load calculation; largest motor load & noncoincident loads

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unsaint34

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When load calculating, if my AC compressor is the largest motor load, do I add the 25% of that AC compressor, even if the AC compressor load was not added into the service load calc because my heating load was larger? Some books say I have to use the 25% of the next largest motor. I don't agree with that. What if my AC compressor was 5hp which would be 6.7KVA (28Ax240V=6720VA) and my next largest motor is a 10A garbage disposal? Then the difference of the 25% additional motor load between the two methods would be 5.5KVA!
 
You stated your own answer to the problem in the first sentence. The Code says to use the largest of the two (for calc.'s) the A/C is out. Consider that one or the other will be running and Neither at the same time, it's not pick one, it's use the largest one.

What you might be confusing is when to use the largest motor calculations, IE when you have mutliple motors in a string on a circuit. While this could be available to use in a house calcualtion.

Here's my argument againest that(in your case)

Your combining devices or equipment that's not on the same circuit generally. Thus using motor calc.'s are out.
 
Your A/C load value, when compared to the heating load value, should already include the extra 25% for the largest A/C motor.

IMO, if the heating load value is the greater of the two, there is no need to make an additional allowance for any motor if the largest A/C motor is also the largest motor supplied.
 
Dennis Alwon said:
I believe that the 25% is only on the units MCA not when using HP
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Let's review the requirements, as they apply:

220.50 Motors. Motor loads shall be calculated in accordance with 430.24, 430.25, and 430.26 and with 440.6 for hermetic refrigerant motor compressors.
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430.24 Several Motors or a Motor(s) and Other Load(s). Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than the sum of each of the following:

(1) 125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A)

(2) Sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A)

(3) 100 percent of the noncontinuous non-motor load

(4) 125 percent of the continuous non-motor load.

Informational Note: See Informative Annex D, Example No. D8.

Exception No. 1: Where one or more of the motors of the group are used for short-time, intermittent, periodic, or varying duty, the ampere rating of such motors to be used in the summation shall be determined in accordance with 430.22(E). For the highest rated motor, the greater of either the ampere rating from 430.22(E) or the largest continuous duty motor full-load current multiplied by 1.25 shall be used in the summation.

Exception No. 2: The ampacity of conductors supplying motor-operated fixed electric space-heating equipment shall comply with 424.3(B).

Exception No. 3: Where the circuitry is interlocked so as to prevent simultaneous operation of selected motors or other loads, the conductor ampacity shall be permitted to be based on the summation of the currents of the motors and other loads to be operated simultaneously that results in the highest total current.
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440.6 Ampacity and Rating. The size of conductors for equipment covered by this article shall be selected from Table 310.15(B)(16) through Table 310.15(B)(19) or calculated in accordance with 310.15 as applicable. The required ampacity of conductors and rating of equipment shall be determined according to 440.6(A) and 440.6(B).

(A) Hermetic Refrigerant Motor-Compressor. For a hermetic refrigerant motor-compressor, the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection. Where no rated-load current is shown on the equipment nameplate, the rated-load current shown on the compressor nameplate shall be used.

Exception No. 1: Where so marked, the branch-circuit selection current shall be used instead of the rated-load current to determine the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, and the branch-circuit short-circuit and ground-fault protection.

Exception No. 2: For cord-and-plug-connected equipment, the nameplate marking shall be used in accordance with 440.22(B), Exception No. 2.

Informational Note: For disconnecting means and controllers, see 440.12 and 440.41.

(B) Multimotor Equipment. For multimotor equipment employing a shaded-pole or permanent split-capacitor-type fan or blower motor, the full-load current for such motor marked on the nameplate of the equipment in which the fan or blower motor is employed shall be used instead of the horsepower rating to determine the ampacity or rating of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate overload protection. This marking on the equipment nameplate shall not be less than the current marked on the fan or blower motor nameplate.
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Sorry for the work on that but I agree using the mca on the unit then the 125% is built in but if you use the HP rating I believe the 125% needs to be added. Are we not saying the same thing. All I am saying is the 125% is already in the MCA but not the other ratings. Are you saying no 125% is needed at all?
 
Dennis Alwon said:
Sorry for the work on that but I agree using the mca on the unit then the 125% is built in but if you use the HP rating I believe the 125% needs to be added. Are we not saying the same thing. All I am saying is the 125% is already in the MCA but not the other ratings. Are you saying no 125% is needed at all?
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No, I am saying the extra 25% for largest A/C motor should already be in the A/C load which is compared to the heating load... whether it is there by way of mca or motor/compressor nameplate times 125%. If the heating load is greater, omitting the A/C load from the calculation, and the A/C has the largest motor, I say the the extra 25% of highest rated motor requirement has been met.
 
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