If you wire two fuses in parallel, is the result a fuse of their combined ratings?

Question

So, I want a 15amp circuit breaker on the headlights of my vintage car (12 volt). I have two 7.5 amp circuit breakers. I could put one circuit breaker on each headlight, but what would happen if I wired the two circuit breakers together in parallel? Could I draw 15amps through the pair of them, or would they trip at 8amps?

Finally, and just out of interest, if you wired a 10amp breaker and aa 5 amp breaker together in parallel, when would they trip?

Answer 1

All of these ideas are probably bad.

Circuit breakers have low resistance. They have to because they're in series with the entire circuit. If they did not have low resistance, then plugging in something with even a moderate amount of current would cause a large-enough drop across the breaker that everything else on the circuit would dim (this is what happens when you plug in a high-current device, like a sweeper, into a circuit with a few lights on it; the lights dim when the sweeper is turned on).

Because of the low resistances, small variances in the resistances may be relatively large. That means that while ideally you would get an even split of current across both breakers, you would probably get a significantly higher amount of current through one breaker than the other. Once that high-current breaker flipped, it would cause all of the current to go through the other which would cause it to flip. In effect, you probably would not ever be able to run 15 amps through the parallel combination for very long because one breaker would certainly carry more than 7.5 amps.

Additionally, breakers do not pop immediately when their current rating is run through them. They break after a certain amount of time, and the amount of time is a function of the current. A 20 amp breaker may pop immediately at 50 amps but may take a few minutes to pop at 20 amps.

Because of these reasons, I don't think it's a good idea to speculate about the parallel combination of a 10amp breaker and a 5amp breaker.

Answer 2

Ted's answer just above this one would be the best explanation so far... the combined rating would be somewhat less than double due to the inevitable variance in resistance between the two fuses.

As for the design of the fuses, they all function by the heat of the current passing through them causing their conductors to heat up and break apart, causing an open circuit. It does take a certain amount of time for this heat to build up however, and cause the reaction.

Some fuses are designed to withstand brief over current situations without blowing (slow-blow fuses), whereas others are quick-acting fuses, designed to melt and open very quickly (almost instantly). The proper choice depends on the application.

For fire prevention, where all the other components are tolerant of brief over-currents before they become a fire hazard (such as house wiring), and where slight/brief power fluctuations are common, that calls for a slow-reacting fuse/breaker element. Solid state electronics (transistors, IC chips and such) are generally more sensitive and therefore require quick-reacting fuses, and preferably some sort of power control to smooth/regulate/limit currents from its power supply.

Cheap computer modems use small capacitors instead of fuses to protect your computer from lightning. When lightning causes a power surge in the phone line, the capacitor shorts out to ground more quickly than your typical fuse. Naturally, if you're like most people, you replace the whole modem rather than replace just the blown capacitor... and the manufacturers want it that way. To make you buy another modem... the jerks. :-)

Answer 3

Ted and Hirsch are right. All other answers above are wrong, for one reason or another.

The actual result depends on the fuses impedances.

The result would be like a fuse which is _certainly_ more than the smaller (coz part of the current goes the other way in all cases), _maybe_ more than the biggest (if the smaller doesn't get too much current and is not blown at that time), and _probably_ less than the sum (coz their impedance is probably not exactly the right values to divide the current in the optimal way 1/3 - 2/3).

What is sure is that no-one knows when it will break, so DON'T do that.

Answer 4

Fuses in parallel summate if wired correctly. Therefore two 7.5 amp fuses would be the equivalent of a 15 amp fuse.

It is always preferable to have jut a single fuse though.

To answer your second question a 10 amp fuse and a 5 amp fuse would trip at 15 amps.


As an extra, the rules are different when working with electrical resistance as the SI unit. When linked in parallel the overall resistance is defined by the following rule:

1/Rtot = 1/Ra + 1/Rb + ... + 1/Rn

Answer 5

The division of current through each of two circuit breakers or fuses wired in parallel will depend on the internal impedance of each breaker. If both breakers have identical impedance, each breaker will pass half the total current. If one breaker has exactly twice the impedance of the other, that breaker will only pass one third of the total current while the other passes two-thirds.

Two identical breakers wired in parallel should carry twice as much current as each single breaker. Unfortunately, their combined current interrupting capacity would be the same as just a single breaker.

In general, you never want to wire two or more dissimilar protective devices in parallel. Two or more dissimilar breakers wired in parallel will trip somewhere between the trip point of the smallest breaker and the combined trip points of all breakers.

- old electric power engineer

Answer 6

your question is confusing...... when did circuit breakers started having the units amp?? i tot circuit breakers were resistors???

Answer 7

They would always trip at the lowest rating

Answer 8

it work the same in paralle...the reason they separate 7.5 each, because for safety when they trip one by one only...so you have spare light for people can see your car........better than your both light out at the same time..

Answer 9

Yes, if both had the same resistance.