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Feeder and Subfeeder Design

A step by step guide for feeder and subfeeder design.

Feeders” are conductors which carry electric power from the service equipment (or generator switchboard) to the overcurrent devices for groups of branch circuits or load centers supplying various loads.

Subfeeders” originate at a distribution center other than the service equipment or generator switchboard and supply one or more other distribution panelboards, branch circuit panelboards, or branch circuits. Code rules on feeders also apply to subfeeders.

Feeders and subfeeders must be capable in carrying the amount of current required by the load, plus any current that may be required in the future.

Selection of the size of a feeder depends upon the size and nature of the known load as computed from the branch circuits, the unknown but anticipated future loads and the voltage drop.

Because feeders & subfeeders are inherently carrying heavy or high ampere loads, most electrical fires originate in these circuits. Electrical fires usually don’t happen after one or two weeks from the time of energization. They usually happen after several years of operation where loads are added indiscriminately and capacities of feeder cables went overloaded, unnoticed.

II. What are to be Designed?

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RELEVANT CODE PROVISIONS:

1.) The feeder conductor ampacity shall be at least equal to 125% of a continuous load. (NEC Section 220-10b).

2.)The total load on any overcurrent device located in a panelboard must not exceed 80% of the rating of the overcurrent device. (NEC Section 384-16c).

3.)Circuit conductors shall be protected against over-current in accordance to their ampacities, but where the ampacity of the conductor does not correspond with the standard ampere rating of a fuse or a circuit breaker, the next higher rating shall be permitted only if this rating does not exceed 800 amperes. (NEC Section 240-3).

4.)Circuit conductors shall be protected against over-current in accordance to their ampacities, but where the ampacity of the conductor does not correspond with the standard ampere rating of a fuse or a circuit breaker, the next higher rating shall be permitted only if this rating does not exceed 800 amperes. (NEC Section 240-3).

5.)These normal ampacities may have to be reduced or derated where there are more than three conductors in a cable or raceway (Note 8 to Tables 310-16 through 310-19). This means a change in ampacities of circuit conductors.

6.)The current permitted to be carried by the branch circuit conductors or feeder may have to be reduced if the load is continuous. This does not mean a change in the ampacities of the conductors but the rule refers to a limit of the load to be carried by the conductors. The change of ampacity of conductors because more than 3 conductors are installed in a cable or raceway is distinctly different from limiting the load. (NEC Section 210-22c).

ILLUSTRATION 1

Problem Determine the minimun size of the conductor that could be used to supply a ligting load of 30,000VA fed by a 240 V, 3-Phase, 3 wire feeder operating at 80% power factor?

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1.) Feeder Amps = (30,000)/(220x1.732) = 78.73 A
2.)Feeder Size = 1.25x78.73 = 98.41 (use three # 3 THW)
Ampacity 100 A
Max Load Allowed = 100 x 0.80 = 80A
3.)Feeder Protection = Feeder Ampacity = 100A
Use 100A CB or Fuse, rated 240 V, 3 poles
4.)The Busbar in the panelboard shall have a minimum ampacity of 100A
5.)The mains of the Panelboard shal be rated 100A,3-poles,240V
6.)The equipment grounding (4th wire), use #6 TW. ( Reminder: the system in this example is a 3-phase 3 wire. The 4th wire is not a current carrying conductor but is intended for equipment grounding)

ILLUSTRATION 2

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Three Line Diagram

Line Current = 96 x 1.732 = 166 A (line current for delta
connected Load)
(note: largest phase current 96 A was used)
Size of Main Feeder Conductor = 1.25 x 166 = 207 A
use 3/0 THW, Ampacity: 200A: Main Breaker Size:200AT, 250AF

But with some reserve capacity in mind:
use 4/0 THW Ampacity:230A (assuming only 3 current carrying conductors in a conduit & not more than 30 deg C Ambient temp)
Main Breaker Size: Use, 225 AT, 250AF

FEEDER CALCULATION WITH UNKNOWN LOADS

NEC Table 220-3(b) lists certain occupancies (types of buildings) for which load densities (lighting & general-purpose receptacles) specified in volt-amperes per square foot.

The PEC also lists down the lighting load densities in terms of volt-amperes per square meter.

In each type of occupancy, there must be adequate feeder circuit capacity to handle the total load that is represented by the product of volt-amperes per unit area times the area of the building. This becomes the most likely connected load in that particular occupancy.

The connected load shall then be multiplied with a demand factor in order to approximate the maximum demand of the load served by a feeder or sub-feeder. The maximum demand plus a load growth factor of 35% (next 5 years), or 50% (for the next 10 years) may be inputted to determine the feeder. It is recommended that a load growth factor be imbedded in the design.

In design practice, a 55%-70% loaded feeder or transformer in a brand new office building is usually acceptable.

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ILLUSTRATION 1

SIZING PARALLEL FEEDERS

Parallel Feeders are feeders with more than one conductor per phase. The ampacity of the parallel conductors is equal to the ampacity of one multiplied by the number of conductors in parallel. Parallel conductors shall be of the same size, length & type.

Parallel feeders must not be misunderstood as “equivalent conductors”. Equivalent Conductors are used in grounding circuits having a different definition.

“Conductors in sizes 1/0 AWG (50 mm2) and larger maybe run in multiples provided the arrangement is such as to assure equal division of total current among all conductors involved.

All of the multiple conductors shall be of the same length, of same conductor material, circular mil area, same insulation type and terminated in the same manner. Where run in separate raceways or cables, the raceways or cables shall have the same characteristics”.

Example:

Ampacity of 2// 500 MCM THW /phase in two separate conduits is 380 A x 2 or 760 Amps

Ampacity of 3// 4/0 THW per phase in three separate conduits is 230 A x 3 or 690 Amps

But if installed in common conduit or raceway, Note 8 must be considered:

Ampacity of 2// 500 MCM THW/phase in one 5 inch conduit is 380 x 2 x 80% or 608 A

Ampacity of 3// 4/0 THW in one 3-1/2 inch conduit is 230 x 3 x 70% or 483 Amps

THE CONDUIT WIRING METHOD

What do “New Work” & “Rewiring of Existing Conduits” mean in conduit wiring method?

Why the code mandated less number of conductors inside conduits in “New Work” than in “Rewiring” works?

In summary, the intent of the NEC is for only 25% conduit fill for “new work” installations – to give allowance for future rewiring without the need for new conduit installations.

The same installations can be filled up to a maximum of 40% fill if there is a need to rewire them.

In no way however, that the conduit be filled up more than 40%.

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