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Why does a power sub panel have to have separate neutral and ground bars other than code satisfaction?

Does a sub panel need a main breaker if there is one at the main?

Also, does the sub panel need a ground wire connected back to the main panel in the same building?

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Bonded ground/neutral

If you have the neutral and ground bonded at a subpanel, then you'll get neutral return current through the ground wire back to the main panel (since there are now multiple paths). Even worse, as @Tester101 points out, if the neutral ever has a fault, everything will continue to work but you'll have all the current on the ground, which also means that you can now be electrocuted by touching the panel chassis, for example.

The ground and neutral must be bonded only at one place (in the main panel) to avoid this.

Subpanel main breaker

A main breaker on a sub-panel is not necessary because this is in the same building (if you are in a different building then NEC 225.31, 225.32, 225.33 apply). That said, having a main breaker in the sub-panel is also acceptable.

For whatever reason (economy of scale, I guess) "main" panels are often sold cheaper. I recently purchased a small 12-breaker panel (which included a main breaker and a couple 15A breakers) for almost half the price of a similarly-sized sub-panel (which didn't include any breakers). It makes absolutely no difference to wiring, you just have to be sure to take out the ground/neutral bonding screw/bar (if pre-installed).

Subpanel ground

The sub panel must absolutely have ground back to the main panel, and it must be appropriately sized for the current rating of the panel (as in, it's the same as or larger size than the hot/neutral wiring). This is regardless of being in the same structure or not.

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    If you have the grounding and grounded (neutral) bonded at the second panel, you will have current on the grounding conductor. If for any reason the grounded (neutral) drops out, all the current will be on the grounding conductor. – Tester101 Nov 12 '14 at 15:50
  • If the second panel is in a seperate building, the main breaker satisfies the disconnect requirement. – Tester101 Nov 12 '14 at 15:54
  • @Tester101 wouldn't a GFCI in the sub panel prevent that? – ratchet freak Nov 13 '14 at 8:31
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    @ratchetfreak prevent what, the current on the grounding conductor? No. The grounding and grounded conductors are bonded at the main panel. If they're also bonded at a second panel, any current on the grounded conductors of the panels branch circuits will use both the grounded and grounding conductor to return to the main panel. A GFCI breaker in the main panel on the feeder to the second panel, shoulda/woulda/coulda trip if the grounded and grounding conductors are boned at the second panel. This is because some of the current will be returning on the grounding conductor. – Tester101 Nov 13 '14 at 10:49
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As stated above current flows from the hot wire through the device and returns on the neutral wire. GROUND is just that GROUND! It is used for one thing and one thing only!!! To provide a short to trip the breaker. I asked the electrical inspector who tagged me on the first time I installed a sub panel in my home. I am not an electrician but understand the concept... The inspector could not answer my question of WHY??? He said because code says you can't! OK...

After careful consideration it came to me WHY! OHMS LAW! The further from the ground bar in the panel the more resistance in the white wire. The further you go the higher the voltage potential on the white wire gets. Ground is not covered in most cases. If you touch the white wire after the lamp you will get a jolt. When you touch the wire at the ground lug you won't.. Just OHMS law. Maybe this will help you understand why ground should not be hooked to white at any place but the ground bar in the main panel. KEEP GROUND GROUND!

  • I see what you're saying but when you apply Ohms law, it doesn't seem that significant... 100 ft of 14 gauge wire has about .25 ohms of resistance. If you run 20 amps through it, that gives a voltage drop of around 5 volts (ignoring connector resistance). If you bonded the ground and neutral at the panel, that would cut the resistance back to the main panel in half, so there'd be around 2.5 volts on the wire. The real problem is if something severs the neutal+ground and leaves the hot still connected, in that case the bare ground would be energized with nearly the full voltage of the circuit. – Johnny Jul 18 '17 at 1:39
  • Well after reading what you said again.. DUH... YES if the ground and neutral were cut... the distant end that is no longer ground (its tied to neutral and BARE) would try to use you as the current loop. THAT WILL CURL YOUR HAIR>... I was only considering that the ground itself would be carrying a current and that current would be greater the further away from the ground lug. What you say is very valid! Thank you! – Tony Jul 18 '17 at 5:43

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