WA Administrators Exam

[wgholt]

I?m still not following. To me, VA means V times A, 1? = V X A & 3? = V X A X 1.732.
For Motor Feeders, or any feeder, there should also be an additional 25% of the largest motor added to the summation; But what establishes the Amperage for mixed Continuous and Non-continuous Motors, to find circuit VA? T430.147-150 state values of full load motor current; Part II of 430 specifies ampacities of conductors, and 230.3(B)(3) outlets for motor loads.
Is this the correct way to calculate O.C.P.D.?s (Inverse Time Circuit Breakers) and total V.A. for Motor Feeder Circuits; Continuous and Continuous with Duty Cycle Motors?
(No Name Plate F.L.A. given)

10 HP (one) 3? 208V Continuous 30.8A @ 100% T430.150
5 HP (two) 1? 208V Continuous 30.8A @ 100% T430.148
2 HP (three) 1? 120V Continuous 24A @ 100% T430.148

30.8 X 250% = 77 = 80A Branch CB 430.52(C)(1) & 240.6
80 + (2 X 30.8) + 24 = 165.6 = 150A Feeder CB 430.62(A) & 240.6

(208 X 1.732 X 30.8) + (2 X 208 X 30.8) + (3 X 120 X 24) + (30.8 X 25%) =
32,557 Total Feeder Circuit VA

The same motor configuration, except: the two (5HP 208V 1-Phase) motors are now classified as ?Intermittent Duty?, 15 Minute Rated motors; and the three (2 HP 120V 1-Phase) motors are now classified as ?Varying Duty?, Continuous Rated motors.

Comparison for largest motor: per (430.24 Exc.1)
10 HP 3? 208V 30.8A @ 125% Continuous = 38.5A 430.24 (Exc.1)
5 HP 1? 208V 30.8A @ 85% Intermittent Duty = 26.18A T430.22 (E)
2 HP 1? 120V 24A @ 200% Varying Duty = 48A T430.22 (E)

10 HP (one) 3? 208V Continuous 30.8A @ 100% T430.150
5 HP (two) 1? 208V Intermittent Duty 26.18A @ 85% T430.22 (E)
2 HP (three) 1? 120V Varying Duty 48A @ 200% T430.22 (E)

48 X 250% = 120 = 125A Branch CB 430.52 (C)(1) & 240.6
125 + (2 X 26.18) + 30.8 = 208.16 = 200A Feeder CB 430.62 (A) & 240.6
(A) By Motor Table FLC (208 X 1.732 X 30.8) + (2 X 208 X 30.8) + (3 X 120 X 24) + (30.8 X 25%) = 32,556 Total Feeder Circuit VA

(B) By Motor Rating (208 X 1.732 X 30.8) + (2 X 208 X 26.18) + (3 X 120 X 24) + (30.8 X 25%) = 30,634 Total Feeder Circuit VA

(C) By Motor Duty (208 X 1.732 X 30.8) + (2 X 208 X 26.18) + (3 X 120 X 48) + (48 X 25%) = 39,279 Total Feeder Circuit VA

Any Help would be greatly appreciated, as the ?Grand fathers? in WA runs out on 7/1, and I take
Part III for the third time on 6/29.

 

[CaseyM]

Re: WA Administrators Exam

I too am having my own difficulties with the Wa state test as you can tell from my previous postings, so I'm definatly no expert but I'll spend some time on your calcs. One thing I noticed is that when you size the breaker for a motor feeder, just like when you size the main breaker for a panel or the like, you size for the total, how ever you find it, of one leg. I.E you have a 400 amp load on a two hundred amp panel because it's devided into two legs, 200 each. The same for motors. When you mix single phase with three phase motors you size the breaker for the highest leg, not the total, just like a main breaker.
In Mike Holtz prep book he shows a line diagram, easy to draw yourself, where you devide your motors into their respective legs, and pick the highest leg to size your breaker.

 

[jwelectric]

Re: WA Administrators Exam

wgholt

I have no idea what the state of Washington does as far as their test. I do know that Ohm?s law plays no part in the motor calculation of a motor as outlined in the NEC.

Start with
430.6 (A) (1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate.

For conductors
430.22 Single Motor.
(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor?s full-load current rating as determined by 430.6(A)(1).
Or
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 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads.

Over current comes out of 430.52
430.52 (C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used.

Feeders are found in 430.62
430.62 Rating or Setting ? Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group.
Where the same rating or setting of the branch-circuit short-circuit and ground-fault protective device is used on two or more of the branch circuits supplied by the feeder, one of the protective devices shall be considered the largest for the above calculations.

As you can see Ohm?s law was not used nor was the square root of three for three phase calculations.

[ June 30, 2005, 02:57 PM: Message edited by: jwelectric ]