Question About Planning Circuits

[muskoka]

I have a pretty basic question: I'm trying to size wiring for my instruments and the total Fused rating is 40A but the actual Spec rating is 12A.

Do I spec the wire size for the fused rating or the specified rating?

[Bill_E]

I'm not sure if I quite understand the question but I'll add $0.02 anyway. First, you need to have a fuse that will blow before the wiring gets too hot or the equipment fails. But in my experience, especially with instruments, the voltage drop is the critical issue. If you have a long run of wire, the voltage drop can be very significant.

I had a case where my VHF radio was installed with wire that was too small. During transmission the current draw was sufficient to cause the radio to break up when the batteries weren't absolutely topped up. (I didn't do the installation, by the way.)

So, I'd put in the smallest gauge wire that I could conveniently pull through the space and that will fit terminal blocks, etc. I guess I think that less voltage drop is always better. Here is a link to some information about ampacity and voltage drops. American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies

There are lots of very good marine electricians on the forum so I hope you'll get some better answers than mine.

Bill

[capt_douglas]

You don't provide any specifics on the piece of equipment that has those requirements.

1) Total fused current could be the combination of your instruments (maybe the sum of all the fuses) or the maximum amount a wiring block can handle (screw size, block size, wire size). If it's a wiring block, then that tells me you can connect loads that total 40A but the connections are rated for 12A maximum.

2) I'd think there could be as much as 40A peak for a few milli/microseconds but the average current draw was 12A. An example would be a motor - the start-up current is generally higher than the operating current. Another example might be an HF radio - the current required to listen is far less than the current required when transmiting.

3) And as BillE observed, if the voltage drops, the current goes up. I don't think the current would rise that much but it is an alternative explaination.

[sailvayu]

The idea of the fuse or breaker is to protect the wire not the device. If you have a 40 AMP fuse the Wire should be able to Carry more than 40 AMPs or the wire will melt down before the fuse blows. Its that simple.

Can't make sense out of the rest of your question but that should help

[capn_billl]

Most 12V wiring is, (or should be), either 12guage or 10 guage for main power wires. Both guages have a rating of more than 40amps.

[AnchorageGuy]

Originally Posted by capn_billl

Most 12V wiring is, (or should be), either 12guage or 10 guage for main power wires. Both guages have a rating of more than 40amps.

Not sure what you are trying to say but it could be confusing to others. 12 volt wiring size is dependent on the distance to and from the power source and the amps that will be carried. This can be a 1/O or a 14 gage. Saying that most is 10 or 12 gage does not take these into consideration. Our "cables" from the main panel to the batteries are #1 because of the distance and maximum amps the wire might need to carry. Chuck

[muskoka]

Let me rephrase the question.

Regardless of what piece of equipment: If the manufacturer says it draws 3.5 amps and should have an online fuse of 10 amps.........do you design the circuit for 10 amps or 3.5 amps.

1. I'm not asking for a lesson in sizing wire x distance = gauge. The question is about the input figures for that calculation.
2. I get the total amps to the wiring block etc. The question is do you calculate on fuses or specified current?
3. There is undersized wiring in every circuit! You can stick a short length of wire in any circuit - it's a fuse.

So, the question is: If you have a piece of equipment (or bunch) which utilizes 3.5 amps, but is fused for 10 amps all per manufacters specs. Do you calculate the wire size at 10 amps or 3.5 amps?

PS - the reason for my question is that the club 'know-it-all' says my radar wiring should be designed for 10 amps which is the fuse size. My electrician says it should be designed for 3.5 amps which is what the manufacturer says it draws. I think the electrician is correct.

[RodeTrip]

You would want your wiring to be good up through the maximum amperage ( fuse limit 10A). It would make sense to me that if you wire for 3.5 amps, then you could have a problem with your wiring prior to the fuse being blown. Essentially setting your wire as your fuse ( it's weaker than the 10A fuse). I am not an expert on this though.

[muskoka]

Originally Posted by RodeTrip

You would want your wiring to be good up through the maximum amperage ( fuse limit 10A). It would make sense to me that if you wire for 3.5 amps, then you could have a problem with your wiring prior to the fuse being blown. Essentially setting your wire as your fuse ( it's weaker than the 10A fuse). I am not an expert on this though.

I'm asking for an experts opinion. I don't think speculation does much good in terms of answering a technical question.

For example, the wire in a 10 amp fuse is tiny compared to any wire feeding that circuit so the logic doesn't follow. If your logic were to be consistent, a 10 amp circuit could be a wire slightly larger than the hairline wire in the fuse. Wire's are sized up to avoid voltage drops - it isn't a case of 5 amps = gauge but rather amps x distance = gauge per the tables for voltage drops.

[sailvayu]

Forget the wire in the fuse it could be a breaker or whatever. If the manufacture calls for 10 max then use 10 amp and set your wire to carry a 10 amp load it is that simple. The 10 amp is max load and you need to be protected for that not the min or working load. Take for example the start up on a AC compressor. it draws 5 amps but start up of 10. If you are not sized for 10 it will trip every time it starts. And if the wire is not sized right you get a voltage drop and your amps go up.

Once again do not over think this if the manufacture calls for a 10 amp use a 10 amp even if working load is 3.5 you need to protect for max not min. That simple

[GordMay]

Originally Posted by muskoka

... PS - the reason for my question is that the club 'know-it-all' says my radar wiring should be designed for 10 amps which is the fuse size. My electrician says it should be designed for 3.5 amps which is what the manufacturer says it draws. I think the electrician is correct.

It seems that your club know-it-all really does; whereas your electrician probably isn't.
The wire must be sized to a minimum of 10 Amps, if protected by a 10A fuse. The actual load doesn't matter.

Gord May
Master Electrician

[muskoka]

Thanks Wayne & Gord,

This is a first for the club 'know-it-all' and I'll stand him a beer tomorrow! Too bad I can't do the same for you gents. Thank you!!!

In all fairness to the sparky, he speaks better English than I do Cantonese so I may have completely misconstrued his answer. But I will drag out the wiring size chart from Calders book and double check prior to him installing the cables! I've been doing the donkey work of conduit installation and drilling holes...

Cheers

PS - Which just goes to show what a wealth of knowledge this forum has!

[muskoka]

Here's another question.

My solar panels are 2 banks of 140 watts (280 W total) which come to the solar controller via two 8 AWG cables, which then step down to a single 10 AWG cable in and out of the charger. The 10 AWG cable is only about 30 cm long. And the techs at Blue Sky say the max. Cable size into the controller can only be 10 AWG.

How does that work? Two massive cables being stepped down into a relatively small wire? There must be 20 amps going through at noon.

[GordMay]

See Post #1
➥ http://www.cruisersforum.com/forums/...boats-372.html

[sailvayu]

The reason for the larger cables is to eliminate voltage drop to the controller which is sensing voltage, I do not understand the #10 max though that sounds odd? I would re read that one and double check