can I run my sub panel from lugs in main breaker panel

Question

I need to run a sub panel box right next to the main panel box to add am40 amp double pole breaker for a range.

I have seen where u can use say a 60 amp breaker and have the wiring coming from the breaker into the subpanel.

Here’s my main question. The main panel has 2’large lugs To receive the main feeds. However the feed or line wires are not attached to them but rather a large double pole breaker that covers Both buses. When I turn the breaker off the lugs Also lose power since the power is coming into box thru the breaker. So could I just tap into the lugs and go into my sub panel that way since it is protected by the main breaker.

Answer 1

OK, so you have a let's say 125A electric service. The main panel has a bus/lug rating of somewhere between 125A and 200A. It doesn't have a main breaker. It has main lugs. The "main breaker" is a regular breaker located where branch circuit breakers would go, except it is 125A (in this example), has a tie-down kit, and labeling that says "Main Breaker".

OK, we can work with that.

Yes, you can come off the main lugs and go to a sub panel. However

• The feeder wires must be at least as big as the service wires (125A in this example).
• The sub panel's bus rating must be at least the service size (125A in this example).

If you're too cheap to do that, then feed the sub panel the normal way with a breaker in the main panel.

But before you go "I ain't buying all that copper" remember to forget what you think you know about aluminum. Aluminum heavy feeder is Good Stuff, and in fact the lugs on both ends are made of aluminum. 1/0 aluminum oughta do 'er for any service size up to 125A. (and back-feed breakers larger than 125A are unlikely).

Answer 2

I know that some panels have feed through lugs that are specifically designed for that purpose. I don't know if the main lugs of a panel using a backfed breaker can be used in the same way or not.

Assuming (big assumption - that needs to determined for the specific panel) that they can be used that way:

• The wire connecting the main panel to the subpanel must be rated for the same (or more) current as the main breaker. There are some exceptions - for example as I understand it, no subpanel feed wire ever has to be larger than the main feed wire, so certain adjustments allowable for the main feed wire can also apply here where they don't apply to ordinary subpanel feeds. But this means that if you have a 100A main breaker then the wires will need to be much larger than would be needed for a 60A subpanel. The cost of the wire is not a problem for a few feet. But bending the wire and piecing everything together may be a little tricky.
• Adding a new 40A load means a new load calculation is likely in order. With a backfed main breaker I'd be a little extra concerned as that tends to be a smaller feed then a regular main breaker. The load calculation relates to both the panel size (as defined by that main breaker) and also to the utility service size.
• Adding a subpanel to add a 40A double breaker seems a bit extreme. Which points to the panel being full. Which along with the backfed main breaker is an indication that you may have a relatively old panel. Old panels are not inherently bad, except when they are (certain specific brands/types are known hazards). Installing a huge subpanel that can actually become a new main panel is worth considering, depending on the size, type and condition of the existing main panel.

Answer 3

This falls into the NEC category of "tap rules". Google it. It's very specific but would be code compliant in your situation. In a nutshell: You can have a tap, but it really needs to be compliant. In a nutshell, esp, with such a short run, as long as the wires are 20% of the ampacity of the supply AND they are protected by a down stream breaker (IE, the one in the sub-panel), you should be good. BUT and here's the big BUT inspectors are not often familiar enough with tap rules. I had to install a small sub in a pump house/small building and the inspector called me out on it. So I had to read him chapter and verse from the NEC to get him to pass it. In my case the tap was 20% of the feeder, less than 10', enclosed in conduit and protected downstream from the feeders via a breaker in the sub.

Answer 4

If the new panel is a "main lug only" then the rating of the sub-panel and wire feeding it will need to be rated for no less than the rating of the main breaker protecting the panel that feeds the new panel.

If the new panel is rated for lower amperage than the main panel then overcurrent protection (a main breaker) that is not larger than the rating of the sub-panel will need to be used in the sub-panel. Normally wire needs to be sized to handle the rating of the overcurrent protection feeding the wire, but there are exceptions in NEC 240.1 that are called Tap Rules. If the wire length is less than 10', protected by conduit terminating in a single breaker it is allowed to be rated for not less than the Section 220 rated load or 10%. So the short sighted answer is you would be allowed to mount a 60A rated panel and feed it with #6 THHN copper wire in 3/4" metal conduit, but realistically you should probably mount a 100A panel fed by #3 (or larger) copper wire in 1.25" (or larger) conduit.