what is available fault current
all right so we hear this term available fault current uh
constantly if you're on facebook forums or you know just talking to people in general maybe
they had a journeyman's exam or master exam or something then they were like this available
fault current thing is something i keep coming across or maybe you're just out in the field and
your boss gives you this placard and he's like hey you're at a commercial building we need to
put the the afc placard on to a service in commercial and other than dwelling services
we have to actually mark what the available fault current for that service is so what does
that even mean all right first let's break out the handy dandy 2020 national electrical code
go into the definitions and uh we have well we have fault current and fault
uh ground fault so what are all these things mean first of all let's just talk about what a fault is
fault is basically an accidental occurrence like uh you know in a tennis match or so i don't know
i don't watch tennis but i feel like i've heard the term fault before uh but fault is just like
oops that's not supposed to happen um so code defines a fault current as the current delivered
at a point on the system during a short circuit condition so not a ground fault not where a hot's
touching a ground specifically a short circuit but we can have a ground fault it's still an
oops or an accidental it's just a ground fault is what we call when a hot touches ground or when it
grounds out we don't call that a short circuit a short circuit is talking about the actual circuit
where there's current flowing to a load so a short circuit happens between hot conductors you know
ungrounded conductors or hot and neutral right so like uh the the un grounded and the grounded
conductor so when we're looking at short circuit current or a short circuit is a type of fault it's
just not a ground fault there's also arc faults so arcing can happen between two conductors we
have series arcs where we have like a break in a wire or at like a terminal where we've
landed something on a terminal in series it can arc so that's a type of fault that causes
a breaker to trip we could have a parallel arc which is between two actual ungrounded conductors
or between two uh like an ungrounded grounded so between two hots or between hot neutral
it is taking a short cut it's basically the same thing as a short circuit it's just an arc that's
happening between two conductors rather than a direct short where they're touching each other
so those are the kinds of faults that you might come across and what they actually mean so what
does code say about available fault current so available fault current or afc is the largest
amount of current capable of being delivered at a point on the system during a short circuit
condition so we're talking about a short circuit condition what is the largest amount of current
that can flow during a short circuit at an electrical service the reason we put that is
that is the closest point to the transformer so from the transformer we have our secondary
conductors that come in they attach to our service entrance conductors come into the service and
boom they land on our breaker everything from the breaker out can still have short circuits
and ground faults and everything like that but the amount of available fault current is going
to be based off of a couple factors and one of them is the conductor size how much current can
travel through a tiny conductor versus a large one a whole lot more current can go through a
large conductor so the availability for current to travel the amount of current that can travel
in on a fault condition in a small conductor that's way out in the field hooked up to like
a toaster or something like that there's not much available fault current that can happen
compared to the amount of current that can float in these service entrance conductors
imagine in a service coming off a transformer we're hooking up like
parallel 500s i mean we're talking like multiple massive conductors on per phase and then phase to
neutral so when we figure out our available fault current we're basically saying at this service if
these two incoming hot conductors line to line not necessarily line to neutral line the line the most
amount of current would happen between two hots not a hot and a neutral because we have double the
voltage between two hots where we only have the single amount of voltage between hot and neutral
so the available fault current is always a line to line measure of if these two hots touch here
what is that current how much current is going to be allowed to flow in this bit of wire from the
breaker or wherever you know essentially from the breaker if they were to short out at the terminals
back to the transformer so much current is going to flow so much more than any other point on
the system because the conductors are the largest you probably have a certain amount of length from
the service so that's another thing to figure out is how long are the conductors because the
the impedance or the opposition to current flow in conductors the longer the current that conductor
is the more material there is for opposition to happen so naturally you have resistance
or impedance i'm just going to call it resistance for ease of understanding but the the amount of
resistance or impedance that you can have in a conductor is going to increase with
the length of that conductor so the how close you are to a transformer what the let-through
current of that transformer is what the voltage is whether or not it's single phase or three-phase
um and the size of those conductors are all going to determine how much current can flow if these
two huge conductors just touch each other that's what available fault current is and we have to
figure that out for every service that's not a residential service so typically three-phase
environments maybe not three-phase but just commercial environments industrial stuff like that
so then if we look into uh 110.24 110.24 talks about available fault current and field marking
of these panels so what it says 110.24a says field marking service equipment at other than dwellings
so not houses shall be legibly marked in the field with the available fault current the field marking
or markings shall include the date the fault current calculation was performed and be of
sufficient durability to withstand the environment involved so like a placard or something that's not
going to like rub off the calculation shall be documented and made available to those authorized
to design install inspect maintain and operate the system and then b goes into modifications so
even if you modify a service you still have to update the available fault current calculation
and put the date and all of that stuff on it so that is available fault current hope that
answered some of your questions please leave comments below any other questions things that
you want me to do videos on i am spending all of this year 2022 going through the comments and just
answering your guys's stuff because holy hell over the five or six years that i've been doing
this now there's been so many suggestions that i can't keep up um so i've been loving doing
this but please ask any of your questions below love you crazies and i'll see in the next one
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