Service Disconnect Number & Rating Calculator

The 2020 single-disconnect change (230.71)

For decades the "six-disconnect rule" let a service be shut off with up to six switches or circuit breakers grouped in one enclosure - the familiar main-lug panelboard with six breakers and no single main. The 2020 NEC rewrote 230.71. Each service now has a single service disconnecting means unless 230.71(B) applies, and where 230.71(B) permits two to six disconnects, they can no longer share a single enclosure.

The reason is safety for the next person in the enclosure: even with all the disconnects switched off, the line-side terminals and busbar ahead of them stay energized. Splitting the disconnects into separate enclosures, barriered vertical sections, or separate compartments keeps that still-energized service busbar out of the enclosure a worker opens to service one of the downstream disconnects. The four permitted configurations are separate enclosures each with a disconnect; panelboards each with a main; a switchboard with one disconnect per barriered vertical section; and switchgear or metering centers with one disconnect per compartment.

Minimum ratings (230.79) and the combined rating (230.80)

Section 230.79 sets four floors for the rating of the service disconnecting means: 15 A for an installation of one branch circuit, 30 A for two 2-wire branch circuits, 100 A three-wire for a one-family dwelling, and 60 A for all other installations. These are minimums, not the answer by themselves - the disconnect must also carry the Article 220 calculated load, and you take the larger of the two. This calculator reports the 230.79 floor; use the Service Size or Electrical Load calculators to compute the load.

When the service uses more than one disconnect, 230.80 ties the pieces back together: the combined ratings of all the switches or circuit breakers must be not less than the 230.79 rating. A one-family dwelling split across two disconnects, for example, needs the two ratings to total at least 100 A. Enter the individual ratings above and the tool sums them and checks the floor.

Worked examples

• One-family dwelling, single main. One disconnect, so the count and arrangement pass automatically. Minimum rating is 100 A, 3-wire (230.79(C)); the actual size is the larger of 100 A and the Article 220 load.
• One-family dwelling, two 60 A disconnects in separate enclosures. Two disconnects each in its own enclosure passes 230.71(B). Combined rating 60 + 60 = 120 A, which meets the 100 A floor (230.80). Compliant.
• Commercial building, three breakers in one main-lug panel. Three is within the six-disconnect maximum, but grouping them in a single enclosure fails the 2020 230.71(B) arrangement rule. Move each into its own enclosure or use a single main.
• Seven disconnects. Exceeds the maximum of six. Reduce the count or split the load across additional services permitted by 230.2.

Frequently asked questions

How many service disconnects can I have?

One, unless NEC 2020 230.71(B) applies, which permits two to six. The change from earlier editions is that the two to six can no longer be grouped in a single enclosure - each must be in its own separate enclosure, barriered vertical section, or compartment. Six is the hard maximum.

What is the smallest service disconnect for a house?

NEC 230.79(C) sets 100 A, 3-wire as the minimum for a one-family dwelling. That is a floor; the disconnect must also handle the calculated load from an Article 220 calculation, so many services are larger. This tool reports the floor, and the Service Size and Electrical Load calculators handle the load.

Does the calculated load change the minimum rating?

The 230.79 floors are fixed regardless of load. The disconnect rating is the greater of the 230.79 floor and the Article 220 calculated load. This tool reports the 230.79 floor only and does not compute the load - that is a separate calculation, cross-linked below.

Which NEC edition is this?

NEC 2020. The single-disconnect change to 230.71 is a 2020 revision; 2017 and earlier editions still permitted up to six disconnects in one enclosure. Always confirm the edition your jurisdiction has adopted.

Related calculators

• Service Size Calculator: size the service disconnect, entrance conductors, and grounding electrode conductor from a known load.
• Electrical Load Calculator: compute the dwelling service load with the NEC 220.82 optional method.
• Breaker Size Calculator: standard NEC 240.6(A) breaker for a load with the 125% continuous rule.
• Grounding Electrode Conductor Size Calculator: size the GEC from the service conductor per NEC Table 250.66.