Cable capcity

[nathanlee]

What's the leeway in terms of current capacity on electrical cable?

i.e. If I have cable rated at 15amps, what could I safely get down it for a short burst of a few seconds or so, perhaps a minute?

I realise the "proper" answer is going to be 15amps, but what's the real life answer if I needed to do it in an emergency?

The reason I ask is that I've used 15 amp cable to feed the switch panel, and off that is the autopilot, which is fused for 10amps, as well as an inverter. The draw on the system should never be more than 15 amps continuous, but it is possible that if I had the VHF keyed up, the inverter on, and the AP cranking the tiller at full power, then it'd peak above 15amps.

 

[pvb]

The "rated capacity" of cable is a bit misleading. What you need to ensure is that the voltage drop along the cable is acceptably low even when everything is switched on. The voltage drop depends on the size of the cable but also, crucially, on the length of the cable.

 

[noelex]

As PVB says usually in a 12v system voltage drop is the limiting factor (so make sure you calculate the effect of this as well. Things like VHF transmitters dont like low voltage), but for short runs current carrying capacity is occasionally the limiting problem.
Both voltage drop, and current carrying capacity have to be OK for the wire to function adequately.

There is not a lot of leeway in current carrying capacity. It is usually specified for a wire on its own (not bundled with others as usually occurs in a boat) at reasonable ambient temperatures (which are generally lower than in the engine bay for example)
So to exceed this capacity is risky even for reasonably short durations.
I would upgrade the wire size.

 

[mitiempo]

It definitely needs upgrading to be able to handle the maximum amperage with everything turned on, taking into account voltage drop. Here's a good calculator for wire sizing. http://boatstuff.awardspace.com/awgcalc.html
Just fill in the blanks.

I am curious what gauge your "15 amp wire" really is. By the ampacity chart below that would be 18awg which is the smallest wire ABYC allows on a boat. It is way too small for anything more than a few led lights as far as I am concerned. The wire I use for most low draw items on a boat, such as lights, fans, etc is 14awg - heavier wire for items like macerators.
This chart doesn't consider voltage drop.

 

[prv]

Your overload is going to be for seconds' duration at worst. Tillerpilots don't draw their max current continually. You don't spend long transmitting on VHF (you're not *allowed* to do it for more than 30 seconds continuously). It's hard to imagine what you'd be using the inverter for in rough enough weather to stress your tillerpilot and demand long chats on the VHF - I'd have thought it was a moored / at anchor only device?

Mains electricity circuits in houses are allowed to use "diversity" in calculating the ratings, ie after having added up all the possible peak loads, you can knock some off for the fact that it is unlikely that everything will be turned on at once. I am not an electrician and have never designed a circuit for which this would apply, so I don't know how much can legally be "knocked off", but I believe it's substantial.

Personally, in your situation I wouldn't worry, especially with short cables (and I'm guessing there ain't room on Kudu for long ones ). If you want to be sure, there are formulae with which you can calculate exactly how much you might be overloading stuff in your worst-case scenario - selling "15 amp cable" is convenient but very much simplified.

Pete
(This reply languished in the editor for a while as I got distracted, so I won't have read any intervening replies )

 

[VicS]

I am curious what gauge your "15 amp wire" really is

Flexible cable rated at 15amps will have a conductor cross-sectional area of 1.5mm² . That is between AWG 15 and AWG 16.

It has a resistance of 11.4 Ω/1000m
If you apply the usual rule of limiting volts drop to 3% (0.36V for a 12V system) it means that the maximum cable length ( and that is the total of positive + negative cable) that you should be running at 15amps is only 2.1m.

The maximum current that you should consider for a short length freely ventilated, ie not bundled with other wires or contained in conduit etc is approx 25 amps

 

[mitiempo]

VicS
Wire between AWG 15 and AWG 16 is not suitable for supplying a switch panel with more than a single circuit - and possibly not that if the run is very long. The inverter mentioned by Nathan, even if of low output, will draw more than the 15 amp cable can handle I would guess.

 

[VicS]

VicS
Wire between AWG 15 and AWG 16 is not suitable for supplying a switch panel with more than a single circuit - and possibly not that if the run is very long. The inverter mentioned by Nathan, even if of low output, will draw more than the 15 amp cable can handle I would guess.

You said, "I am curious what gauge your "15 amp wire" really is."

