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Commercial mains wiring: 12ga wire for 30 amp load?

  • Thread starter Esther & Fester Bestertester
  • Start date
E

Esther & Fester Bestertester

Jan 1, 1970
0
NEC reference says that 12ga THHN wire can carry 30A max
current. Motor load is currently drawing constant 20A (230v
1ph circuit). Breaker is 30A.

(A bit of history: Previous motor (running from 208v) was
drawing less (15A?) current until machine broke. New
machine's motor (running from 230v via boost
autotransformer) is drawing 20A (measured at breaker).
Motor spec plate says 16A.)

Is 12ga wire sufficient, or should I bump it up to 10ga?

Would the autotransformer account for the increased current
draw?

FBt
 
NEC reference says that 12ga THHN wire can carry 30A max
current. Motor load is currently drawing constant 20A (230v
1ph circuit). Breaker is 30A.

(A bit of history: Previous motor (running from 208v) was
drawing less (15A?) current until machine broke. New
machine's motor (running from 230v via boost
autotransformer) is drawing 20A (measured at breaker).
Motor spec plate says 16A.)

Is 12ga wire sufficient, or should I bump it up to 10ga?

Would the autotransformer account for the increased current
draw?

FBt


You have 2 issues. Foiust, is the motor supposed to be running on 230
or 208? That couild be the extra current.
If the motor is really exceeding the nameplate amps I would
investigate futher.

The second question is ampacity of the conductor 12ga THHN is only
good for 30a if you have all 90c terminations and that is just not
going to true so you have to use 25a. That is still 125% of the 20a
you measured so that is OK.

For you "20a voters", article 430 allows you to use the 310.16
ampacity on a dedicated motor circuit, not 240.4(D) which says 20.
It also allows a breaker sized to 175% of FLA so the 30a breaker is
OK.
 
I

Ignoramus26750

Jan 1, 1970
0
NEC reference says that 12ga THHN wire can carry 30A max
current. Motor load is currently drawing constant 20A (230v
1ph circuit). Breaker is 30A.

It is my understanding that wire must be matched to the breaker. So,
if the breaker is 30A, the wire should be rated for 30A as well. That
means 10 gauge.

i
(A bit of history: Previous motor (running from 208v) was
drawing less (15A?) current until machine broke. New
machine's motor (running from 230v via boost
autotransformer) is drawing 20A (measured at breaker).
Motor spec plate says 16A.)

Is 12ga wire sufficient, or should I bump it up to 10ga?

Would the autotransformer account for the increased current
draw?

FBt


--
 
G

GregS

Jan 1, 1970
0
It is my understanding that wire must be matched to the breaker. So,
if the breaker is 30A, the wire should be rated for 30A as well. That
means 10 gauge.

It does not have to be matched. Only on how big the breaker can be.
The wire can be bigger than the breakers rating.

greg
 
I

Ignoramus26750

Jan 1, 1970
0
It does not have to be matched. Only on how big the breaker can be.
The wire can be bigger than the breakers rating.

I agree.

i
 
K

Ken Taylor

Jan 1, 1970
0
Ignoramus26750 said:
I agree.

i
The breaker should be greater than the cable capacity so that the cable
doesn't burn out to protect the breaker.

Cheers.

Ken
 
I

Ignoramus26750

Jan 1, 1970
0
The breaker should be greater than the cable capacity so that the cable
doesn't burn out to protect the breaker.

That was funny!

i
 
J

Jim Thompson

Jan 1, 1970
0
The breaker should be greater than the cable capacity so that the cable
doesn't burn out to protect the breaker.

Cheers.

Ken

Eh? The breaker trip rating should be _lower_ than the current
capacity rating of the wire.

...Jim Thompson
 
J

John Hudak

Jan 1, 1970
0
Ignoramus26750 said:
It is my understanding that wire must be matched to the breaker. So,
if the breaker is 30A, the wire should be rated for 30A as well. That
means 10 gauge.
NO, not according to NEC and wiring protection practices. You size the
breaker to be 80% of the rated FLA of the conductor. You are protecting
the wire with the breaker, not the motor...If the wiring overheats you
got a fire.
John
 
Exactly!

