thanks in advance.

 

[Blue Seas]

Ah,

I seem to have found a possible explanation for anyone interested. There is a difference in 'voltage drop' allowances of either 3% (critical systems) or up to 10% if non-critical and there is a good reason for not being able to adhere to the 3% rule. So I am being locked out not by Amperage but by the 3% voltage drop in some calculators.
Nigel Calder's website has a good calculator (link below) that not only tells you that you are reaching the limits but also 'why' you are doing so be it amps, 10% or 3% drop. A very useful tool compared to the static graphs.
HTH.

ISO Wire Size Calculator

 

[melvynpatrick]

Another good website. Explorist life. They have a wire calculator. It’s in American so you have to convert awg nto mm2. They have a very good YouTube channel too

 

[PaulRainbow]

Start with 1.5mm. Is it rated for the load ? Is the voltage drop OK ? If yes to both use it.

If 1.5mm isn't enough, go to 2.5mm. Is it rated for the load ? Is the voltage drop OK ? If yes to both use it.

You won't have much left, maybe one or two things on 4, 6, 8 or 10mm, then the heavy stuff like battery cables, inverter, bow thruster etc.

No need for 50,000 sizes of wire.

 

[Blue Seas]

Start with 1.5mm. Is it rated for the load ? Is the voltage drop OK ? If yes to both use it.

If 1.5mm isn't enough, go to 2.5mm. Is it rated for the load ? Is the voltage drop OK ? If yes to both use it.

You won't have much left, maybe one or two things on 4, 6, 8 or 10mm, then the heavy stuff like battery cables, inverter, bow thruster etc.

No need for 50,000 sizes of wire.

Yep, that's exactly what I was trying to achieve as per my OP of using only 2.5 or 4 unless bigger was required. ABYC say that the breaker rating can be used as an alternative to the load size and although that puts me somewhat over the top on many runs I'd rather the simplicity as you say. All the beefy stuff is on separate circuits outside of the main distribution panel so I'll stick to the original plan. It was the 3/10% variation that fooled me - some are set by default to 3% and some the other way round.
Much appreciated Paul

 

[Boathook]

I've always oversized the wiring to reduce the voltage drop to the absolute minimum and then fused the wire for the appliance(s).

If I was totally rewiring a boat I might try to size the wiring a bit more, but for one off jobs I not to bothered.

 

[Seastoke]

Well my opinion is what is the breaking capacity of your breaker , so take a 10 amp one , if there is a dead short the cables need to carry the inrush amperage before the breaker trips or they could be damaged.