Electrical Level 4 Module 1
Continuous duty load
A load that operates without interruption for more than 3 hours straight
Standard calculation for multi-family dwellings
Additional demand factors may be applied to the portion of the neutral load contributed by electric ranges and the portion of the total neutral load greater than 200A
NEC Section 220.61(B)
Allows a demand factor of 70% for sizing a neutral conductor
Interior lighting @ 3VA/ft^2
An electrical load for a school building that has 200,000 ft^2 of floor area and a 480Y/227V service
Note:
Any loads that are continuous duty are multiplied by 125% to determine the overcurrent protection device and conductor size. All others are calculated at 100%.
CM
Area of the conductor in circular mils
One exception: a storage warehouse that is occupied infrequently
Demand factors may be applied to various non-continuous loads according to the respective section of the NEC
Commercial and Industrial Lighting Loads
Each load must be designated as either continuous or non-continuous. *most loads are considered continuous
NEC Table 220.42 - the demand lighting load
First 20,000 @ 50% = 10,000VA 48,000 - 20,000 = 28,000 @ 40% = 11,200VA TOTAL: 21,000 VA
VD = square root 3xLxRxI/1,000
Formula for VD of a three-phase feeder with a noncontinuous load
I= VA/V x square root of 3 or I = VA/V x 1.732
Formula for balanced three-phase circuit feeder ampacity
I = VA/V x square root 3 (1.732)
Formula to be used if connected load is less than NEC requirement
NEC Section 220.12
General lighting loads by occupancy-provides unit load per ft^2/m^2 according to the type of occupancy
The 100' (30 m) tap rule NEC Section 240.21(B)(4)
Limited to installations in a high bay manufacturing building with wall heights over 35' (11 m)
Loads for Restaurants
More than six units of kitchen equipment with a total connected load of 80kVA (all units are 208V, three-phase equipment)
Show Window Loads
NEC Section 220.43(A) - Requires that the lighting load for the show window feeders be calculated at 200VA per linear foot of show window (660VA per linear meter)
Optional Calculations for Schools
NEC Section 220.86
When the overcurrent protection for a conductor is located at the load end instead of at the supply side of a conductor
NEC generally considers to be a trap
What is the adjusted ampacity for each of six No. 4/0 THHN copper current-carrying conductors in a single conduit?
See NEC Table 310.15(B)(16) for ampacity and NEC Table 310.15(B)(3)(a) for adjusted ampacity by 80%
Calculations for the Primary Feeder
Start with the lighting load- NEC Table 220.12 requires a minimum general lighting load for office buildings to be based on 3-1/2VA/ft^2 (39VA/m^2)
NEC Table 220.56
The 80kVA kitchen equipment load is subject to the application of a 65% demand factor, which reduces it to a demand load of 52kVA.
The wiring systems for marinas and boatyards are designed using the same NEC rules as for other commercial occupancies EXCEPT FOR
The application of several special rules dealing primarily with the design of circuits supplying power to boats
Compute the floor area of the building using the outside dimensions of the building
The first step in determining the service load for a given type of occupancy
2VA x 240ft^2 x 100 units = 48,000VA
The general lighting load @ 2VA/ft^2 (22VA/m^2)
Farm Load Calculations with Multiple Buildings
The load to be included at 100% demand, however, cannot be less than 125% of the largest motor and no less than the first 60A of the total load
30A found in NEC Section 555.19(A)(4)
The smallest sized locking and grounding type receptacle that may be used to provide shore power for boats
1.732
The square root of 3
Name one reason the current-carrying conductor size would be adjusted.
There are more than three current-carrying conductors in a raceway per NEC Section 310.15(B)(3)
What are the formulas to calculate VD for single-phase circuits with resistive loads?
VD = 2xLxRxI/1,000 and VD = 2xLxKxI/CM
Voltage drop rule of thumb (A)
VD for feeders should not exceed 3% to the farthest outlet and 5% for the combo of feeders and branch circuits to the farthest outlet.
NEC Table 220.86 -demand factors for feeders and service-entrance conductors for schools and 220.87 -optional method for determining existing loads
What are at least two of the dwelling and non dwelling unit demand factors
Voltage Drop rule of thumb (B)
When combined, the max VD allowed for the branch circuit is usually 3%, then VD allowed for feeder can be no more than 2%.
There is no overcurrent device within the transformer itself
Why are almost all secondary sides of transformers classified as trap conductors?
