4.5 Parallel Conductors, Dwelling Service Sizing & Conditions of Use

Why Conductors Are Paralleled

Above roughly 400 A a single conductor per phase becomes impractical: 600 kcmil is heavy, stiff, hard to bend into a gutter, and suffers increasing skin effect. Running two or more smaller conductors per phase in parallel is standard practice. The Code permits it, but under conditions strict enough that any asymmetry — one leg longer, one lug different — is a violation, because unequal impedance means unequal current sharing and one conductor overheating while the others loaf.

The Paralleling Rules — 310.10(G)

310.10(G)(1) General. Aluminum, copper-clad aluminum, and copper conductors of size 1/0 AWG and larger, comprising each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor, shall be permitted to be connected in parallel — electrically joined at both ends.

310.10(G)(2) Conductor Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall comply with all of the following:

1. Be the same length;
2. Be made of the same conductor material;
3. Be the same size in circular mil area;
4. Have the same insulation type;
5. Be terminated in the same manner.

Read the scope carefully: the rules apply within each phase, not between phases. The A-phase conductors must match each other; they do not have to match the neutral. It is entirely legal to parallel two 250 kcmil copper conductors per phase alongside a single smaller neutral, provided the two A-phase conductors are identical to each other and the two B-phase conductors are identical to each other.

310.10(G)(3) Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, or grounded circuit conductor shall not be required to have the same physical characteristics as those of another phase.

A critical practical rule follows: if you run parallel sets in separate raceways, each raceway must contain one conductor of each phase and the neutral. Putting all the A-phase conductors in one ferrous raceway and all the B-phase in another creates enormous inductive heating in the steel and unbalanced impedance. In a nonferrous raceway or in nonmetallic conduit the heating problem is smaller, but the Code rule still stands.

310.10(G)(4) Ampacity Adjustment. Conductors installed in parallel shall comply with the provisions of 310.15(C)(1). Each parallel conductor counts individually. Three phases paralleled twice equals six current-carrying conductors if they share a raceway — an 80 percent adjustment.

310.10(G)(5) Equipment Grounding Conductors. Where parallel equipment grounding conductors are used, they shall be sized in accordance with 250.122. Sectioned equipment grounding conductors smaller than 1/0 AWG shall be permitted in multiconductor cables under the conditions in that subsection.

Remember 250.122(F): where conductors are run in parallel in multiple raceways, an equipment grounding conductor must be installed in each raceway, and each one must be sized in compliance with 250.122 based on the rating of the overcurrent device protecting the circuit — not divided among the raceways.

Worked Example — Parallel Feeder

A 600 A feeder is run as two parallel sets of 350 kcmil THWN-2 copper per phase in two separate PVC raceways, protected by a 600 A device. What equipment grounding conductor is required in each raceway?

From Table 250.122, a 600 A overcurrent device requires a 1 AWG copper equipment grounding conductor. Under 250.122(F), each of the two raceways must contain a full-size 1 AWG copper EGC — you do not halve it to 4 AWG per raceway.

Dwelling Services and Feeders — 310.12

The 2020 NEC replaced the old Table 310.15(B)(7) with a percentage rule in 310.12. For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings supplied by a single-phase, 120/240-volt system:

310.12(A) Services. For a service rated 100 A through 400 A, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating.

310.12(B) Feeders. For a feeder rated 100 A through 400 A, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating.

310.12(C) Feeder Ampacity. The feeder conductor ampacity shall not be required to be larger than the service conductor ampacity.

Worked Example — 200 A Dwelling Service

Service rating ........................ 200 A
83 percent allowance .................. 200 x 0.83 = 166 A
Required conductor ampacity ........... 166 A
Table 310.16, 75 C column, copper ..... 2/0 AWG = 175 A   -> acceptable
Table 310.16, 75 C column, aluminum ... 4/0 AWG = 205 A   -> acceptable
                                        3/0 AWG Al = 175 A -> also acceptable

The familiar rules of thumb — 2/0 copper or 4/0 aluminum for a 200 A dwelling service — come out of this calculation. Note the limits on the allowance: it is only for single-phase 120/240 V, only for dwellings, only for the entire load of the dwelling or unit, and only from 100 A through 400 A. A 208Y/120 V apartment service does not qualify, and neither does a subpanel feeder that serves only part of the dwelling load.

The Grounded (Neutral) Service Conductor

The neutral is sized by two separate requirements, and the larger governs:

1. 220.61 — the calculated maximum unbalanced load, with the permitted demand reductions (covered in Chapter 8 of this guide).
2. 250.24(C) — the grounded conductor run from the service point to the service disconnect shall not be smaller than the size required by Table 250.102(C)(1), based on the size of the ungrounded service-entrance conductors. Where the ungrounded conductors are paralleled, 250.24(C)(2) sets the requirement per raceway.

So a 200 A dwelling service with 2/0 copper ungrounded conductors has a Table 250.102(C)(1) minimum of 4 AWG copper for the grounded conductor, even if the calculated unbalanced load would allow something smaller.

Other Conditions of Use in 310.10

• 310.10(E) Shielding. Solid dielectric insulated conductors operated above 2000 V in permanent installations shall have ozone-resistant insulation and be shielded, with specified exceptions.
• 310.10(H) Direct-burial conductors must be of a type identified for that use — for example, USE-2 or UF.
• 310.15(D) Bare or Covered Conductors. Where bare or covered conductors are used with insulated conductors, their allowable ampacity shall be limited to that permitted for the adjacent insulated conductors.