Back Feed Breaker for a Square D Service Box
An electric panel is backfed when the main breaker is mounted in the cluster of breakers on the branch bus, instead of being separate, vertical, and above or below the branch-circuit breakers. It is essentially providing power "backwards" into the distribution bus. The breaker is usually, but not always, in the upper left position, and required to be clearly marked as "MAIN BREAKER." A backfeed main breaker must also be mechanically secured in place. The red tab at the breaker above is a rated securing device, and it will only be visible with the dead front removed.
One additional requirement for a backfed main breaker is that the "maximum amps per stab" does not exceed what the manufacturer allows. A stab is another name for a bus bar finger. This maximum is listed in the data plate. If you backfeed a panel with a 100-amp service breaker, and the maximum allowed by the manufacturer is 125 amps per stab, then the max you can install on the other side of the stab is 20 amps. This is why the slots opposite a backfeed main breaker are often blank. To learn more, see our blog post What is a "bus stab" in an electric panel?
So, the things to verify when examining a backfeed panel are:
- Is the main clearly marked as "MAIN BREAKER"?
- Has the breaker been mechanically secured in place?
- Does the sum of the amperages of the main breaker and the one beside it come to equal or less than the "maximum amps per stab" listed on the manufacturer's data plate?
Even a electric panel with a regular main breaker may be backfed by a breaker below it that is used for connecting a generator to the panel for use during a power outage. There are specific safety requirements for this type of backfeed, and they are intended to eliminate the possiblity of the both the main breaker and the generator backfeed breaker are on the same time. See our blog post Why did my generator hookup get tagged as defective by the home inspector? for more.
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Here's links to a collection of our other blog posts about ELECTRIC PANELS:
• What causes copper wires to turn green or black in an electric panel?
• What is the maximum number of circuit breakers allowed in an electric panel?
• When should a corroded or damaged electric panel cabinet or disconnect box be replaced?
• What is a tandem circuit breaker?
• When did arc fault circuit interrupter (AFCI) breakers first become required?
• Can an electric panel be located in a closet?
• Can an electric panel be located in a bathroom?
• Can you add circuit breakers by different manufacturers to an electric panel if they fit?
• My circuit breaker won't reset. What's wrong?
• What is a split bus electric panel?
• How do I identify a combination AFCI (CAFCI) circuit breaker?
• What does a circuit breaker with a yellow or white test button indicate?
• What is the maximum gap allowed between the front of a recessed electric panel box and the wall surface surrounding it?
• What are the requirements for NM-cables entering an electric panel box?
• Why is a fuse box/panel an insurance problem for homebuyers?
• Why is bundled wiring in an electric panel a defect?
• What is the difference between GFCI and AFCI circuit breakers?
• Why are old electrical components not always "grandfathered" as acceptable by home inspectors?
• What happens when you press the "TEST" button on a circuit breaker in an electric panel?
• What is a Dual Function Circuit Interrupter (DFCI)?
• What is the difference between a Combination Arc Fault Circuit Interrupter (CAFCI) and an Arc Fault Circuit Interrupter (AFCI) circuit breaker?
• What is the difference between "grounded" and "grounding" electrical conductors?
• What does it mean when a wire is "overstripped" at a circuit breaker?
• Why is an old fuse panel dangerous?
• Who is the manufacturer of those "bad" electric panels?
• Why is the circuit breaker stuck in the middle?
• What is a double tap at a circuit breaker?
• What is the right electric wire size for a circuit breaker in an electric panel?
• What is the life expectancy of a circuit breaker?
• My circuit breaker won't reset. What's wrong?
• Why do some breakers in my electric panel have a "TEST" button on them?
• What is the right size electric panel for a house?
• What do I need to know about buying a whole house surge protector?
• What is the maximum allowed height of a circuit breaker (OCPD) above the floor?
• What is the maximum height you can mount an electric panel above the floor?
• What is the code required clearance in front of an electric panel?
• What is the main bonding jumper and where do it find it in an electric panel?
Visit our ELECTRIC PANELS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.
Source: https://www.howtolookatahouse.com/Blog/Entries/2019/7/what-is-a-backfeed-main-breaker-at-an-electric-panel-1.html