Industrial Calculations: Feeder Loads -- Part 3

Now it's time to use what we've learned in this column over the last several issues and calculate the service load for feeders No. 1 through No. 6 in our hypothetical industrial plant. See the April and May 2001 articles for amperage values of feeders No. 1 through No. 6. Refer to the Figure for a one-line diagram of the electrical equipment.

When computing the load for an existing electrical system or when designing an entirely new system an important tool is a good, up-to-date line diagram of the system.

Calculate the service load in amps for each voltage

Refer to the following Code sections when performing your calculations: Secs. 230-42(a)(1), 430-24, and 440-34.

Step 1: Calculate the amps of each feeder and the total load [Sec. 220-2(a)].

This involves adding up the individual loads each feeder must supply. Determine these loads by looking at specification sheets and estimating receptacle loads. From previous parts of this series, we have the following values:

Feeder No. 1 (277/480V)=684.65A
Feeder No. 2 (13.8kV)=27.875A
Feeder No. 3 (120/208V)=534.525A
Feeder No. 4 (277/480V)=15A
Feeder No. 5 (277/480V)=588.05A
Feeder No. 6 (2400/4160V)=405A
Total load=2255.1A

Step 2: Calculate kVA of feeders 1, 3, 5, and 6, based on supply voltage and load.

To do this, multiply the current by the voltage. Then, multiply that answer by the square root of three because this is a 3-phase system now you have your VA. Divide your answer by 1000 so you can express the value in kVA.

Feeder No. 1.
kVA=684.65A x 480V x 1.732/1000
kVA=569.19kVA

Feeder No. 3.
kVA=534.525A x 208V x 1.732/1000
kVA=192.57kVA

Feeder No. 5.
kVA=588.05A x 480V x 1.732/1000
kVA=488.88kVA

Feeder No. 6.
kVA=405A x 4160V x 1.732/1000
kVA=2918.07kVA

Step 3: Calculate amps of each feeder, based on supply voltage and load.

To do this, convert kVA to VA by multiplying your kVA values by 1000. Because it's 3-phase, next multiply your voltage by the square root of three. Divide your first answer by your second answer, and you have your current. Be sure to round decimals up, not down no matter how small the “extra” is.

Feeder No. 1.
I=kVA x 1000/(V x 1.732)
I=569.19kVA x 1000/(13,800V x 1.732)
I=23.82A

Feeder No. 2.
Amps at 13,800V=27.88A

Feeder No. 3.
I=kVA x 1000/(V x 1.732)
I=192.57kVA x 1000/(13,800V x 1.732)
I=8.06A

Feeder No. 4.
Amps at 13,800V=15A

Feeder No. 5.
I=kVA x 1000÷(V x 1.732)
I=488.88kVA x 1000/(13,800V x 1.732)
I=20.46A

Feeder No. 6.
I=kVA x 1000/(V x 1.732)
I=2918.07kVA x 1000/(13,800V x 1.732)
I=122.09A

Solution: Summing the individual amps for each feeder yields a total amperage of 217.31A.

Step 4: Determine which 90°C MV conductors to use.

Current of 217.31A at 13.8kV requires No. 4 cu., per Table 310-77. Note, we are not using Table 310-16 because we are dealing with MV.

Size the transformer to supply the service equipment loads

Select the transformer based on the available standard size, the amount of loading you want on the transformer (for example, 60% to 80%), planned additional load, and other engineering factors. In our example, let's assume this facility plans for a load decrease in the future, and you are going to load your transformer close to 100%. Such a scenario can happen when a company retools for more efficient production processes or changes its focus entirely.

Step 1: Find kVA.
kVA=I x kV x 1.732
kVA=217.31A x 13.8kV x 1.732
kVA=5194.06kVA

Step 2: Select the transformer. Based on the Code, you need a transformer bank that can handle 5194.06kVA at 13.8kV.

What is the full load current (FLC) of the transformer?

You'll need this information to size your main switch and breaker, as well as calculate your fault current.

FLC=kVA x 1000/(V x 1.732)
FLC=5194.06kVA x 1000/(13,800V x 1.732)
FLC=217.31A

Solution: The FLC of the transformer is 217.31A.

What is the available fault-current (AFC) at the terminals of the transformer?

You'll need this information to determine your overcurrent protection coordination, as well as order the right transformer. This transformer has a 2% impedance. Don't confuse this with your system impedance, which is a different quantity. We normally express system impedance in ohms. If someone has already done a network analysis on your project, you may have a value shown as the admittance, expressed in units called siemens. To convert to ohms, invert the value ohms and siemens are reciprocals of each other.

AFC=FLA of transformer/Z
AFC=217.31A/.02
AFC=10,865.5A

Solution: The AFC at the terminals of the transformer is 10,865.5A.

Sizing elements of each feeder

Before you can order switchgear, you need to determine what size breakers you will need for each feeder. You also need to determine conductor ampacities. Remember, you must select conductors rated for MV application.

Feeder No. 1. (684.65A)

The OCPD for the panelboard will be 700A. Why would the breaker not be smaller than 684.65A? Remember, you are sizing your breaker for the load the circuit must handle. Size your conductor large enough to handle the current the breaker will allow. If you remember that the breaker defines the circuit, you will find it easy to keep this straight. Use three No. 4/0 THWN cu. conductors per phase (parallel conductors) for the panelboard, based on Table 310-77.

Feeder No. 2 (27.875A)

The OCPD for the switchgear will be 30A (based on calculated load) or 50A (based on conductors). As shown in the April issue, the conductors for the switchgear will be three No. 6 cu. conductors per phase (parallel conductors) based on Table 310-77, which shows No. 6 to be the minimum size for 5001V to 35kV.

Feeder No. 3 (534.525A)

The OCPD for the primary of the 208/120V transformer will be 50A. This transformer has a 15:1 winding ratio, and 1/15 of the secondary current is 35.5A. Table 310-77 requires No. 6 cu. conductors for the primary of the transformer. If you were sizing the conductors on the secondary side of this transformer, you would refer to Table 310-16 (because of the voltage level).

Feeder No. 4 (15A)

The OCPD for the future load will be 15A. Conductors for the future load will be No. 6 cu., based on Table 310-77.

Feeder No. 5 (588.05A)

The OCPD for the secondary connection will be 600A. In May, we decided to use three No. 3/0 THWN cu. conductors per phase (parallel conductors), based on Table 310-16.

Feeder No. 6 (405A)

The total load for the feeder is 405A. The OCPD for the secondary connection will also be 405A. You could pull two 4/0 THWN cu. conductors in parallel to supply this load, per Table 310-16.

Lessons learned

Always develop a one-line diagram when calculating feeder loads. This will help you clearly see the distribution system and avoid calculation errors. If you are using engineering software or even a spreadsheet to do your calculations, this step will help you avoid the “garbage in/garbage out” problem. You must work methodically, first determining your loads and then determining what size transformers you need to supply them. Size your overcurrent protection so you can run your loads, and size your conductors large enough to handle the current load demand.