Answering electrical exam questions correctly that require you to use the noncoincident load rule is key to preparing to pass your electrical exam. Can you answer the following question with ease?
Question:
A dwelling service supplies a 5 hp. 240 volt single-phase air conditioning unit and 6.5 kVA 240 volt single-phase electric heating unit. What is the total load on a 240 volt/120 volt single-phase service for these units?
A. 6500 volt-amps
B. 6720 volt-amps
C. 7260 volt-amps
D. 7650volt-amps
Solution:
B. 6720 volt-amps
Refer to Article 220.60-Noncoincident Loads. This article indicates that where it is unlikely that two or more non-coincident loads will be in use simultaneously, it shall per permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service.
Since the central air-conditioning and the central heating unit will not be operating simultaneously, these units are considered noncoincident loads. When calculating the service load contribution for these units only the largest of the two loads need be included.
In order to compare which of the two loads is largest, both loads must be in the same unit of measure. Convert the air conditioning load rating provided in voltage and horse-power into the equivalent rating of 6,720 volt-amps.
Since 6,720 volt-amps of air conditioning load is larger than 6,500 volt-amps of electric heating load, the heating load would be omitted.
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