Ground Bar vs. Neutral Bar in Panel

I was looking in a residential service panel today. 150A, 30 circuits, 15 per side.
For the circuits that run down the right side of the panel there is a bus bar that holds the ground wires and a separate bus bar for the neutrals. The ground wire to the water meter was attached to the neutral bar.
For the circuits that run down the left side of the panel there is a single bus bar and the grounds and neutrals from each circuit are paired up under each screw.
A strap connects the neutral bus bar on the right side to the neutral/ground bus bar on the left side.
Why are there 2 separate bus bars on one side but a combination bus bar on the other? Aren't they all electrically at the same point? If so, why separate them on one side?
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On 03/21/2014 12:12 AM, DerbyDad03 wrote:

Generally, in the first panel after the meter, the grounds and neutrals may land on the same bar.
For the second panel (sub-panel), the neutrals and grounds must be kept separate.
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Thanks for that information, but it only answers half of my question. This part remains unanswered:
Why are there 2 separate bus bars on one side but a combination bus bar on the other?
In other words, based on your answer, it's easy to wire the box as the first panel because it doesn't really matter where you connect the grounds. However, if the panel was used as a sub-panel then the grounds for any circuit coming into left hand side would have to be run around the box to the ground bar on the right hand side.
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On Fri, 21 Mar 2014 04:12:51 +0000 (UTC), DerbyDad03

There should also have been a green tinted screw or a dedicated strap bonding the neutral bar to the can and grounding bar since this seems to be the service panel.

This is a hazardous practice. A loose or faulty connection could cause the ground to become hot through the neutral with current flow. Most panels allow more than one grounding conductor per grounding bar terminal. Neutral bars sometimes are listed for more than one neutral. I have never seen a panel approved to mix neutral and grounding conductors in the same terminal.

This is commonly done to supply the needed number of neutral connections and usually allow the circuits to be reasonably close to each other.

Think grounding conductors to grounding bar, neutrals to neutral bar but if a service panel the grounding conductors may be terminated to the properly bonded neutral bar.
--
Mr.E

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On 3/21/2014 12:12 AM, DerbyDad03 wrote:

ground/neutral screw?

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wrote:

Some/most grounding bus bars are listed for up to 3 wires but those are only for grounding wires. All neutrals must have their own terminal. (one per screw).

I am not sure why the installer chose to wire the panel this way but the ground and neutral under a single screw is a violation.
It is also a violation to use a bus bar that is only connected to the enclosure for neutral wires but if there is a conductor between that and the bar that the service neutral and grounding electrode conductor connect (wire sized to 250.66) or a strap) it is OK. You can't run neutral current through the enclosure alone.
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On 3/20/2014 10:12 PM, DerbyDad03 wrote:

In addition to what others have said... My guess is that a ground bar was added to give more total spaces to land ground and neutral wires.
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