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How to fuse 12v circuit for appliances

chadmichael

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Feb 26, 2022
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I'm installing appliances in my 12v mobile solar system. Some of my appliances, like the ventilation fan, have a very small gauge wire on the appliance itself - 16 AWG. I'm running my circuites in 10 AWG. The fan draws 5Amp max.

If I use this calculator, it says that 10 AWG with 2% loss can be up to 24 feet. With 5% loss, it could be a 60 foot circuit. It also says that the 16 AWG can only be 6 feet at 2% loss, and 15 feet at 5% loss.

Question 1: Is it important to strive for the 2% here?

As for the fuse... it protects the wire ... which wire? do I do math on both wires, like:

14 feet of 10 AWG can carry 10 Amps, so I'd probably choose a 7.5 Amp fuse (can you use a fuse equal to the rating of the wire?)

6 feet of 16AWG can carry 10 Amps also, so I would likely choose the same 7.5 Amp fuse for that

Question 2: Does it work like that? :)
 
Question 1: Is it important to strive for the 2% here?
No.... for the fan you can aim for 5% or less and be just fine.

As for the fuse... it protects the wire ... which wire? do I do math on both wires, like:

14 feet of 10 AWG can carry 10 Amps, so I'd probably choose a 7.5 Amp fuse (can you use a fuse equal to the rating of the wire?)

6 feet of 16AWG can carry 10 Amps also, so I would likely choose the same 7.5 Amp fuse for that

Question 2: Does it work like that? :)

The fuse protects the wire, but there is no need to put a fuse that is larger than what the load needs.

Your fan is 5A. You don't want to run a fuse at it's blow point so the fuse for a 5A load should be 1.25*5=6.25 (min). Round that up to 7A.
So your fuse should be 7A. 7A is also the current to use for calculating the minimum safe size for the wire. However, the wire may need to be larger to manage the voltage drop. (In normal operations it will not be running 6.25 or 7A so use the actual load (5A) for calculating the voltage drop. )
 
Not exactly.
Talking about dc systems.
The length of the wire, the voltage, with the amperes expected is used to calculate voltage drop. Some devices such as the small fan are fine with more voltage drop, per ABYC 10%. Critical devices recommend not more than 3%
Ampacity is different. The length of the cable doesn't effect ampacity. Using marine grade cable. 10 awg can carry 60 amperes, if it is 2 feet or 20 feet still can carry 60 amperes. I would chose a 60 ampere fuse. That doesn't mean the voltage drop will not be too high if the cable is long. 16 awg can carry 25 amperes.
Now every time a wire run changes to a smaller awg, a different fuse is required.
Getting back to your fan. If you run 10 awg from the fuse to the fan with nothing else on the circuit you must fuse to the smallest awg, 16 awg. I would use 25 ampere or 20 ampere fuse.
 
Not exactly.
Talking about dc systems.
The length of the wire, the voltage, with the amperes expected is used to calculate voltage drop. Some devices such as the small fan are fine with more voltage drop, per ABYC 10%. Critical devices recommend not more than 3%
Ampacity is different. The length of the cable doesn't effect ampacity. Using marine grade cable. 10 awg can carry 60 amperes, if it is 2 feet or 20 feet still can carry 60 amperes. I would chose a 60 ampere fuse. That doesn't mean the voltage drop will not be too high if the cable is long. 16 awg can carry 25 amperes.
Now every time a wire run changes to a smaller awg, a different fuse is required.
Getting back to your fan. If you run 10 awg from the fuse to the fan with nothing else on the circuit you must fuse to the smallest awg, 16 awg. I would use 25 ampere or 20 ampere fuse.

When you say that when the AWG changes a different fuse is required... you don't mean "another fuse" right? Just that the fuse on the block has to be stepped down for the smaller wire, yeah?
 
@chadmichael As you can see, @Zill and I have very different approaches on choosing fuse size. I fuse based on load size even if the wire can handle more current. @Zil fuses based on wire size even if the load does not need that much current.

My approach is based on the fact that I prefer to keep the current to a minimum....particularly in a short circuit situation. As an example if a strand of wire is loose and touches the other pole, you might have a resistive short that is running 15A and a 20A fuse would not blow.

As is so often the case on the web, you are getting two different answers and there is no easy way for you to know who is correct.
All we can do is give you our answers and supporting reasoning and you have to decide what to do.
 
Voltage loss equals heat. But at 5 amps I'm not sure you could measure that amount of heat. I dunno, I've never tried. I tend to use the larger size wire.

Note that according to the Wire Gauge Calculator that I use, a 2% voltage drop @ 5 amps is appropriate for 24' one way distance or 48' round trip. The 7 amp fuse is acceptable.
 
