20

I am adding an attached garage to my house. I'm going to be putting in a 100 Amp subpanel which is about 60 feet away and run through the basement joists.

What gauge wire do you recommend for either aluminum and copper? Also, do you recommend one or the other?

  • 3
    Is the subpanel 60' away from the main panel, or is the chase between the panels 60'? i.e. is it 60' as the grow flies, or as the cable runs? – Tester101 Jun 26 '13 at 10:59
19

When determining feeder conductor size, you'll want to consider the "lowest temperature rating of any connected termination, conductor, or device" as per National Electrical Code (NEC) Article 110.14(C).

While the cable/wire may be rated at 90°C, you'll likely find that the terminals are rated at 75°C, or not labeled at all. 110.14(C)(1)(a) tells us, that since we're working with 100 amperes or less. We should use the 60°C column of Table 310.15(B)(16) to determine the conductor size, unless the equipment is listed and labeled for a higher temperature.

National Electrical Code 2014

ARTICLE 110 Requirements for Electrical Installations

110.14(C)(1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.15(B)(16) (formerly 310.16) as appropriately modified by 310.15(B)(6).

(a) Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following:

(1) Conductors rated 60°C (140°F).

(2) Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used.

(3) Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.

(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.

Since the cable will run from a breaker in the main service panel, to either a breaker or lugs in a subpanel. We have to consider the temperature rating of...

  • The conductors
  • The terminals in the main panel where the conductors will connect.
  • The terminals in the sub panel where the conductors will connect.

We'll then use the lowest value, or 60°C if any of the above are not labeled. Once we know the size of the overcurrent devices, and the lowest temperature rating, we can use Table 310.15(B)(16) to determine the conductor size and material we'll need.

Table 310.15(B)(16) - Highlighted

This will give us the current carrying conductor size required for our feeder.

But wait...

If you're working in a one-, two-, or multi-family dwelling unit, is Article 310.15(B)(7) applicable?

National Electrical Code 2014

ARTICLE 310 Conductors for General Wiring

310.15 Ampacities for Conductors Rated 0–2000 Volts.

(B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.15(B)(16) through 310.15(B)(19), and Ampacity Table 310.15(B)(20) and Table 310.15(B)(21) as modified by (B)(1) through (B)(7).
The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, provided the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).

(7) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders.

(a) For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(7), shall be permitted as 120/240-volt, single-phase service-entrance conductors and service lateral conductors.

Table 310.15(B)(7)

NO.

Notice the codes says

"shall be permitted as 120/240-volt, single-phase service-entrance conductors and service lateral conductors".

After reading the definition of these terms, it's clear that this does not apply to the wire between the main panel and a subpanel.

Service-Entrance Conductors, Overhead System. The service conductors between the terminals of the service equipment and a point usually outside the building, clear of building walls, where joined by tap or splice to the service drop or overhead service conductors.

Service-Entrance Conductors, Underground System. The service conductors between the terminals of the service equipment and the point of connection to the service lateral or underground service conductors.

Service Lateral. The underground conductors between the utility distribution system and the service point.


tl;dr

Conductors and all terminals rated at or above 75°C.

Use 3 AWG copper or 1 AWG aluminium for the current carrying conductors.

Conductors rated at or above 75°C, terminals rated at 60°C or unlabeled.

Use 1 AWG copper or 1/0 AWG aluminium for the current carrying conductors.

Conductors and terminals rated at 60°C.

Use 1 AWG copper or 1/0 AWG aluminium for the current carrying conductors.

Conductors rated at 60°C, terminals rated higher than 60°C

Use 1 AWG copper or 1/0 AWG aluminium for the current carrying conductors.

  • It should be pointed out that 310.15(B)(7) excludes 60°C cable (only 75°C and 90°C cables are listed). – HerrBag Jun 26 '13 at 16:23
  • In the tl;dr section, I advise replace conditions 2-4 and solutions 2-3 with the single word "Otherwise"... unless you can think of a good reason not to do that. – Harper Mar 19 '16 at 20:03
  • What about the length of run aspect of the OP's question? Are there voltage drop concerns to consider? What Code needs to be considered in that regard (if any)? – the_meter413 Mar 21 '17 at 18:57
  • 1
    @the_meter413 there's no code in the NEC about reducing voltage drop, only a suggestion to reduce it to less than 5% (I think) at the point of use. The conductors suggested in the answer should not have that large of a voltage drop, so it shouldn't be a concern in this case. However, keep in mind that these are minimum standards. The wire can always be larger if you think voltage drop will be a problem. – Tester101 Mar 21 '17 at 19:08
  • One of the best,answers.I have seen on this topic. Love all the work you put into it. Blows the answers i was getting away. – Robert Moody Jun 1 at 6:27
5

You need a 3-3-3-5 copper SER cable to carry 100 amps (that's three #3 gauge for the two hot wires and the neutral and a #5 gauge for the ground).

enter image description here Source

If you want to use aluminum, you need at least #2 gauge or perhaps #1.

  • Why are you using the 75°C column? – Tester101 Jun 26 '13 at 11:19
0

Section 310.15 (7)(a) in the 2011 NEC goes on to say:

"For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits for feeders, or both, all loads that are part of associated with the dwelling unit."

This is pretty clear that 4-4-4-6 copper is good for 100 amps to a sub panel.

...by table 310.15(B)(7).

  • It's not likely that a sub panel supplies "all loads that are part of or associated with the dwelling unit.". Also note that the wording of this section has been changed in the 2014 version of the code, which will hopefully make it more clear. – Tester101 Feb 26 '16 at 17:16
  • The section you are citing would be used to size wire "between the main disconnect and the panelboard that supplies". i.e. I had a house that had one main breaker at the end of the house on the garage (since that is where the service was) that fed the breaker panel in the basement. So, the wire that ran between the main and the breaker panel through the garage could be downsized according to this section. – ArchonOSX Feb 26 '16 at 21:02

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