Section 215-2(b) FPN No. 2. Conductors for feeders as
defined in Article 100, sized to prevent a voltage drop exceeding 3 percent
at the farthest outlet of power, heating, and lighting loads, or combinations
of such loads, and where the maximum total voltage drop on both feeders
and branch circuits to the farthest outlet does not exceed 5 percent, will
provide reasonable efficiency of operation.
The NEC does not contain any requirements that ungrounded or
grounded conductors be sized to accommodate voltage drop.
Voltage drop affects the efficiency
of the equipment, but is not a safety issue.
The NEC recommends [FPN's to 210–19(a), 215–2(b),
and 310–15] a maximum of 3% voltage drop for branch circuits, a maximum of
3% voltage drop for feeders, but a maximum of 5% voltage drop overall for
branch circuits and feeders combined.
Each branch circuit must meet NEC requirements in determining
the computed loads:
Sections 220–3(a), 220–10(b), and 384–16(c). The NEC
requires the branch circuit computed load for conductor sizing to be sized
at 125% of the continuous load, plus the noncontinuous load (100%).
The general lighting load is first computed at 3 volt-amperes per
square foot of outside area not including the garage or porches.
Table 220–3(b), general use receptacles not greater than
20 amperes are included in the 3 VA per foot of the general lighting load.
The number of receptacles in the system are determined by the
locations of the receptacles. Each receptacle is rated at 180 va. A
receptacle must be located within 6 feet of any wall area measured along
the baseboard including around corners. This means the cord of an appliance
in the center of a 12 foot span would be able to reach a receptacle either
direction. There must be a receptacle within any wall space 2 foot of more.
There must be a ground fault receptacle in any bathroom. There must be
ground fault receptacles within any 4 feet of kitchen counter top and do
not install face up. There must be two ground fault receptacles outside,
one in front and one in back. There must be one ground fault receptacle
in the garage area. There must be one receptacle circuit in the utility
room for the washer.
Branch Circuit Calculations:
Section 220–3(c)(6). The NEC does not have a specific rule
that says 10 receptacles on a 15 ampere circuit or 13 receptacles on
a 20 ampere circuit. For 15 ampere circuits, the calculation is as follows:
120 volts x 15 amperes = 1800 VA/180 VA = 10 receptacles, and for 20 ampere
circuits, 120 volts x 20 amperes = 2400 VA/180 VA = 13 receptacles.
Many people think that when calculating the number of receptacles the
load should be reduced by 80%, but this is not the case! Yes, the NEC limits
the load on receptacle circuits to 80% for portable appliances [210–23(a)]
and, yes, the maximum continuous load on a branch circuit shall not exceed
80% of the circuit rating (protection device) [220–3(a) and 384–16(c)],
but these rules don't apply to the number of receptacles per circuit. I
wish the NEC were more specific, but it's not.
There are many Articles that apply to branch circuits, such as
210; Overcurrent Protection, 240; Wiring Methods, 300; Conductors, 310;
Appliances, 422; Electric Space Heating Equipment, 424; Motors, 430;
and Air-Conditioning, 440.
A summary of Article 220 branch circuit rules is as follows:
Voltage for calculation shall be 120, 120/240, 208Y/120, 240, 480Y/277,
and 480 [220–2(a)].
When a combination of continuous and noncontinuous
loads are present, the branch circuit conductor is sized based on 125% of
the continuous load and 100% of the noncontinuous load [220–3(a)].
Branch circuits to inductive lighting must be sized according to the
VA rating, not the wattage rating [220–3(c)(3)].
The number of receptacles permitted on a branch circuit for commercial
occupancies is based on 180 VA per receptacle [220–3(c)(6)].
The minimum number of branch circuits is determined from the total
connected load. Example: An 1,800-square-foot house requires 3–15 ampere
circuits for general lighting and receptacles, calculated as follows:
1800 x 3 VA = 5,400 VA/(120 volts x 15 amperes) = 3 circuits [220–4(a)].
A minimum of two 20-ampere branch circuits are required for dwelling
unit countertop receptacles [220–4(b)].
A minimum of one 20 ampere circuit is required for dwelling unit
laundry receptacle outlet(s) [220–4(c)]. General lighting and receptacles
loads must be evenly distributed between the branch circuits [220-4(d)].
Branch circuit calculations for ranges, cooktops and ovens are according
to Section 220-19 Note 4 in conjunction with Section 210-19(b).
There are many Articles that apply to branch circuits, such as
210; Overcurrent Protection, 240; Wiring Methods, 300; Conductors, 310;
Appliances, 422; Electric Space Heating Equipment, 424; Motors, 430; and
Air-Conditioning, 440.
Article 225 covers outside circuits:
Article 225 contains the following requirements:
A readily accessible disconnect is required at the remote building,
located either outside or nearest the point of entrance inside [225–8].
There shall be no more than six disconnects mounted in a single
enclosure, or up to six separate enclosures [230–71]. The disconnects
must be grouped and each disconnect must be marked to indicate the load
served [110–22 and 230–72].
Article 225 does not contain any requirements for grounding,
but Section 250–24 contains the requirements for remote buildings and
structures. The basic rule is that a ground wire is not required if the
grounded conductor (neutral) is used to ground the separate building
disconnect. However, a ground wire is permitted to the separate building.
Article 225 contains the same requirements for outside overhead
conductors as Article 230 for overhead service conductors: The minimum
clearance for the point of attachment is 10 feet but it may need to be
higher to maintain the minimum clearances specified in 225–18 [225–16].
Overhead conductors must maintain clearances from ground and roofs
according to Sections 225–18 and 225–19.
Source:
http://ourworld.compuserve.com/homepages/boyce_smith/HOMEPAGE.HTM
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