FUSE SIZING PRIMER
Written by John Swiatek
This primer focuses on the fusing requirements for power outlets during typical use. I am always concerned fusing a 12V vehicle circuit above 10A. If you need more current than that it may be best to add more outlets. If you must fuse a single circuit at 15A then certain precautions must be taken.
Here is why: First, fuse ratings can be a bit misleading. A 10A ATO (automotive) fuse will conduct 11 amps for 100 hours minimum. At 13.5 Amps a 10A ATO fuse can take as long as 10 minutes to blow. It is not like once you draw 10 amps "poof" the fuse is gone.
The primary function of a fuse is to protect the wire, connectors or "load" from overheating and starting a fire. Fuses will not protect sensitive electronics from damage. Below are typical single conductors' "free-air" current capacities for various gauge wires. Current carrying capacity is defined as the amperage a conductor can carry before melting the insulation. The numbers provided are for 80C (176F). All Powerlet wiring harnesses are constructed with 14AWG 105C wire and 105C PVC sleeving for added reliability.
Wire Capacity - Data Compliments of Alpha Wire Company:
- 22awg => 8A
- 20awg => 10A
- 18awg => 15A
- 16awg => 19A
- 14awg => 25A
NOTE: The practical current rating for a wire is much lower; it is also subjective. For instance, with the same current there will be more loss in an 18awg wire than in a 14awg wire. According to the chart both may operate a couple heated vests safely, however, with 14awg wire you will "feel" warmer since there will be more power delivered to the vest (less power lost in the wiring). A practical limit for 18awg may be about 8 amps while a 14awg conductor may be practical to around 15 amps.
Keep in mind that connectors also carry a current rating. The BMW type connectors used in the Powerlet products are rated at 16 amps. The fuse should not be higher than the "weakest link".
With that said here is the main concern: if you fuse higher than 10A, lets say at 15A (the next larger size), the circuit resistance may not allow the fuse to blow during a short circuit. Keep in mind a 15A fuse can conduct 20.25A for up to 10 min. The resistance of said circuit must drop below 0.60 ohm to blow a 15A fuse, where V=12V, I=20A, and R = V/I =12/20 = 0.6 Ohms. It is not difficult to accumulate 0.60 ohm of resistance in a vehicle circuit. Add together all of the wire and connector resistances then throw in corrosion at the connection points. 20awg wire has a resistance of about 1 ohm per 100 ft. Twenty foot of wire may sound like a lot but it adds up fast (there and back).
Therefore I am always concerned fusing a 12V vehicle circuit above 10A.