how big a breaker

if you have 4 amps of draw can you use a 5 amp breaker or is it too close to tripping seems there should be a percentage above the draw.

Yes, a 5A breaker would be fine. Remember, you are protecting the wire with the breaker, not the device that is being powered.

Bill, IMO, you're working with the wrong premise.

An easy rule of thumb is for loads to be 80% of wire capacity, and circuit protection to be 80% of wire capacity. Having said that, there's a lot more to your question than you asked. Lamp load? Inductive load? Resistive load?

Breakers *typically* respond much slower than *typical* fuses, so are *typically* more tolerant of surge overload.

Having said all *that*, breakers and fuses protect *wire*; not devices (contrary to what cheezy device mfgrs try to tell you). Size the wire to easily handle the load with tolerable voltage drop, and size the circuit protection to protect the wire. Done.

Bill v,

Circuit breakers often aren't well understood. A 5 amp circuit breaker won't immediately trip at 5 amps. AC 43.13B calls out MIL-C-5809 for circuit breakers, an example circuit breaker which follows MIL-C-5809 is the Klixon 7274 series. The data sheet on this circuit breaker provides a current vs. time graph. You can see the data sheet here:
https://www.sensata.com/sites/default/files/a/sensata-7274%20series-high%20performance%20circuit%20breaker-datasheet_0.pdf

Some notes from reviewing the data sheet:

A 5A CB won't trip until 115% (5.75A) of the rated current at 25C, and even then it would only trip after some period of time. As the temperature gets hotter, the CB could trip as low as 4.5A at 71C after the same period of time.

To trip a CB within 2 seconds, a current of twice the rated load is required. This means that the CB doesn't necessarily have to be sized for the inrush current provided the inrush current isn't extreme.

There is a voltage drop associated with any CB. The voltage drop increases as the rated current of the CB decreases.

Thanks,
Levi

thankyou

thank you for the for the responses. the reason for the original question is I have two led lights in the wing tip that use one amp each so two amps of draw I size the wire but want one switch so I need at least a 5 amp breaker to power both sides. if I get a short then I would send 5 amps down the wire before the breaker trips which is ok but if I need the next size larger breaker (7 1/2) then I need to up the wire size (in my understanding). not for the load but for the fact I have more then one run of wire for the same breaker and might get a short in one run.

There should be little if any startup surge with LEDs. You should size the wire for the load current and voltage drop (based on wire length). You should protect the wire with a breaker sized to protect the smallest wire attached to that breaker.

And not to start a 'primer war' on circuit protection, but an ATO blade fuse will cost about 1/10 the price of an a/c breaker, and weigh about 1/20 (maybe 1/100?). And probably be more reliable. If everything is sized properly, you'll never have a nuisance trip on that circuit, so you'd never need to attempt a reset in flight.

rv7charlie said:

There should be little if any startup surge with LEDs..

Click to expand...

Actually, the inrush current in most LED drivers is HUGE! The circuit behaves like a dead short. However, the duration of the inrush is very, very short, so your circuit protection typically won?t be hindered.
Still, in the commercial world where thermal/magnetic breakers are used, most engineers are not allowing circuit loading beyond 50% of the breaker rating to avoid false tripping.

I am not sure what our typical aviation suppliers breakers use for technology. Thermal only? If so, you would never get a false trip due to inrush. If they are magnetic, there is a remote possibility.

I absolutely agree with you on your other statements.

JonJay said:

Actually, the inrush current in most LED drivers is HUGE! The circuit behaves like a dead short. However, the duration of the inrush is very, very short, so your circuit protection typically won’t be hindered.
Still, in the commercial world where thermal/magnetic breakers are used, most engineers are not allowing circuit loading beyond 50% of the breaker rating to avoid false tripping.

I am not sure what our typical aviation suppliers breakers use for technology. Thermal only? If so, you would never get a false trip due to inrush. If they are magnetic, there is a remote possibility. I absolutely agree with you on your other statements.

Click to expand...

I just came to understand why some LED's suck up current initially, when turned on. Many LED lights have a dedicated power supply, driving the LED(s). The power supply can draw a high current momentarily as the capacitors charge and suck up a lot of current initially. It may be milliseconds but there could be a spike.

The solution is "inrush current limiters". You can buy them from Digikey or Mouser or Newerk, a few bucks... min shipping might get you if you don't order other items. Some LED products include an inrush current limiter... you have to check.

Amazon has them but they are expensive. However in this example shipping is free. "Aqua-Rite Thermistor AS32 2R025.( Pack of 2) Inrush Current Limiter 2 Ohm Ohm ±25% 1.18" (30mm)" These are 25 amps, but they have them in different limits.

Some LED's are low current even when you turn them on initially, typically they are driven directly off the buss and only have resistors to drop the current to the required amount for a LED. No inrush current limiter is needed. LED landing light probably does need it, especially if you are using a WIG WAG...