Connected load

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ktm400sx

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Is it possible to derate connected load for a MCC feeder? ie if I have 410 amps connected load (motor full load amps) can I use a 400 amp breaker?
Thanks for any help
 
Probably not. You have to go by ?calculated load,? not ?connected load.? In many cases, the calculated value is lower than the connected value. But for motors, it doesn?t always work out that way. For starters, you have to add 25% of the largest motor, when calculating the load.

Your best hope would be if two or more motors are interlocked in a manner that prevents one from running, if another is running. You don?t have to count both loads from a pair of loads that is non-coincident.
 
Two more tidbits.

Number one is that you do not ?derate? a load. You can sometimes apply a ?demand factor? for a group of loads. But the only thing that gets ?derated? is the ampacity of a conductor.

Number two is that you should not be getting your motor load current from the nameplate. Look at tables 430.248 and 430.250. This might or might not make your situation better for you.
 
You need to find out what the "demand" load is for the MCC. If it is an industrial facility then the client should have knowledge of the process and which motors will be on at one time. If it is another facility then someone (HVAC guy, building maint, etc.)should be able to give some input. If not, assume a 100% demand and size accordingly. The smallest MCC I have ever seen is 600 amps so why not assume that the MCC could grow and size the feeder for the max it could be?
 
Thanks for all the answers...is there a formula or equation for figuring the demand?
Iam going to install a larger than needed feeder, but I do need to know how to figure demand load.....

Some of the motors (2 of the largest) only run about 2 % of the time
 
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I?ve taken Article 220 (2002 NEC) and deleted all ?residential/dwelling? sections - as indicated by an ellipsis (?). For some of the remaining Subsections I retained only the title to indicate they don?t usually apply to industrial facilities. I retained the critical cross-references to external Sections that effect industrial / commercial design and the key ?basic? rules within the Article.

I draw particular attention to Section 220.10. Since I have also left the references to ?? applicable demand factors permitted by Parts II, III, or IV?? (220.14 and 220.21) how would you size the Load? Cite your Code references.

Remember: "noncoincident loads" are mutually exclusive such as electric space heating and air conditioning or "running" and "spares." They are not "probabilities,? "diversity? or ?demand? factors.

ARTICLE 220 Branch-Circuit, Feeder, and Service Calculations.
This article provides requirements for computing branch-circuit, feeder, and service loads.
Exception: Branch-circuit and feeder calculations for electrolytic cells as covered in 668.3(C)(1) and (4).
220.2 Computations.
(A) Voltages. Unless other voltages are specified, for purposes of computing branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.
(B) Fractions of an Ampere. Where computations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped.
220.3 Computation of Branch Circuit Loads.
Branch-circuit loads shall be computed as shown in 220.3(A) through (C).
(A) Lighting Load for Specified Occupancies.
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(B) Other Loads - All Occupancies. In all occupancies, the minimum load for each outlet for general-use receptacles and outlets not used for general illumination shall not be less than that computed in 220.3(B)(1) through (11)
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(C) Loads for Additions to Existing Installations.
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(2) Other Than Dwelling Units. Loads for new circuits or extended circuits in other than dwelling units shall be computed in accordance with either 220.3(A) or (B), as applicable.
220.4 Maximum Loads.
The total load shall not exceed the rating of the branch circuit, and it shall not exceed the maximum loads specified in 220.4(A) through (C) under the conditions specified therein.
(A) Motor-Operated and Combination Loads. Where a circuit supplies only motor-operated loads, Article 430 shall apply. Where a circuit supplies only air-conditioning equipment, refrigerating equipment, or both, Article 440 shall apply. For circuits supplying loads consisting of motor-operated utilization equipment that is fastened in place and has a motor larger than hp in combination with other loads, the total computed load shall be based on 125 percent of the largest motor load plus the sum of the other loads.
(B) Inductive Lighting Loads. For circuits supplying lighting units that have ballasts, transformers, or autotransformers, the computed load shall be based on the total ampere ratings of such units and not on the total watts of the lamps.
...
II. Feeders and Services
220.10 General.
The computed load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, as determined by Part I of this article, after any applicable demand factors permitted by Parts II, III, or IV have been applied.
FPN: See Examples D1(A) through D10 in Annex D. See 220.4(B) for the maximum load in amperes permitted for lighting units operating at less than 100 percent power factor.
220.11 General Lighting.
The demand factors specified in Table 220.11 shall apply to that portion of the total branch-circuit load computed for general illumination. They shall not be applied in determining the number of branch circuits for general illumination.
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220.12 Show-Window and Track Lighting.
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220.14 Motors.
Motor loads shall be computed in accordance with 430.24, 430.25, and 430.26 and with 440.6 for hermetic refrigerant motor compressors.
220.15 Fixed Electric Space Heating.
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220.21 Noncoincident Loads.
Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time, in computing the total load of a feeder or service.
220.22 Feeder or Service Neutral Load.
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III. Optional Calculations for Computing Feeder and Service Loads
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220.34 Optional Method - Schools.
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220.35 Optional Calculations for Determining Existing Loads.
The calculation of a feeder or service load for existing installations shall be permitted to use actual maximum demand to determine the existing load under the following conditions:
(1) The maximum demand data is available for a 1-year period.
Exception: If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (measure of average power demand over a 15-minute period) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be periodic in nature due to seasonal or similar conditions.
(2) The maximum demand at 125 percent plus the new load does not exceed the ampacity of the feeder or rating of the service.
(3) The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.
220.36 Optional Calculation - New Restaurants.
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IV. Method for Computing Farm Loads
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Computing loads per Article 220 creates a modest ?safety factor? for electrical designs. We?re lucky; civil/structural and mechanical safety factors are often 5, 10, 20 and ocassionally 25.

I'd add a brief comment to Charlie's first comment. The NEC does not actually require the loads to be interlocked.
 
rbalex said:
I'd add a brief comment to Charlie's first comment. The NEC does not actually require the loads to be interlocked.
True. But if you do have an interlock, it might make it easier for you to convince the AHJ that the loads are non-coincident.
 
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