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I'm a little unfamiliar with circuit breaker specs and confused by this circuit breaker datasheet:

https://www.waytekwire.com/datasheet/46974.pdf

On the second page, it says that it takes more than 1000 seconds for the circuit breaker to trip at the rated current (100%).

So does that mean that a 30A fuse breaker will trip more than 16 min after a 30A current runs through it?

I understand that current spikes are usually pretty high over a short period of time, but my application is a solar panel that might accidentally produce too much current gradually (like the clouds slowly clearing) for my MPPT charge controller to handle.

Is there a better option I could use?

Edit: For those confused about why I would need a fuse on my MPPT, this is the source I used to understand the problem (but with fuses instead of circuit breakers): https://www.windynation.com/jzv/inf/how-properly-fuse-solar-pv-system

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    \$\begingroup\$ what makes you think MPPT charge controller can't handle it? THat's its whole purpose to is regulate input to Vmp and output I charge control, to satisfy both needs at all times. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 30 '17 at 0:51
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This is most likely a thermal circuit breaker. In order to trip, the sensing element has to heat to a certain temperature. This is slow, and not very accurate. Also it is much too slow to protect aganinst shorts.

In electrical installations, normally thermal magnetic breakers are used. These use a thermal sensing element to detect low values of overcurrent (trips in 1 second for 10x overload) backed up by a magnetic actuator (a coil) which ensures the huge currents caused by a short circuit are interrupted as fast as possible (less than 20ms for 100x overload).

Your solar panels don't have the same enormous current capability than the power utility, so let's forget about that for a moment.

my application is a solar panel that might accidentally produce too much current gradually (like the clouds slowly clearing) for my MPPT charge controller to handle.

This makes absolutely no sense. Your MPPT controller should only draw the current it can handle from the panel. Unless you exceed its input voltage rating by putting too many panels in series, but you probably checked for that. Breakers or fuses' job is to protect connections and cables against fire. Most likely you're trying to solve a non-existent problem.

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Ideally, a circuit breaker should never trip while carrying its rated current.

Looking further down the graph to where it should trip, we find that a 30 Amp breaker will trip in about 10 seconds if carrying 60 Amps (200% of rating). A short pulse of 200% overload (say, 5 seconds) will not cause a trip, but a continuous 200% overload will, after about 10 seconds.

The spec sheet states that that is a thermal breaker - something inside has to heat up before it will trip. There are also magnetic breakers which will operate much faster than the thermal types.

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  • \$\begingroup\$ >"Ideally, a circuit breaker should never trip while carrying its rated current." But they do. It is common for a breaker to trip at it rated current after a period of time, as stated in the question. \$\endgroup\$ – Scott Geier Aug 1 '17 at 14:11
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So does that mean that a 30A fuse breaker will trip more than 16 min after a 30A current runs through it?

Yes. The breaker will trip after 16 minutes with 30A through it. It will even take less time if the breaker is installed in a location with an ambient temperature above 50 degrees C.

  1. Find the power rating of your solar panels circuit. (Panel rating) * (Industry Standard Max Multiplier 1.25) * (NEC requirement for continuous load 1.25) * (Number of panels).
  2. Size your wire using the power rating: enter image description here NOTE: The wire ampacities in the table above must be derated by the ambient temperature and the number of current carrying conductors in a conduit.

  3. Size your breaker such that the breaker rating is less than or equal to the wire rating.

i.e. If you have figured panel circuit current to just under 30A, and thus selected 10 AWG wire (see the *note at the bottom of the table.) The put a 30A breaker on the circuit to protect it.

The point of all this:

The breaker is installed to protect the wire from getting too hot which causes damage to the insulation. Breakers current ratings are specified at a current level which will trip after a specified time because the insulation can tolerate the current for a period of time without degradation.

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