The answer is 1.5 mm² ! That happens to be between AWG 15 and AWG 16.

The rest of my reply is about volts drop and as you can see in the case in point this becomes a more relevant consideration than current carrying capacity.

If the OP were to be running 10 feet or 3m equal to a total cable length of 20 feet or 6m
To fall within the 3% volts drop he would have to use 4mm² cable which is AWG 11 to the nearest
(AWG 10 according to THIS TABLE)

 

[mitiempo]

If the OP were to be running 10 feet or 3m equal to a total cable length of 20 feet or 6m
To fall within the 3% volts drop he would have to use 4mm² cable which is AWG 11 to the nearest
(AWG 10 according to THIS TABLE)[/QUOTE]


Without knowing the maximum amperage of the inverter (in addition to the distance run) a wire gauge cannot be selected.

 

[NigelFortune]

Double it up !

If you have any question about the current limit of the cable and you still have some left why not double the cable up. Mark it clearly so as to let anybody who is not familiar with the boat just in case !

 

[maxi77]

I must admit my gut reaction is to wire the invertor in independantly of the other items, why risk your VHF power supply in this way.

 

[VicS]

Without knowing the maximum amperage of the inverter (in addition to the distance run) a wire gauge cannot be selected.

The OP presumably knows both of these!
He can, therefore, look up a suitable cable size in the link in my post.

I must admit my gut reaction is to wire the invertor in independantly of the other items, why risk your VHF power supply in this way.

I agree. I'd wire an inverter ( via a fuse ) directly to the battery isolator or bus bar.

They used to recommend that a VHF was wired in this way too.

 

[LadyInBed]

Maximum current through aluminum and copper wire

 

[pvb]

Maximum current through aluminum and copper wire

In the context of 12v systems on boats, your link is completely meaningless.

 

[VicS]

In the context of 12v systems on boats, your link is completely meaningless.

That's a bit harsh. The maximum safe current is still relevant esp with (very) short runs where voltage drop is not the overriding factor.

 

[claudio]

A 15amp cable should have a correctly sized breaker to protect the cable. The maximum cable overload is determined by the characteristics of the breaker.

A 'real life' answer is, when the breaker trips, the load is too much.

 

[pvb]

That's a bit harsh. The maximum safe current is still relevant esp with (very) short runs where voltage drop is not the overriding factor.

True, the maximum safe current would be relevant, but the link referred to didn't really show that. It shows copper wire at 75 and 90 degrees, but many people won't realise that ordinary PVC-insulated cable is typically only rated at 75 degrees max, or that cables grouped together (or run through hot spaces such as engine rooms) are subject to lower safe ratings. And, frankly, I wouldn't be happy about my wiring routinely getting up to 75 degrees, which is why I originally posted that voltage drop ought to be the basis of specifying cable on 12v boat systems.

 

[savageseadog]

In the context of 12v systems on boats, your link is completely meaningless.

Quite agree. The maximum current you can put through wire is set by the temperature that the insulation can stand which will be a combination of heat generated by the current plus ambient temperature. You could run the wire red hot with Teflon insulation if you want! Forget all that, you don't want any heat (or as little as possible), the basis for choosing a wire size on a low voltage system is set by the acceptable drop in voltage. The 3% figure is a reasonable one equating to .36V which is a good objective. In any event don't go below 1.5mm sq for any load however low. There was a statement on this thread about diversity and peak currents on things like tillerpilots/Autohelms. The current to consider is the peak current, the last thing you want is for an Autohelm voltage to drop under load when pressed. Take the peak or maximum current as the requirement. Don't forget the round trip for the current, the current has to pass through the load back to the battery negative so the negative arrangements are just as important.

 

[nathanlee]

Thanks folks. Some very helpful replies in here.

I guess I'm going to have to change the power supply cables then. I used AWG 10 from the charger to the battery, and from the battery to the main fuse/shunt, but that wouldn't fit in the conduit.
I think I'm just going to have to bite the bullet and route the cable on the outside of the conduit.


By the way, the inverter is only 150w, and the VHF is on a direct circuit to the battery.

Cheers

 

[st599]

In the context of 12v systems on boats, your link is completely meaningless.

Plus, it's a little bit of knowledge which may lead to danger, nowhere does it tell you how to derate the cable for the environment it is installed in e.g. multi cable trunking.