Wiring Size Please Help

[alexrunic]

Hi,
i have noticed my feed from my battery to the switchboard is rather small, so i am going to replace it. What size do i need the total possible load i have calculated by adding up the values of all the breakers is 60 amp? (length of cable approx 3 meters)

i have looked for the last hour on the net and have got many different answers from 3.2 mm - 13.6 mm .

Thanks

Alex

 

[whipper_snapper]

[ QUOTE ]
i have calculated by adding up the values of all the breakers is 60 amp? (length of cable approx 3 meters)

[/ QUOTE ]

That will hopefully very significantly over estimate the max load.

But a 6m (2-way) run for 60A and a 5% drop needs 6AWG.

 

[BAtoo]

According to the formula I use if you are running 60A through a 3m cable and will accept a voltage drop of 0.5V then you can use 6mm; if you want only 0.25V then its 12mm.

However just think about it -

are you likely to run at 60A - if you do you wont do it for long as (in a typical battery set-up) your batteries will be flat in 1-2 hours!!

If you are worried re a short circuit then its the fuse that takes care of that - if you short all of your instruments at once then you have problems!1

I may be wrong!!

 

[tillergirl]

For 5% voltage drop - length of cable is 6 metres (including the return), max 60 amps on circuit Calder's book gives 4 AWG which equals 19 sq mm.

For 10% voltage drop, 8AWG or 8sq mm

Of course you will never draw 60 amps - unless something has gone wrong!

 

[Bilgediver]

I may be wrong!!

***************************************

Your answer was a bit ambiguous like the answers the questioner finds on Google

At least for safety sake advise us if you are quoting wire diameter as per our american kissin cousins or wire cross sectional are as per UK

Your answer is unclear and this could also be the reason for the apparent ranges of size on Google.

 

[tillergirl]

There. That's clear init?

 

[alexrunic]

Ok then maybe i was wrong to say 60 amps what sort of figure should i be working on then? sorry electrics not one of my strong points. also cable size?

 

[whipper_snapper]

[ QUOTE ]
Ok then maybe i was wrong to say 60 amps what sort of figure should i be working on then? sorry electrics not one of my strong points. also cable size?

[/ QUOTE ]

Add up the loads you are supplying!

 

[VicS]

Here is a link to a table. The distance to use is the total run from battery to panel and back again. So if they are 3metres, say 10ft, apart that's 20 feet total.

For the current, estimate the maximum load you will have on at any one time. You are the only one who can do that by adding up the amps ( calculated from watts/volts when necessary) for all the items you will have on at any one time.

Round up to a figure in the table, but you know it wont exceed 60 amps.

I make that 4awg for 60 amps and 20 ft, 6 AWG for 30 or 40 amps, 8 AWG for 20 or 25 amps.

There is a link on the page to another table to convert AWG to mm². You'll note that the wire sizes are pretty heavy. That's because it is all based on an acceptable volts drop not on what will safely carry the current.

AMPS & WIRE GAUGE for 12 VOLT CIRCUITS

 

[Quandary]

Like the poster I know very little about electrics, but I understood that a cable could not be too big, Is this true?
Since this is the main load carrier and electrical cable is relatively cheap, I would have used the biggest size that I could, taking account of terminal crimping restrictions and space, on the presumption that the voltage drop is less in larger cables. ( substantially larger than the old cable)
If this is not so please tell me because I have been fitting extras with over sized cables for years on this premise.

 

[VicS]

No technical objections to going oversize, just gets progressivley more difficult to route them and to make the terminations.

 

[co256]

I would listen to the advice of "tillergirl" and "VicS"

It's a circuit and as such whatever the length of cable from the battery to the thing it powers you need to allow for the return distance to the battery...... hence you double! 6 metres not 3 metres!!!

AWG means "American Wire Gauge" it's just a unit of measurement.

The only advice I would add is to buy the best quality marinised, tinned cable you can afford to buy and if there's a size choice always go one size bigger!

All the best.

 

[whipper_snapper]

[ QUOTE ]
I would listen to the advice of "tillergirl" and "VicS"

[/ QUOTE ]

Oh fine, ignore me, I don't mind.

There is no point in massively over sizing wire. Good quality wire IS expensive. And harder to route neatly. Adding up the values of your breakers is very unlikely to equate to the load. You just need to add up the max power requirements of al the bits you are powering. Add a healthy margin, but don't overdo it. After all, worse case is that the drop exceeds 5% by a bit.

 

[alexrunic]

Thank you everyone for your ideas. i think i have also been getting the dia mm and the cross sectional area mixed up fundamental i know /forums/images/graemlins/blush.gif

 

[VicS]

[ QUOTE ]
getting the dia mm and the cross sectional area mixed up

[/ QUOTE ] Easy to get confused because so many suppliers say mm when they mean mm² .

You've got some heavy cable in use if you've been using 21mm diameter instead of 21mm² xsectional area /forums/images/graemlins/grin.gif

The AWG to mm² table that I mentioned also gives the diameter in mm and inches and the resistance per 1000m, fwiw.

 

[savageseadog]

I would recommend 6- 10mmsq for feeds to a switch panel. Probably worth running two or three with corresponding negative returns and splitting the load. Fuse them near the main switch at 20A each which protects against a short circuit. Could divide (for example) as 1)interior stuff including lighting, 2)nav lights and instrumentation, engine panel and tiller etc.

It's better that way because the varying loads are at a lower maximum and each circuit can carry a lower fuse rating and will provide better protection.

 

[DaveS]

While there are some advantages to this approach, one point worth watching if low rated upstream fuses or CBs are used is the danger of losing discrimination between upstream and downstream devices. Basically a fault on a final circuit should take out the final circuit's protective device without tripping / blowing the upstream device protecting the feeder. If the devices' respective ratings are too close together, e.g. a 20A fuse upstream of a 15A fuse, then both are likely to blow.

It's really a subject in its own right, and is complicated by the fact that different types of fuses and circuit breakers each have their own characteristics. These should be documented, and this info. should be checked to ensure that the maximum I^2t let-through of the downstream device is less than the minimum initiating I^2t of the upstream device. A useful rule of thumb, however, and with all the usual caveats, is that for devices of the same type there is unlikely to be a problem if the current rating of the upstream device is twice or more that of the highest rated downstream device it supplies.