Typically you protect a #12 wire with a 20 amp or less breaker, and a
#10 wire with a 30 amp or less breaker. For #14 wire a 15 amp breaker
is the maximum, but a 10 amp breaker provides an added margin of
safety.

The terminal design on many breakers intentionally make it difficult
(although not impossible) to attach a wire that is undersized for their
rating.

Harry C.
 
C

CJT

Jan 1, 1970
0
Fred said:
Nonsense- if you hook say 40A 6ga to a 20A breaker, that 40A will trip
the breaker almost immediately...
Even without a load?

:)
 
K

Ken Taylor

Jan 1, 1970
0
Jim Thompson said:
Eh? The breaker trip rating should be _lower_ than the current
capacity rating of the wire.

...Jim Thompson
--
Damn! That was what I meant! Thanks Jim.

Ken
 
M

Member, Takoma Park Volunteer Fire Department

Jan 1, 1970
0
scada said:
Ignoramus26750 wrote:

True, but why would anyone undersize the wire? If it's a 30A breaker - you
best be using at least 10AWG wire! Even larger if youre 50' or more from the
source. Why would you Jeopardize burning out the motor from a large voltage
drop in the wire? Stall current (starting current to break inertia) is
typically 20* running I...
scada said:
Ignoramus26750 wrote:

True, but why would anyone under size the wire? If it's a 30A breaker - you
best be using at least 10AWG wire! Even larger if your 50' or more from the
source. Why would you Jeopardize burning out the motor from a large voltage
drop in the wire? Stall current (starting current to break inertia) is
typically 20* running I...

Motor Circuits are unique animals. In general the Over Current
Protective Device for a motor branch circuit is sized to permit the
motor to start without nuisance tripping of the breaker or fuse in
question. The OCPD serves only to provide fault protection to the
circuit. Overload protection is provided by the Motor Overload
Protective Device. This may be a thermal protector on the motor or it
may be heaters sized to the motor running current and installed in the
motor starter. The circuit conductors are sized at 1.25 times the full
load current of the motor it serves based on the ampacity in the NEC
table. The note to the table reads "except as otherwise permitted by
this code" and it is that phrase that allows for full current ampacity
of conductors to be used in sizing motor branch circuits and feeders.

In other words the requirement to protect number twelve copper
conductors with an OCPD not greater than twenty amperes does not apply
to motor circuits.
 
J

James Sweet

Jan 1, 1970
0
Esther said:
NEC reference says that 12ga THHN wire can carry 30A max
current. Motor load is currently drawing constant 20A (230v
1ph circuit). Breaker is 30A.

(A bit of history: Previous motor (running from 208v) was
drawing less (15A?) current until machine broke. New
machine's motor (running from 230v via boost
autotransformer) is drawing 20A (measured at breaker).
Motor spec plate says 16A.)

Is 12ga wire sufficient, or should I bump it up to 10ga?

Would the autotransformer account for the increased current
draw?

FBt


I always use #10 for 30A circuits, particularly if you're running a
motor load which will pull a large inrush current. On a long run the
motor can stall before it gets up to speed if the wiring is too small.
 
D

Don Bowey

Jan 1, 1970
0
Nonsense- if you hook say 40A 6ga to a 20A breaker, that 40A will trip
the breaker almost immediately...

Where did the poster to whom you are commenting, say he would put a 40A load
on a 20A breaker?

He said, "The wire can be bigger than the breakers rating.
 
E

Esther & Fester Bestertester

Jan 1, 1970
0
Thus spake [email protected]:
You have 2 issues. Foiust, is the motor supposed to be running on 230
or 208? That couild be the extra current.
If the motor is really exceeding the nameplate amps I would
investigate futher.

Nameplate: 230V, 16.5A.

I presume that if an autotransformer boosts the voltage
10%, it must draw 10%(?) more current in order to do so. Is
this correct? (Nothing is free.)
FBt
 
G

Glen Walpert

Jan 1, 1970
0
Thus spake [email protected]:


Nameplate: 230V, 16.5A.

