It refers to a "standard" phase shift for star delta transformers. As I
recall, the standard is that primary A phase leads secondary A phase by 30
degrees. The way that the A phase as indicated on a drawing is not really
that important- as long as it is marked -but there is likely a standard.
It is related to field wiring applications. If everyone does it this way,
there is less likelihood of misconnection, particularly for some protective
Sure, one can connect in such a way that the secondary a,b,c phase voltages
lag or lead primary A,B,C by a number of different angles. However, life is
a lot easier if a single standard is used, within a utility and between
utilities and manufacturers. It can eliminate a lot of expensive mistakes.
It is not a lack of knowledge of what is going on but rather a knowledge of
what possible phasing problems (in differential protection for example or
even fault analysis) can occur that leads to setting a standard. --
Don Kelly email@example.com
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Quite right. Things like differential relay protection across transformer
bank must have both sides connected for the same phase-shift. Gets really
tricky when the power transformer is delta-wye and has a phase shift of its
own :-). Mis-connections don't always show up until you start to
significantly load the transformer, then you get protective trip and spend
some downtime tracing it all out (btdt).
I am in a situation where (aside from being retired) I have little use
for my PE license. Most of my need for a license has been to use the
title of "Engineer." Rather than learning standards that I am never
going to use, I would rather do the extra work required by starting from
I have a situation now, for example, where I am checking out someone's
fault current calculations as a favor without charging. I do not have
much experience with residential wiring, but I am willing to attest to
accuracy of the calculations if I am provided with a schematic
specifying all the conductors and their connections to equipment and
I would rather do the same but effectively the manufacturers have come up
with a standard so that you know what you are getting before you get it and
can plan accordingly. In an academic situation- not of importance. I can
calculate fault currents if I have information-. certainly knowing that the
wiring is #12AWG Copper does allow use of standard information such as
resistance per foot, which is needed for your household fault analysis- you
don't go out and measure the resistance of the conductor but you do look up
this information in a table. This transformer information is the same sort
of thing-of importance to differential protection and in application of
symmetrical components in fault analysis (unbalanced fault on the secondary
of a 3 phase transformer- what is the primary current in each leg?).
Don Kelly firstname.lastname@example.org
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I thought that residential fault analysis would assume zero ohms in the
branch wiring. You can't really predict where the fault will occur so you
assume it's immediately downstream of the circuit breaker. Only the supply
transformer impedance and the service drop wiring go into the fault current.
That's one reason why the utility doesn't put in really large transformers
in developments. Too large a transformer and the available fault current is
higher than 10kA and typical service panel equipment needs to be upgraded.
At least, that was my understanding....
By specification, I meant lengths and size of the conductors. The
jurisdiction involved specifies the impedance of the wires (for a given
size) including the effects of magnetic conduit. That makes my job one
of checking multiplication and the like. In this case, they do not
distinguish between resistive and inductive contribution to the
impedance. The big problem is for the client to find out what
transformer impedance is from the nameplate or the power company. The
only real thing I have to do is watch out for the gotchas. For example,
the client indicated #2 wire when he meant #2/0. Nevertheless, I would
my comfort limit for anything much more complicated.
I suspect this would mean 'at the load terminals of the overcurrent
protective device' (as in load side terminals and line side
terminals), not 'at the terminals of the load'. I encourage you to
use that interpretation if possible.
It would not make sense to include the branch wiring. This would
quite simply be an incorrect way to carry out fault current
calculations, in my opinion. It would be (theoretically) unsafe
because it calculates fault currents lower than what the OCPD may be
called on to interrupt. This is a life safety as well as a property
damage issue. I suggest that it would in general be inappropriate for
a PE to carry out a fault current calculation this way. One might get
away with it because a fault that causes a catastrophic failure of an
OCPD and thereby some collateral damage is probably not common in a
typical residential application.
In your other post you mention watching out for the gotchas. Any
running motors in a premises will contribute to fault current when a
fault occurs. This contribution is often added to the utility
contribution at the main service for simplicity.
I am a little surprised that a person would use the nameplate
impedance of the utility transformer. What if the utility replaces it
in the future with a lower impedance unit? Here we are required to
assume certain lowish impedances for utility transformers. Plus
infinite primary of course.
Ignoring phase angles of impedances is also not the greatest idea. It
will tend to underestimate required interrupt ratings. Hopefully not
by much in your application. It may end up being compensated for by
the infinite primary assumption.
Then it is safe to use to treat the "load terminals" as the house service
panel, rather than at various points inside the house. With typical
distribution transformers having about 5% impedance, the maximum fault
current will be 20 times transformer KVA rating divided by rated voltage
and this would ber good enough in practice. Since a fault on any circuit
can occur just past the individual circuit breaker, this will be a good
indication of worst case conditions.
Sorry- it looks like you are into something more than residential wiring
when you meention 2/0. Just what are you looking at that requires this for
internal wiring (and in raceways)? You may be getting into code /cookbook
issues. I agree that the transformer impedance is the major factor and
uncertainties in this may be greater than uncertainties in the conductor
Because of possible misinterpretation of mere words,I am asking for a
schematic diagram of all the conductors and circuit breakers among other
things. I would want to have branch breakers trip without tripping main
The jurisdiction wants to know what the transformer impedance is.
Getting from nameplate information seems good enough for them. Who am I
to argue. I expect that the conductor impedance will be high enough to
limit current flow to a value breakers can handle. Transformers have
Typically 5% (0.05 per unit)for distribution transformers under
150-200KVA -basically leakage reactance as you say. Max fault KVA for
bolted fault will be its KVA rating divided by the per unit rating so that
if the transformer is 100KVA the expected maximum bolted fault KVA will be
something like 2000KVA for single phase. -
That all seems reasonable enough, but you never know. I look upon this
as n information gathering job to be performed by someone else. As a PE,
all I have to do is take that information and certify that calculations