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

[Glen Walpert]

On Thu, 10 Nov 2005 22:40:32 -0800, Esther & Fester Bestertester
<anon@bogus.com> wrote:

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

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.

 

[Glen Walpert]

On Fri, 11 Nov 2005 04:28:09 GMT, "Member, Takoma Park Volunteer Fire
Department" <postmaster@takomaparkfire.org> wrote:

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!

 

[Fred Bloggs]

Glen Walpert wrote:

On Thu, 10 Nov 2005 22:40:32 -0800, Esther & Fester Bestertester
anon@bogus.com> wrote:


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


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.

Since he went with an autotransformer, he should derate the wiring for
NEC "general use" and that puts 12ga THHN at 20A. He needs to reinstall
at 10ga- maybe- depending on circuit run length. 10ga would be good for
up to 100', 8ga would be recommended for over 150', 6ga for up to 300'
etc at 2.5% voltage drop..This may explain why he went with an
autotransformer- burned the old motor out with overload at low voltage
and upped the breaker from 20A motor time delay to 30A to eliminate LRA
trips. He should therefore reinstall the 20A breaker with the 10ga or
larger and get rid of the autotransformer unless the motor specifically
calls out 230V which is rare these days.

 

On Thu, 10 Nov 2005 22:40:32 -0800, Esther & Fester Bestertester
<anon@bogus.com> wrote:

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.

 

[Rich, Under the Affluence]

On Fri, 11 Nov 2005 15:44:53 +1300, Ken Taylor wrote:

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote in
On Fri, 11 Nov 2005 08:44:44 +1300, "Ken Taylor" <ken123@xtra.co.nz
....
The breaker should be greater than the cable capacity so that the cable
doesn't burn out to protect the breaker.

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

Damn! That was what I meant! Thanks Jim.

What does Homer Simpson use to bake bread?

v



v



v



v



v




v




v



v



Doh!

;-)
Rich

 

[keith]

On Sat, 12 Nov 2005 22:06:47 +0000, Rich, Under the Affluence wrote:

On Fri, 11 Nov 2005 15:44:53 +1300, Ken Taylor wrote:
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@My-Web-Site.com> wrote in
On Fri, 11 Nov 2005 08:44:44 +1300, "Ken Taylor" <ken123@xtra.co.nz
...
The breaker should be greater than the cable capacity so that the cable
doesn't burn out to protect the breaker.

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

Damn! That was what I meant! Thanks Jim.


What does Homer Simpson use to bake bread?

Marge.

--
Keith

v



v



v



v



v




v




v



v



Doh!

;-)
Rich

 

[Jasen Betts]

["Followup-To:" header set to sci.electronics.design.]
On 2005-11-11, Esther & Fester Bestertester <anon@bogus.com> wrote:

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.

frequency? what mechanical load is it driving?

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

yeah 10% more voltage out than in means 10% more current in than
out (+ losses).

--

Bye.
Jasen

 

[Esther & Fester Bestertes]

Thus spake Jasen Betts:

Nameplate: 230V, 16.5A.

frequency? what mechanical load is it driving?

60hz. 4hp cap. start motor driving 2-stage compressor,
light duty cycle (20-25%).

FBt

 

[Long Ranger]

Would the autotransformer account for the increased current
draw?

A measurement between the load and transformer should tell you where the
extra current is going.

 

[Jasen Betts]

["Followup-To:" header set to sci.electronics.design.]
On 2005-11-13, Esther & Fester Bestertester <anon@bogus.com> wrote:

Thus spake Jasen Betts:

Nameplate: 230V, 16.5A.

frequency? what mechanical load is it driving?

60hz. 4hp cap. start motor driving 2-stage compressor,
light duty cycle (20-25%).

Try a smaller pulley on the motor (or larger on the compressor)

That'll increase the duty cycle but reduce the load on the motor
(and thefore the current it draws)

--

Bye.
Jasen