Circuit breaker numbers

E_Dawg

Moderator
Ya know those numbers on the breakers? Is that the normal amp draw of the circuts they are supposed to protect, or is the the maximum apmerees it will accept before popping?
 

Center_Mid

Well-Known Member
I thought it was the normal amp load, so that if you're running on a battery you can figure out the load and do a rough amp-hour computation.

I don't know for sure - lookin' it up when I get home!
 

EatSleepFly

Well-Known Member
They are the same as fuses. The number you see on them is the maximum number of amps it can take. Any more load than that, and out it comes. (Or, in the case of a fuse...it blows).
 

bluelake

Well-Known Member
yes, if you lose your alternator... then adding up all of the circuits that are "on" will give you the amp-hours you are using.

Unfortunately, in a LOT of older airplanes i fly in the numbers are worn off. I suspect that this is because students (and I was one of them) read the checklist "circuit breakers in" and run their fingers across it. So, its good to use the POH while sitting in your armchair to review some of the major load items... that way in flight if you lose your alternator, you'll know which ones to shed even if the numbers are rubbed off.
 

E_Dawg

Moderator
It either shows the max permissible load or the normal amp load. If it's the latter you can add it up, if it's the former you can't. Anyone know for sure?
 

n2o2diver

New Member
It is the MAX AMPS before that breaker will POP, has nothing to do with how much the circuit is drawing, but you can be assured it is less than the number on the breaker.
 

MikeD

Administrator
Staff member
[ QUOTE ]
It is the MAX AMPS before that breaker will POP, has nothing to do with how much the circuit is drawing, but you can be assured it is less than the number on the breaker.

[/ QUOTE ]

Not necessarily. Most CBs are designed to pop at a load rating of nominal plus about 10% in order to compensate for the higher amperes on the initial current draw; this value varying depending on the particular circuit being protected. Breakers that regularly get pulled and reset, such as overnight shutdown, fueling, etc could actually "weaken" and pop at lower amps than advertised due to the simple fact of the breaker spring weakening from constant use. Can be a pain if you have a highly used breaker (regularly pulled) that seemingly pops for no reason, causing you to troubleshoot the associated system when the breaker itself is the real problem.
 

C650CPT

Well-Known Member
What it really is = It represents the number of times that you can reset that particular breaker.



No I'm just kidding you, it is as stated the level of protection provided to that circuit. As MikeD said the engineers figure in some additional protection for initial draws, but bigger items that have considerable draw are protected by current limiters, which are "slow blow" fuses. The current limiter protects the airplane's electrical system from shorts, once a CL is blown it cannot be reset, it must be replaced by an A&P mechanic.
 

sixpack

New Member
One unfortunate thing about threads, is that it's hard to undo incorrect information. (including mine, if I screw up this summary). I suspect SkyGuyEd didn't get a solid answer. Here's my summary...

[ QUOTE ]
CORRECT:
They are the same as fuses. The number you see on them is the maximum number of amps it can take. Any more load than that, and out it comes. (Or, in the case of a fuse...it blows).

INCORRECT:
yes, if you lose your alternator... then adding up all of the circuits that are "on" will give you the amp-hours you are using.

PAINFULLY CORRECT, BUT YOU CAN IGNORE THIS DETAIL:
Not necessarily. Most CBs are designed to pop at a load rating of nominal plus about 10% in order to compensate for the higher amperes on the initial current draw.

HUMOROUS:
What it really is = It represents the number of times that you can reset that particular breaker... ...No I'm just kidding you

[/ QUOTE ]
 

MikeD

Administrator
Staff member
[ QUOTE ]
One unfortunate thing about threads, is that it's hard to undo incorrect information. (including mine, if I screw up this summary). I suspect SkyGuyEd didn't get a solid answer. Here's my summary...

PAINFULLY CORRECT, BUT YOU CAN IGNORE THIS DETAIL:
Not necessarily. Most CBs are designed to pop at a load rating of nominal plus about 10% in order to compensate for the higher amperes on the initial current draw.




[/ QUOTE ]

But I like pain!
 

bluelake

Well-Known Member
Well now,

I dont mind at all being wrong and I also am a fan of pain, but what would be even more useful than Sixpack's play by play-slash-editorial on the accuracy of this thread's responses would be his OWN offering on what the numbers on the CB's mean
:)

I officially change my response to this: when the alternator fails... the bigger the number.. the more soul searching ya'ought to do decide if ya' REALLY need whatever is on that circuit.

There.. hows that
:)
 
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