Oversizing a breaker

Merry Christmas
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I'm having a disagreement about oversizing a breaker formotors. The guy is telling me that you oversize the breaker for startupamperage but I told him that's not right and that you size it according to thenameplate. Could someone please tell me the code article for this, I would lookit up myself but I don't have access to a code book right now, plus I wouldlike other people?s opinion.
 

david luchini

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Staff member
Location
Connecticut
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Engineer
Table 430.52 allows an inverse time c/b to be sized at 250% (or the next size up) for the full load current of the motor as listed in the Art. 430 tables. The motor nameplate amps would not normally be used for OCPD sizing.
 

kwired

Electron manager
Location
NE Nebraska
You are both right. It is sized larger than usual for other types of loads because of starting current. Nameplate is still used to determine what that current may be. The starting current will be higher than the full load current listed on the nameplate though.

As mentioned 250% of nameplate current is NEC acceptable protection for inverse time breakers, but there can be exceptions if this level does not hold during starting.
 

david luchini

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Location
Connecticut
Occupation
Engineer
You are both right. It is sized larger than usual for other types of loads because of starting current. Nameplate is still used to determine what that current may be. The starting current will be higher than the full load current listed on the nameplate though.

Doesn't 430.6(A)(1) tell you to use the Table value rather than the Nameplate current? Or has this changed in 2011? (We're still on 2005 NEC in CT.)
 

kwired

Electron manager
Location
NE Nebraska
Doesn't 430.6(A)(1) tell you to use the Table value rather than the Nameplate current? Or has this changed in 2011? (We're still on 2005 NEC in CT.)

You are probably correct, unless a special motor not listed in the tables.

I generally use slide calculator provided by Square D, especially when sizing a Square D made breaker - their results are usually less than the allowed 250% and I can't remember a motor tripping a breaker unless it had a something wrong with it, driven load was plugged, etc.
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
No Changes:

(A) General Motor Applications. For general motor applications,
current ratings shall be determined based on
(A)(1) and (A)(2).
(1) Table Values. Other than for motors built for low
speeds (less than 1200 RPM) or high torques, and for multispeed
motors, the values given in Table 430.247, Table
430.248, Table 430.249, and Table 430.250 shall be used to
determine the ampacity of conductors or ampere ratings of
switches, branch-circuit short-circuit and ground-fault protection,
instead of the actual current rating marked on the
motor nameplate. Where a motor is marked in amperes, but
not horsepower, the horsepower rating shall be assumed to
be that corresponding to the value given in Table 430.247,
Table 430.248, Table 430.249, and Table 430.250, interpolated
if necessary. Motors built for low speeds (less than
1200 RPM) or high torques may have higher full-load currents,
and multispeed motors will have full-load current
varying with speed, in which case the nameplate current
ratings shall be used.

(with exceptions, of course) :D

(2) Nameplate Values. Separate motor overload protection
shall be based on the motor nameplate current rating.
 
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