When you say that when the AWG changes a different fuse is required... you don't mean "another fuse" right? Just that the fuse on the block has to be stepped down for the smaller wire, yeah?
You might want to review this:
 
Ampacity is different. The length of the cable doesn't effect ampacity. Using marine grade cable. 10 awg can carry 60 amperes, if it is 2 feet or 20 feet still can carry 60 amperes. I would chose a 60 ampere fuse. That doesn't mean the voltage drop will not be too high if the cable is long. 16 awg can carry 25 amperes.

Wish I understood this bit better. :)
 
@chadmichael As you can see, @Zill and I have very different approaches on choosing fuse size. I fuse based on load size even if the wire can handle more current. @Zil fuses based on wire size even if the load does not need that much current.

My approach is based on the fact that I prefer to keep the current to a minimum....particularly in a short circuit situation. As an example if a strand of wire is loose and touches the other pole, you might have a resistive short that is running 15A and a 20A fuse would not blow.

As is so often the case on the web, you are getting two different answers and there is no easy way for you to know who is correct.
All we can do is give you our answers and supporting reasoning and you have to decide what to do.
I appreciate all thoughtful opinions. I'm an engineer in the real world and know full well how context and usage, not to mention design goals, impact what is correct or incorrect.
 
Wish I understood this bit better. :)
There are two separate things to consider when choosing wire size:

1) Safety. A wire must be sized such that the current through it won't heat it up and cause problems. The Ampacity of the wire tells you how much the wire can handle without heat problems. Note that the ampacity of the wire is highly dependent on the insulating material. If the insulating material breaks down at a very high temperature, the wire can get hotter before there is a problem. Consequently, wires with high temp rated insulation are rated for higher current. Also note that the ampacity is not dependent on length of the wire. If a 1 foot section of wire can handle 5 amps without a heat problem..... a 100 foot section of the same wire can handle 5 Amps without a heat problem.

2) Voltage drop. Voltage drop IS dependent on length. The 100 foot section of wire mentioned above may be safe from fire, but the voltage drop may be so high that the appliance does not work correctly.

When calculating wire size, I do this.

1) Load-current x 1.25 = minimum fuze size.
2) For safety, the MINIMUM wire size must be large enough to handle the current of the blow point of the fuse.
3) Use load current and length to calculate wire size for voltage drop. If the wire size for voltage drop is larger than the minimum wire size for safety, use the larger size.

EDIT: Added the following.
I have never seen a situation in power systems where the minimum wire size for voltage drop is less than the minimum wire size for safety.
 
Last edited:
There are two separate things to consider when choosing wire size:

1) Safety. A wire must be sized such that the current through it won't heat it up and cause problems. The Ampacity of the wire tells you how much the wire can handle without heat problems. Note that the ampacity of the wire is highly dependent on the insulating material. If the insulating material breaks down at a very high temperature, the wire can get hotter before there is a problem. Consequently, wires with high temp rated insulation are rated for higher current. Also note that the ampacity is not dependent on length of the wire. If a 1 foot section of wire can handle 5 amps without a heat problem..... a 100 foot section of the same wire can handle 5 Amps without a heat problem.

2) Voltage drop. Voltage drop IS dependent on length. The 100 foot section of wire mentioned above may be safe from fire, but the voltage drop may be so high that the appliance does not work correctly.

When calculating wire size, I do this.

1) Load-current x 1.25 = minimum fuze size.
2) For safety, the MINIMUM wire size must be large enough to handle the current of the blow point of the fuse.
3) Use load current and length to calculate wire size for voltage drop. If the wire size for voltage drop is larger than the minimum wire size for safety, use the larger size.

EDIT: Added the following.
I have never seen a situation in power systems where the minimum wire size for voltage drop is less than the minimum wire size for safety.
As an interesting aside about current and voltage drop that has almost nothing to do with this thread: Because of the difficulty in managing voltage drop, many industrial controls and the old analog phones are specified by current, not voltage.

The analog phone would require a specific current range to operate and as you talked into the phone it would modulate the amount of current. The central switching system that might be miles away would see the 'sound' as modulation in the current (not modulation in voltage) That way the system would operate regardless of the voltage drop between the central switching system and the phone.

Industrial controls do something very similar. One common standard is the 20mA current loop. A 20ma Current Loop sensor will adjust the output current to something between 0A and 20ma based on the value of whatever it is measuring. Since the current is the same anywhere in the circuit, the receiver would get an accurate value regardless of the voltage drop between the sensor and the receiver. This also happens to be very resistant to any RF noise that might be in the system.
 
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