S.C. & G.F. for an a/c unit and a motor
- Thread starter cppoly
- Start date
- Status
david
Senior Member
- Location
- Pennsylvania
What type of insulation? # 10 from the 75 deg. ampacity column would be good for 35 amps
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Not sure why the MCA is not 125% of the 20.3 FLA.
Using the 20.3 amps and the 5.4 amps you would end up with (20.3*125%) + 5.4 = 30.76 amps minimum conductor size. Still couldn't go above the maximum of 35 amps for the OCPD.
Using the 20.3 amps and the 5.4 amps you would end up with (20.3*125%) + 5.4 = 30.76 amps minimum conductor size. Still couldn't go above the maximum of 35 amps for the OCPD.
I should add that these are two unrelated pieces of equipment. The motor conductors are a tap to the feeder conductors supplying the a/c unit and motor. In regards to multiple connections, which article are you supposed to follow 430 or 440? The motor is 430 and the a/c unit is 440.
Both of these articles have different rules for s.c. and g.f. overcurrent protection.
Both of these articles have different rules for s.c. and g.f. overcurrent protection.
curt swartz
Electrical Contractor - San Jose, CA
- Location
- San Jose, CA
- Occupation
- Electrical Contractor
I'm guessing the 20.3 is for multiple motors (compressor + fans). The largest motor FLA *125% + the FLA of the other motors.
Since the 125% of the largest motor should already be part of the A/C unit MCA you shuld be able to just add the FLA of additional motor to be supplied by this feeder. I would say the minimum feeder size would be 24.1 + 5.4 = 29.4 amps
- Location
- New Jersey
- Occupation
- Journeyman Electrician
You could be right, I should have assumed that some information was missing from the OP.
Gregg Harris
Senior Member
- Location
- Virginia
- Occupation
- Electrical,HVAC, Technical Trainer
I would agree already in the MCA and adding the FLA of the fan tap = you 30 amp
- Location
- Connecticut
- Occupation
- Engineer
You need to know what the individual motor currents within the A/C unit are. The feeder s.c and g.f. protection shall be not greater than the largest rating or setting for any motor or motor-compressor in the group, plus the sum of the full-load currents of the other motors in the group. See 430.62(A).
You need to know the individual motor currents within the A/C unit to determine the conductor size as well. You can't tell from the information given if the largest motor is the fan, or if it is within the A/C unit.
So if a feeder is supplying an a/c unit (containing a compressor + multiple fans) and a tap is made on this feeder to supply a motor, the s.c. and g.f. protection for the both of these has to be calculated by the method in article 430 for multiple motors?
Let me ask, would taking the MFS of the a/c unit + 100% of the FLC of the tapped motor then rounded down work as another method?
Let me ask, would taking the MFS of the a/c unit + 100% of the FLC of the tapped motor then rounded down work as another method?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I agree, but isn't it pretty likely that the two factors already given, the MCA of the AC unit and the FLC of the second motor enough information as Curt suggested?
The supply conductors being a feeder has not yet been established. Assuming the motor "tap" is after the A/C unit's SC/GFPD and no additional SC/GFPD for the motor, it would be considered a branch circuit. Under this scenario, that would be a violation of 430.53(A), (B), and (D) regarding protection of the smallest motor. Will require at the very least appropriate SC/GFPD inline with the motor tap. The other option is to make the supply a full-fledged feeder and provide separate SC/GFPD for each unit.
Smart $, I should have added there is branch circuit protection for the motor at the point of the tap just a few feet away. The motor is provided with 15A sc and gf protection. I'm looking for the feeder sc and gf protection that supplies these two units.
Technically, without separate A/C unit SC/GFPD after the motor tap, it is a branch circuit, not a feeder. The motor SC/GFPD is considered supplementary OCPD underArticle 240. The appropriate section for proper motor SC/GFPD is 430.53 as I previously mentioned. Without separate SC/GFPD for the A/C unit, its nameplate maximum OCPD ratings apply and cannot be altered by any consideration for the additional motor on the circuit. The minimum circuit ampacity rating up to the point of the motor tap will need to be adjusted for the added motor (which has already been covered).
Last edited:
Minimum would be A/C unit MCA plus motor FLC (plus upsize to standard OCPD rating). Any increase in rating would require sizing feeder conductor to match OCPD rating (conductor ampacity at least one ampere greater than next lower standard size where OCPD is 800A or less).
- Location
- Connecticut
- Occupation
- Engineer
You can use a 40A SC/GFPD with #10 conductors for this example.
I agree.
40A is the correct standard rating for the maximum OCPD per 430.62. A/C unit is largest at 35A plus sum of other motors FLC at 5.4A yields 40.5A total (cannot be increased to next larger standard size).
Conductor ampacity must equal or exceed A/C MCA (24.1A) plus motor FLC (5.4A) yielding 29.5A. So #10@60?C will do, provided there is no derating which takes its ampacity below 29.5A.
Correction...
The maximum OCPD is with using the minimum size conductor. OCPD rating can be increased as long as conductor size is increased to where its ampacity is at least one ampere greater than the next smaller standard OCPD rating (800A or less)... [430.62(B)].
- Status