Branch circuits in Subpanels [RE-wrenches]

[Dan Rice]

Ray,

The issue with passing only the line conductors through the conduit nipple
between panels is induced eddy currents within the conduit nipple. The
changing magnetic field(s) in the conductor(s) passing through the conduit
nipple induces an electric current within the surrounding metallic conduit.
If both line and neutral of the given circuit pass through the conduit, then
the opposite effects of each wire's changing magnetic fields cancel one
another out, and there is no net effect on the surrounding conduit. This is
why all current carrying conductors of a particular circuit must remain
together inside metallic raceways. The problem is, the induced eddy currents
in the surrounding conduit can result in the conduit heating up, which I
believe is due to the conduit material's internal resistance to these eddy
currents, which can be rather high. I am sure that there are engineering
calcs that can be performed to ascertain how significant this heating effect
may be based upon the wire size, AC frequency, amperage, conduit size,
material type, length, phase of the moon and which side you part your hair
on.

I'm not sure how you feel about them, but there are new connectors that take
the place of wire nuts. Similar to "backstab" push-in connections to AC
light switches and receptacles, these connectors are small-ish blocks that
you push the stripped ends of solid conductor wire into. I think these
connectors can be purchased with different numbers of holes to accept two to
several conductors. They may help you to save room in the primary load
center as you splice on extensions. I've not used them in place of wire
nuts, and I never back stab receptacles when wiring AC circuits, so I can't
vouch for this approach, but it may be worth considering. It's an
alternative to crimp butt splices, and these connectors are listed for this
type of purpose anyway...

I hope that helps.

Dan Rice
Abundant Sun, LLC

----- Original Message -----
From: "Ray Walters" <walters at taosnet.com>
To: <RE-wrenches at topica.com>
Sent: Friday, October 20, 2006 6:54 PM
Subject: Branch circuits in Subpanels [RE-wrenches]


|
| Hi All;
|
| I have a question related to the grid/tie back up question. When we add
| a sub panel for circuits to be powered by the inverter, we install a
| short 1" to 3 " by 2 " diam nipple to connect the sub panel to the
| existing load center. In many cases the existing branch circuit wiring
| is too short to run directly over to the sub panel, so we wire nut
| extensions to make the run over to the new sub panel. To avoid too many
| wire nutted connections, we have considered just running the hots over
| to the adjacent sub panel, and leave the neutrals and grounds connected
| (as before) to the existing main load center. We run #6 or #4 over to
| connect the subpanel's ground and nuetral bars to the main load center.
| (yes, we observe the "nuetral bonded to ground in only one place" rule)
| Now, I realise code specifies that hots, neutrals, and grounds on branch
| circuits have to be run in the same raceway, and I wouldn't consider
| this, if the load centers were not side by side. What are the potential
| problems here? Basically we're trying to observe another section of the
| code that specifies "professional, workmanship like manner": and wire
| nutting all the hots, neutrals, and grounds to jump 2 ft over to an
| adjacent box looks like a real mess, not to mention the potential
| dangers of all those extra connections crammed into the sides of the
| existing load center.
| Obviously, if its new construction, we can get the branch wires left
| long enough to do this right. However, this is an existing service
| situation. What do you all think or do in these situations? (I've also
| considered crimped butt splices, but IMHO, they are not reliable on
| solid core wire.)
|
| Thanks very much,
|
| Ray Walters


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