Fuse box in the engine area

[cmedsailor]

Quite recently I begun a thread on how to install a second switch panel, but thinking again I realised that the additional “equipment” (fans at the rear cabins, cockpit lights etc) to be installed are nearer to the engine area rather than the chart table where the instruments panels is. So, with my extremely limited (!) knowledge on electrics I would like to ask the following:

Could I add a fuse box, instead of a switch panel (besides all equipment have their own on/off switch) INSIDE the engine area? This is a hot area with diesel around etc.

If I have understood well I will need a relatively thick red (positive) wire going to the fuse box and then thinner red wires going to the various equipment. Similarly, a relatively thick black (negative) wire going to a negative bus is needed and thinner black wires going to the equipment too. I hope that I will be able to connect both the thick red and black wires after the main switch for services (red) and negative (black).

Do I need an additional large fuse on the thick red wire going to the block? If yes, how many amperes? Something at least equal to the total of the various equipment added?

Are there are any fuse boxes with negative bus just to make the work easier (not sure if a larger amount of wires on same box will make it easier or not though)?

Can you recommend some specific fuse boxes?

Am I missing something very obvious to more experts?

Thanks

 

[prv]

You've got it all about right. If you work out the maximum draw for the whole fusebox (sum of all the loads is easiest) and the distance that the cables will need to run from the master switch to the fusebox, then someone will be able to work out the proper cable size for you. This run should really have a fuse at the input end, just in case the cables get damaged and short together; the size of this should be somewhere between the maximum draw and the current-carrying capacity for the chosen size of cable. If there's already a suitable-sized fuse somewhere upstream (for example on top of the battery) then the run doesn't need its own.

Blue Sea have a range of fuseboxes which would fit your needs well, some including the negative busbar:

https://www.bluesea.com/products/category/16/61/Fuse_Blocks/ST_Blade

There are significantly cheaper (but not as nice) ones available if budget is a concern.

Pete

 

[Lon nan Gruagach]

Well, looks like your questions and the answers here have done the job. Yes you seem to be right on the money.

Regards a main fuse for all this:

The simple answer is:
Add up all the potential loads and install cabling that can cope with that, protected by a fuse where the cable gets its power.

A more robust installation would add up the values of all the fuses in your new panel and use that figure for the main supply. This is a bit overkill since it assumes you have a fault or near fault condition on all circuits [but is the requirement for mains distribution]

There are some mitigating factors that let you reduce the size of the main cable and fuse:
If, say, your new panel supplies power to a pump for the head, a pump for the was basin and a pump for the shower (which is in the same room) then you can reasonably assume that not all 3 will be in use at the same time and you could just consider the largest of these loads. This is called diversity, there are tables for it in domestic situations. If you add up all the 13A sockets on a house ring main it exceeds the capacity of that main BUT the fuse/ breaker is small enough to protect the cable.

 

[cmedsailor]

Thanks for the answers. What about my question for installing a fuse box in the engine area? Is that OK to do?

 

[Lon nan Gruagach]

Thanks for the answers. What about my question for installing a fuse box in the engine area? Is that OK to do?

Nothing wrong with that.

 

[PaulRainbow]

Thanks for the answers. What about my question for installing a fuse box in the engine area? Is that OK to do?

Car engine bays are full of fuse boards and electronics. Keep it away from as much heat as possible and it'll be fine.

You're pretty much covered by the previous replies, i'd just add that :

You should really be using tinned wire.

Use wiring for the maximum current that the fuse panel can handle.

Fit a fuse that protects the wire at it's max capacity, or thereabouts. The fuse protects the wiring to the fuse panel, not the equipment connected to the panel.

http://www.12voltplanet.co.uk/cable-sizing-selection.html

 

[stephenh]

"What about my question for installing a fuse box in the engine area? Is that OK to do?"
"Nothing wrong with that. "

However - from the web :-

Temperature Derating of Fuses
Ambient temperature of the application must be considered when choosing the current rating of the fuse.
When a fuse is applied in an ambient temperature exceeding the standard 23°C, the fuse current rating should be derated (a higher amp rating with higher temperatures).
Fuses are temperature dependant so the higher the ambient temperature the quicker the fuse will blow.

I only know enough to be dangerous !

VicS or someone will turn up and explain more fully......

Good luck

 

[coopec]

cmedsailor

Your query in regard to the red (positive) being heavy enough reminded me of some things I had to consider when I did my wiring. I used one of the tables I found on the internet which indicated the cable required for different amps/distances.

http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

This is not going to happen to you but it does highlight the consequences when things go wrong with your electrics!

http://www.pantaenius.com.au/en/news-events/news/newsartikel/news/gone-in-180-seconds.html

Clive

 

[prv]

I used one of the tables I found on the internet which indicated the cable required for different amps/distances.

http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

There are also other tables and online calculators that work in mm² rather than American wire sizes, which is more convenient at least here in Europe. However, they're often a bit vague as to whether they're based on the round-trip distance (length of positive cable plus length of negative cable, which may not be the same) or one-way distance (supply position to load position, assuming positive and negative are of equal length).

