Originally Posted By: rpalac
This post was automatically imported from our archived forum.
Hi Gals and Guys,
(is that proper…I hope)
There are several ways that a sub panel can be wired. The wiring coming from the primary panel can be run from the same bus bar connections if they are properly rated for it. Most of the time the wiring from the primary panel is fed from a breaker suck as a 220volt 60amp or what ever your wiring and sub panel are rated at. (220v 100amp, 440v-200amp,on,and on, and on)
The wiring feeding into the second or correctly said, sub-panel, can also be landed the same way. It can either be connected to main lugs if you have them, a main breaker if so equipped or back fed thru a breaker making it the acting main. (The breaker doesn't care which way the voltage and current are coming it still reacts the same).
The grounding is different.
The primary or main panel has a bonded neutral and ground. This just means that they are both physically connected together by some means. Many panels have a bonding strip of wire or a bonding screw that physically goes through the neutral buss bar and connects to the grounded can of the panel. Some panels actually have a bar that does the same operation of connecting the ground bar to the neutral bar.
[That by the way is the definition of a bonded ground.........when the grounding bar (a.k.a. ground bus) is connected with the grounded bar (a.k.a. neutral bus bar) by a conductor].
The sub-panel or all sub panels are to be isolated from this method.
This means when you land each branch circuit into the sub-panels they have to have all of the grounding conductors (bare wires) landed on the grounding bus bar.
When you land all of the neutrals (grounded conductors-white wires) they are to be landed on a separate bus bar called the grounded or neutral bus.
You should now realize there is a difference of grounded and grounding.
There is a reason for this that is lengthy in explanation and has to do with current, impedance and EMF. Basically you can over heat the line serving the sub panel and burn up the service under the right conditions.
The way you can tell if the neutral bus is bonded or not is primarily a visual inspection. Many manufactures supply a screw that goes through the neutral bus bar and physically connects to the frame or can of the panel. Since the can is directly connected to the grounding bus you intern make a bonded connection.
You can electrically test for this as well but much more involved on a wired panel.
Gosh I hope this helped a little.