Whether you are taking the master or journeyman electrical exam knowing the difference between a connected or nameplate rated load verses a demand load can determine whether you pass your exam. Can you answer exam questions that require you to know the difference?
The term “connected load” is the total nameplate rating of the appliance(s), whereas the term “demand load” refers to a reduction from connected load.
What is the connected and demand load on the service for 10-14 kW ranges supplied by a 208 v/120 v three-phase, 4-wire wye service?
Step #1 Find the connected load:
10 ranges x 14 kW per range = 140 kW connected load
Step #2 Find the demand load:
Refer to NFPA Article 220.55 and Table 220.55 which provides demand factors and loads for household electric ranges.
Article 220.55 indicates that “Where two or more single-phase ranges are supplied by a 3-phase, 4-wire feeder or service, the total load shall be calculated on the basis of twice the maximum number connected between any two phases.”
Since the service is supplied by a three-phase, 4-wire wye, the range loads can be balanced between all three phases.
Phase A Phase B Phase C
4 Ranges 3 Ranges 3 Ranges
Chose the phase with the largest number of ranges to multiply by two connections:
4 ranges x 2 connections = 8 ranges (# of ranges on which service size is based)
Apply Note 1 of T. 220.55
14 kw-12 kW = 2 kW difference.
Increase 5% by 2 kW difference which turns 5% into 10%.
Increase kW demand in Col. C of T. 220.55 for 8 ranges by 10%.
23 kW x 1.10 = 25.3 kW demand load
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