I presume that if an autotransformer boosts the voltage
10%, it must draw 10%(?) more current in order to do so. Is
this correct? (Nothing is free.)
FBt

Correct, except that there will also be an additional bit of current
draw due to losses in the autotransformer, which will probably be over
95% efficient:

For 208V in, 230V out at 16.5A out to motor and 95% autotransformer
efficiency:

Iin = (16.5*(230/208))/.95 = 19.2A

Which is slightly under your measured 20A, which indicates that you
are running slightly over 100% load on your motor if my voltage
guesses are right. Hopefully the nameplate service factor is at least
1.15 continuous (if the motor runs continuously).

BTW congratulations on responding to one of only 2 posters who
understand your situation (the other being Tom Horne of the Tacoma
Park VFD). But consider that anyone responding to your question here
cannot be aware of all pertinent circumstances such as possible
derating requirements for high ambient temperature or more than 3
current carrying conductors in a conduit, which a competent local
electrical designer would take into consideration, as well as
considering the length of the wiring run and resulting voltage drop.

Since you did not seem to understand the very basic fact that the 90 C
wire ampacity rating is provided only to allow for various derating
requirements, and the usable ampacity cannot exceed the rating for the
conductor at the max temp ratings of the connections at either end
(probably 75 C max for your circuit breaker, which will be labeled
with temp rating), I suggest you find a competent local electrical
designer or inspector to examine your installation. The $100 or so
could save a lot more in the long run.
 
G

Glen Walpert

Jan 1, 1970
0
Motor Circuits are unique animals. In general the Over Current
Protective Device for a motor branch circuit is sized to permit the
motor to start without nuisance tripping of the breaker or fuse in
question. The OCPD serves only to provide fault protection to the
circuit. Overload protection is provided by the Motor Overload
Protective Device. This may be a thermal protector on the motor or it
may be heaters sized to the motor running current and installed in the
motor starter. The circuit conductors are sized at 1.25 times the full
load current of the motor it serves based on the ampacity in the NEC
table. The note to the table reads "except as otherwise permitted by
this code" and it is that phrase that allows for full current ampacity
of conductors to be used in sizing motor branch circuits and feeders.

In other words the requirement to protect number twelve copper
conductors with an OCPD not greater than twenty amperes does not apply
to motor circuits.

There are also other exceptions where the characteristics of an
overload protected hard wired load are allowed to protect the
conductors instead of the circuit breaker, such as for welding
machines with limited duty cycle. And then there are loads which do
not require overload protection at all, such as certain emergency fire
pumps, where the wiring burning up due to overload is deemed less
hazardous than the pump shutting down early in a fire emergency. The
NEC is a complex beast, not easily understood in its entirety!
 
Nameplate: 230V, 16.5A.

That is what the NEC says to use for the circuit to the motor. They
probably said 16 ".5" so you wouldn't use 14 ga wire and a 40a brealer
<legal>
That is usually a question on the inspector test.
FLA=16 with internal O/L protection, what is the min wire size & max
breaker. Answer #14cu, 40a.
Now days breakers are usually HACR so you don't need one that big to
handle the locked rotor on start up but the code hasn't changed.

The 14ga = 15a, 12ga = 20a 10ga = 30a we all know is really 240.4(D)
and is aimed at the circuits that are likely to have receptacles,
where the installer has no control over what gets plugged in so the
80% safety factor is built into the breaker size limit. Folks will
keep plugging things in till the breaker trips, then unplug the clock
if that will let it hold.
 
S

scada

Jan 1, 1970
0
John Hudak said:
NO, not according to NEC and wiring protection practices. You size the
breaker to be 80% of the rated FLA of the conductor. You are protecting
the wire with the breaker, not the motor...If the wiring overheats you
got a fire.
John

True, but why would anyone undersize the wire? If it's a 30A breaker - you
best be using at least 10AWG wire! Even larger if youre 50' or more from the
source. Why would you Jeopardize burning out the motor from a large voltage
drop in the wire? Stall current (starting current to break inertia) is
typically 20* running I...
 
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