Adding fuse capacity

[gtmoore]

I'm after a bit of wiring advice - I'd like to fit a couple of water pumps to my 29ft sloop. One is a shower pump and the other is for pressurised water. My existing 8 fuse box and separate switch panel are a bit 'busy' with all the toys I've added recently - this is all low current stuff like map light, HH VHF charger, gas alarm etc etc. I really need to look at replacing these with a proper combined board and CB's but until I do I had planned on installing a second separate fuse box. I was thinking of a small 4 fuse one that I would run a separate (fused) feed from the battery switch for that I could just take power off for my pumps.

The total quoted current draw for the pumps is around 10 amps each and by their nature they would most likely be running at the same time. The run from battery switch to fuse box is approx 2 metres. I was planning to use 6.0mm cable from the battery for this.

Do I need to uprate the negative cabling from the battery or can I just connect the negative wires from the pumps to my existing negative busbar?

Having read the Fire thread recently, I've become a bit more cautious and just wondered if someone could tell me if this seems a reasonable thing to do.

Thanks in advance

Gavin

 

[bedouin]

Difficult to answer without more information about what is connected to the existing fuse box. Since the same current flows in both the -ve and +ve feeds it makes sense to have equivalent capacity in both - so if you are adding a new cable to the +ve then it makes sense to add one to the -ve as well.

There are two considerations when sizing cables, the first is the current handling capacity and the second is the voltage drop. For current handling, the total current draw of all devices powered by the cable should not exceed the capacity of the cable - that is the critical safetly issue. Fuse size should reflect the cable, not the proposed load. Voltage drop is not likely to be a serious issue when connecting the battery to the switch panel - and is not a safety issue

 

[pvb]

Cables and fuses...

If you're going to add a (temporary) fuse box, you might consider getting one which takes automotive-style blade fuses, rather than those nasty ceramic fuses with tapered ends. The contacts seem more reliable.

6 sq mm cable will be fine for a 2 metre run to the fuse box, based on running 2 pumps together. You should upgrade your negative cable between battery and busbar - either replace with bigger cable or add a 6 sq mm cable in parallel with the existing one.

The right cable size from the fuse box to each pump will depend on the length of cable involved. You measure the total cable length (ie to the pump and back to the fuse box - so if the pump is 5 metres from the fuse box, you'd have 10 metres of cable). The idea is to keep the voltage drop along the cable to a reasonable level, so that the pump gets enough voltage to work properly. Ideally, you wouldn't want to drop much more than half a volt. For a 10 amp pump running on 10 metres of 6 sq mm cable, the voltage drop would be 0.33 volts, which is fine. If you reduced the cable size to 3 sq mm, you'd double the voltage drop to 0.66 volts, which would probably be just about be OK. Obviously, longer cable runs increase the voltage drop pro rata, and vice versa (too much Latin there!).

 

[gtmoore]

To be honest I don't have a lot of current draw within the existing system and the thickness of the negative cable would suggest that it was over specified. I've just been down to my local autoelectrical store (to have some battery leads made up for some Mega fuses) and while there asked to see some 40amp cable. Im sure that the positive cable feeding the fuse box and the negative to the busbar that I have in the boat at the moment are far thicker than what I've just seen yet my total current fuse values are less than 40amps.

I guess if the supplying cables are thick enough I could just piggyback my new fusebox off it. Is there a method I can use to identify the existing wire capacity?

Thanks for the advice

Gavin

 

[gtmoore]

Re: Cables and fuses...

Thanks - yes I was going for the bladed type. The existing one is the ceramic kind which as you say is not ideal.

To be honest the entire 12v electrical system on my boat is less than ideal. It was rewired recently (before I bought it) but for some reason the fusebox is mounted under the chart table which is about the least accesible position for it. In addition this of course means there are now 8 wires routed to the switch panel which is mounted well above the chart table, some of said wires which need to then come back down to be routed through lockers and to pumps etc!! I intend to replace all this with a single fuse/switchboard when I'm feeling brave enough to face untangling the spaghetti hiding behind the existing board!!

The new pumps should be located less than 2m away so this should keep the wiring sizes sensible.

Thanks again for the advice.

Gavin

 

[bedouin]

All other things been equal, the capacity of the wire is proportional to the cross sectional area of its conductors - roughly 10A per 1 square millimetre of copper (maybe a little less to be on the safe side).

So either measure the size of the conductor (NOT with the insulation on) or approximate it by comparing it with conductors of known capacity.

On the basis of KISS (Keep it simple - stupid) I would piggyback the new fusebox of the old if the cables are up to it - there is no point in making the wiring more complex than it needs to be.

 

[pvb]

Afraid you\'re wrong there...

The current carrying capacity of wire is certainly proportional to the cross-sectional area. But in a boat, it's essential to consider the voltage drop, because cable runs tend to be quite long.

Whilst a 1 sq mm cable can handle 10amps over a very short distance (say 1 metre), it would result in a big voltage drop over longer runs (eg 2 volts over a 10 metre cable run). Voltage drop doesn't just affect the equipment being supplied, it results in the cable becoming heated and, in the worst case, overheated.

For cables powering electronic equipment, fuse panels and navigation lights, it's best to plan for no more than 3% voltage drop (ie about a third of a volt on a 12volt supply). For other applications, a bigger voltage drop can be tolerated, but certainly no more than 8-10% (say 1 volt on a 12 volt supply).

 

[bedouin]

Re: Afraid you\'re wrong there...

If you read the whole thread you will see that these comments only relate to the safe capacity of the cable, and not voltage drop.

This rating is not affected by the length of the cable run, since it relates to the safe heat dissipation per unit length (assuming the cable is not coiled in an enclosed space). So while running the cable at full load over a long run may cause an unacceptable voltage drop, it is not a safety issue. Properly installed you can run any length of a cable at its full rated capacity for an indefinite period of time - that is close to being a definition of the rated capacity.

When considering the wiring to a switch panel you have to consider two issues:

(a) the maximum possible current draw

and

(b) the maximum acceptable voltage drop

Now for (a) you have to be conservative and rate the cable for every possible connection being run at full load. For (b) you can make more realistic assumptions as to what equipment will be run simultaneously.

As a rule of thumb I would recommend wiring to the switch panel rated at least 4x normal working load, assuming a short run of 1-2 metres, but that is going well beyond the original question.

 

[Anchorite]

Re: Voltage drop calcs

Here is the formula to calculate the voltage drop using copper wire:
Voltage drop = Amps x Cable run (there and back) x 0.017 (copper resistance)
divided by cable section in mm2.
Example: 10 amp pump with 5 meter run (=10 meters there and back), cable
section 2.5 mm2.
Calc: 10 x 10 x 0.017/2.5 = 0.68 Voltage drop. This is 5.7% on a 12 volt system.
If you substitute 4 mm2 cable the drop will be 0.4 volts, or 3.5%.
Generally voltage drop for electronic gear should not exceed 3%, for fans and
lights 10% is a max.
6 mm2 cable is often difficult to fit onto connectors and takes up a lot of room
inside the panel. Perhaps more important than the cable is the quality of the
connections at/to the terminals (tinned, soldered, crimped, varnished).