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

[Esther & Fester Bestertes]

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

 

On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester
<anon@bogus.com> wrote:

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.

 

[Ignoramus26750]

On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester <anon@bogus.com> wrote:

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

--

 

[GregS]

In article <mVLcf.2961$Pk6.145@fe39.usenetserver.com>, Ignoramus26750 <ignoramus26750@NOSPAM.26750.invalid> wrote:

On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester
anon@bogus.com> wrote:
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.

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

(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

 

[Ignoramus26750]

On Thu, 10 Nov 2005 19:10:05 GMT, GregS <szekeres@pitt.edu> wrote:

In article <mVLcf.2961$Pk6.145@fe39.usenetserver.com>, Ignoramus26750 <ignoramus26750@NOSPAM.26750.invalid> wrote:
On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester
anon@bogus.com> wrote:
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.

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

 

[Ken Taylor]

"Ignoramus26750" <ignoramus26750@NOSPAM.26750.invalid> wrote in message
news:h8Ncf.11444$p56.2893@fe73.usenetserver.com...

On Thu, 10 Nov 2005 19:10:05 GMT, GregS <szekeres@pitt.edu> wrote:
In article <mVLcf.2961$Pk6.145@fe39.usenetserver.com>, Ignoramus26750
ignoramus26750@NOSPAM.26750.invalid> wrote:
On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester
anon@bogus.com> wrote:
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.

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

The breaker should be greater than the cable capacity so that the cable

doesn't burn out to protect the breaker.

Cheers.

Ken

 

[Ignoramus26750]

On Fri, 11 Nov 2005 08:44:44 +1300, Ken Taylor <ken123@xtra.co.nz> wrote:

"Ignoramus26750" <ignoramus26750@NOSPAM.26750.invalid> wrote in message
news:h8Ncf.11444$p56.2893@fe73.usenetserver.com...
On Thu, 10 Nov 2005 19:10:05 GMT, GregS <szekeres@pitt.edu> wrote:
In article <mVLcf.2961$Pk6.145@fe39.usenetserver.com>, Ignoramus26750
ignoramus26750@NOSPAM.26750.invalid> wrote:
On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester
anon@bogus.com> wrote:
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.

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

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

 

[Jim Thompson]

On Fri, 11 Nov 2005 08:44:44 +1300, "Ken Taylor" <ken123@xtra.co.nz>
wrote:

"Ignoramus26750" <ignoramus26750@NOSPAM.26750.invalid> wrote in message
news:h8Ncf.11444$p56.2893@fe73.usenetserver.com...
On Thu, 10 Nov 2005 19:10:05 GMT, GregS <szekeres@pitt.edu> wrote:
In article <mVLcf.2961$Pk6.145@fe39.usenetserver.com>, Ignoramus26750
ignoramus26750@NOSPAM.26750.invalid> wrote:
On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester
anon@bogus.com> wrote:
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.

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

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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[John Hudak]

Ignoramus26750 wrote:

On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester <anon@bogus.com> wrote:

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.

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

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

 

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.

 

[scada]

"John Hudak" <jhudak@sei.cmu.edu> wrote in message
news:4373A684.9050008@sei.cmu.edu...

Ignoramus26750 wrote:

On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester
anon@bogus.com> wrote:

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.

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...

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

 

[Fred Bloggs]

GregS wrote:

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

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

 

[CJT]

Fred Bloggs wrote:

GregS wrote:



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


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

Even without a load?

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.

 

[Ken Taylor]

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote in
message news:t097n15fbgf98vke2pirjcctee18irq773@4ax.com...

On Fri, 11 Nov 2005 08:44:44 +1300, "Ken Taylor" <ken123@xtra.co.nz
wrote:

"Ignoramus26750" <ignoramus26750@NOSPAM.26750.invalid> wrote in message
news:h8Ncf.11444$p56.2893@fe73.usenetserver.com...
On Thu, 10 Nov 2005 19:10:05 GMT, GregS <szekeres@pitt.edu> wrote:
In article <mVLcf.2961$Pk6.145@fe39.usenetserver.com>, Ignoramus26750
ignoramus26750@NOSPAM.26750.invalid> wrote:
On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester
anon@bogus.com> wrote:
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.

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

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

Ken

 

[Member, Takoma Park Volun]

scada wrote:

"John Hudak" <jhudak@sei.cmu.edu> wrote in message
news:4373A684.9050008@sei.cmu.edu...


Ignoramus26750 wrote:


On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester

anon@bogus.com> wrote:

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.


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...



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






scada wrote:
"John Hudak" <jhudak@sei.cmu.edu> wrote in message
news:4373A684.9050008@sei.cmu.edu...


Ignoramus26750 wrote:


On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester

anon@bogus.com> wrote:

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







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 gage.


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 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.
--
Tom Horne

 

[Asimov]

"Fred Bloggs" bravely wrote to "All" (11 Nov 05 00:24:42)
--- on the heady topic of "Re: Commercial mains wiring: 12ga wire for 30 amp load?"

FB> From: Fred Bloggs <nospam@nospam.com>
FB> Xref: core-easynews sci.electronics.design:522117
FB> sci.electronics.misc:132449 sci.electronics.repair:348125
FB> alt.engineering.electrical:142556

FB> GregS wrote:

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

FB> Nonsense- if you hook say 40A 6ga to a 20A breaker, that 40A will trip
FB> the breaker almost immediately...


And perhaps permanently...

A*s*i*m*o*v

.... A fail-safe circuit will destroy others.

 

[James Sweet]

Esther & Fester Bestertester wrote:

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.

 

[Don Bowey]

On 11/10/05 4:24 PM, in article 4373E4C9.7070006@nospam.com, "Fred Bloggs"
<nospam@nospam.com> wrote:

GregS wrote:



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

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.

 

[Esther & Fester Bestertes]

Thus spake gfretwell@aol.com:

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

 

[GregS]

In article <MSGID_1=3a167=2f133.0_43741186@fidonet.org>, "Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> wrote:

"Fred Bloggs" bravely wrote to "All" (11 Nov 05 00:24:42)
--- on the heady topic of "Re: Commercial mains wiring: 12ga wire for 30 amp =
load?"

FB> From: Fred Bloggs <nospam@nospam.com
FB> Xref: core-easynews sci.electronics.design:522117
FB> sci.electronics.misc:132449 sci.electronics.repair:348125
FB> alt.engineering.electrical:142556

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

FB> Nonsense- if you hook say 40A 6ga to a 20A breaker, that 40A will trip
FB> the breaker almost immediately...


And perhaps permanently...

A*s*i*m*o*v

.... A fail-safe circuit will destroy others.

Let me try another approach. If you attach a 10 guage wire to a 20 amp breaker, a 10 amp load will
opperate more effeciently and have more voltage, and the wire will run cooler. The ground will also
have a better connection to the main box.

greg