[RVG8tor]

From all of my learning on the subject, it is drummed into you that the circuit breaker or fuse protects the wire not the appliance. Then I read this note in my wire harness kit from Van's...

High Performance Airflow Performance Fuel Pump = PN; 3090050

Problem:
The 5A switch breaker used to protect the circuit Q1 (18AWG) for the fuel boost pump many nuisance trip particularly during start-up or in a warm environment.

Solution;
If this fuel pump will be used substitute a 7.5A switch circuit breaker in place of the 5A (OP-10 Rev 6, OP37-13 Rev1) or (OP-10 Rev 7, OP37-13 Rev 2)

All of the above seems logical until this....

WARNING! Do not substitute a 10A breaker for the 5A breaker since the 18 AWG Q1 wire in you harness cannot handle 10A for the length of the wire required (RV-10 especially).

I don't' understand how length plays into the CB size to protect the wire, I know it can limit the voltage you get at the appliance due the the resistance in the line. My run for the line should not be more that 6 feet (RV-8A), it does not seem like this is a factor. I planned a 10 amp breaker for this line, which is the recommended size for the 18 AWG wire. I must be forgetting something I once read but did not retain which is not uncommon for me. Thanks for any help with this.

Cheers

 

[WingsOnWheels]

Best said by someone else: "AC-43.13 also specifies a maximum allowable VOLTAGE DROP across a wire. The
maximum allowable voltage drop according to the FAA for 14V systems is 0.5 volts. All
wires have some resistance, and when current is drawn through a wire, there will be some
voltage drop across the wire. It is this voltage drop that causes the wire to heat up, since
power is equal to voltage drop times current flow (P = V * I). If the wire is long
enough, the voltage drop across the wire will exceed 0.5V long before the maximum
current capacity of the wire is exceeded. This voltage drop will at some point cause the
equipment being powered to quit working reliably, just as though the battery voltage
were too low." - EXP BUS TECHNICAL BULLETIN

 

[WingsOnWheels]

18awg wire is rated fo 9amps continous (in a bundle). 10 amp would exceed the capacity of the wire.

Also, when you are working your electrical system, try to think in reverse. What CB do you need for your appliance? What wire has a capacity equal to or greater than the CB. Your CB size is then determined by your need and the wire size derived from that - maybe that logic will makes more sense.

 

[RV10Man]

The lower the voltage, the higher the current. Length of the wire lowers the voltage thereby increasing the current (amps).
This example doesn't have anything to do with wire length but it shows with lower voltage the current increases. A 120/240 volt (dual voltage) motor pulls 5.5 amps on 240v or 11 amps on 120v.
I'm sure this is why a lot of aircraft use 28v systems, they can run smaller wire reducing weight.

Marshall Alexander (licensed electrician)
OKC, OK

 

[RVG8tor]

AC43

Hmmm, my AC43 Table 11-3 shows 10amp breaker for 18AWG wire.

18awg wire is rated fo 9amps continous (in a bundle). 10 amp would exceed the capacity of the wire.

Also, when you are working your electrical system, try to think in reverse. What CB do you need for your appliance? What wire has a capacity equal to or greater than the CB. Your CB size is then determined by your need and the wire size derived from that - maybe that logic will makes more sense.

 

[RVG8tor]

How does V drop relate to CB size

I understand the concept of voltage drop, and if the wire is too long the voltage drop will cause the appliance fail to function. In the example I have, Van's has you use an 18 AWG wire to run the fuel pump, they say don't use a 10 amp fuse due to the long run, I understand the appliance may fail to function due to resistance, but why don't they say use 16 awg wire if the run is so long, what does using a 7.5 amp breaker on an 18 AWG wire have to do with voltage drop and protecting the wire. If there is a short in the wire will the breaker not pop with 7.5 or 10 amp breaker?

I still would like to know the above questions but looking at my AFP manual it says the pump draws 5 amps and should be protected with a 7-10 amp breaker. They further state to use 16 awg wire, I am not really sure why Van's has you use 18 AWG wire. Looking at the chart in AC43 18 AWG wire could carry 5 amps at a length of 15 feet before exceeding a voltage drop of .5V.

I sure find these electron matter confusing at times.

Looking at the chart in AC43 Figure 11.2, and 18 AWG wire ruining 9 amps would have a maximum length of 8 feet at continuous flow before the .5 voltage drop limit.

Best said by someone else: "AC-43.13 also specifies a maximum allowable VOLTAGE DROP across a wire. The
maximum allowable voltage drop according to the FAA for 14V systems is 0.5 volts. All
wires have some resistance, and when current is drawn through a wire, there will be some
voltage drop across the wire. It is this voltage drop that causes the wire to heat up, since
power is equal to voltage drop times current flow (P = V * I). If the wire is long
enough, the voltage drop across the wire will exceed 0.5V long before the maximum
current capacity of the wire is exceeded. This voltage drop will at some point cause the
equipment being powered to quit working reliably, just as though the battery voltage
were too low." - EXP BUS TECHNICAL BULLETIN

 

[WingsOnWheels]

Looking at the chart in AC43 Figure 11.2, and 18 AWG wire ruining 9 amps would have a maximum length of 8 feet at continuous flow before the .5 voltage drop limit.

I was also referencing 11.2 @ 8ft (you said the run was ~6ft) This gives you just under 10amps, I rounded down to the nearest amp.

In regards to protecting the wire vs device: In most cases, it is a failed device (not a failed/shorted wire) that will cause an overload in the circuit. In that event, by selecting a wire of greater capacity than the breaker you will protect the wire from overheating since the breaker will trip before the wire reaches maximum load (possibly burning up a bundle).