Pete

 

[pvb]

There are also other tables and online calculators that work in mm² rather than American wire sizes, which is more convenient at least here in Europe. However, they're often a bit vague as to whether they're based on the round-trip distance (length of positive cable plus length of negative cable, which may not be the same) or one-way distance (supply position to load position, assuming positive and negative are of equal length).

A good rule of thumb for calculating voltage drop is my TOT rule. “TOT” stands for “Treble One Twenty”, and is all you need to remember. The 3 ones and the twenty work as follows:-

* 1 amp going 1 metre along a 1 sq mm wire gives a 20 mV drop.

That’s your basic figure, and all you need do now is change the 20mV figure in proportion to the other changes. So, for example:-

* an increase in current increases the 20mV proportionally

* an increase in length increases the 20mV proportionally

* but an increase in wire size decreases the 20mV proportionally.

For example, let's consider a 1200W windlass which will draw about 100A. This 100A load increases the 20mV to a 2000mV drop. Say you've got 20 metres of wire in the circuit (10 metres there on one conductor, and 10 metres back on the other conductor), which further increases the drop by a factor of 20, bringing it to 40000mV. That’s the theoretical drop you’d experience if you used 1 sq mm wire. Using 50 sq mm wire will reduce the drop by a factor of 50. Dividing 40000mV by 50 means that the 50 sq mm wire will give you a drop of 800mV (0.8V).

Now before the purists start, this is only a rule of thumb. It’s about 95% accurate, which I reckon is close enough for leisure boats (after all, your 12 volt source probably fluctuates by 15%). But, and this is important, it’s easy to remember, and easy to work out without needing to resort to a calculator (which is why I believe it’s easier to think in millivolts, as there’s less chance of confusion with the decimal point).

 

[nigelmercier]

Fit a fuse that protects the wire at it's max capacity, or thereabouts. The fuse protects the wiring to the fuse panel, not the equipment connected to the panel...

Why does this nonsense keep being regurgitated?

The wire must be the correct size for the equipment based on the length of and the current draw, the fuse must be sized just big enough so it will not blow in normal use. The current carrying capacity of the wire is therefore irrelevant in selecting a fuse.

 

[PaulRainbow]

Why does this nonsense keep being regurgitated?

The wire must be the correct size for the equipment based on the length of and the current draw, the fuse must be sized just big enough so it will not blow in normal use. The current carrying capacity of the wire is therefore irrelevant in selecting a fuse.

He's fitting an additional fuse panel in the engine compartment, which will supply power to a variety of appliances. What size fuse should he fit ?

 

[PaulRainbow]

http://assets.bluesea.com/files/resources/instructions/Wiring-Diagram-5025_5030.pdf

 

[VicS]

He's fitting an additional fuse panel in the engine compartment, which will supply power to a variety of appliances. What size fuse should he fit ?

The supply to the fuse panel should be fitted with a fuse not exceeding maximum total capacity of the panel. The supply cable to the panel should be rated to carry this current safely. In practice with 12 volt a system the cable will actually be be rated to avoid excessive volts drop as well as safe current carrying capacity and may therefore be significantly heavier than needed for safe current capacity alone .

The individual fuses in the panel should be appropriate/ as recommended for the equipment they supply. The cables to the various pieces of equipment will be rated not only to carry these currents safely but also to avoid excessive volts drop.

 

[PaulRainbow]

The supply to the fuse panel should be fitted with a fuse not exceeding maximum total capacity of the panel. The supply cable to the panel should be rated to carry this current safely. In practice with 12 volt a system the cable will actually be be rated to avoid excessive volts drop as well as safe current carrying capacity and may therefore be significantly heavier than needed for safe current capacity alone .

The individual fuses in the panel should be appropriate/ as recommended for the equipment they supply. The cables to the various pieces of equipment will be rated not only to carry these currents safely but also to avoid excessive volts drop.

That's what i said Vic (i just used less words )

 

[VicS]

That's what i said Vic (i just used less words )

You said this

Fit a fuse that protects the wire at it's max capacity, or thereabouts. The fuse protects the wiring to the fuse panel, not the equipment connected to the panel.

Read more at http://www.ybw.com/forums/showthread.php?458147-Fuse-box-in-the-engine-area#ecAkU6w5MifMGHlf.99

Which is wrong if you mean the max safe capacity of the wire used because the wire will probably be oversized for the current it will carry in order to limit volts drop .

If you match the fuse to the max safe current capacity of the wire it may not adequately protect the equipment supplied. In this case that is the fuse panel