National Electrical Code 2014

Article 250 Grounding and Bonding

II. System Grounding

250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).

(1) General. The grounding electrode conductor connection shall be made at any accessible point from the load end of the overhead service conductors, service drop, underground service conductors, or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.

This means that the grounded (neutral) from the service must be connected to ground, and that the connection can be made by bonding the neutral bus bar to the grounding electrode.

(5) Load-Side Grounding Connections. A grounded conductor shall not be connected to normally non–current carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article.

This means that the grounded (neutral) conductors should only be grounded at the main service disconnnect.

If the main service panel happens to be the same place that the grounded (neutral) conductor is bonded to the grounding electrode, then there is no problem mixing grounds and neutrals on the same bus bar (as long as there is an appropriate number of conductors terminated under each lug). If the two bus bars are not connected; as would be the case anywhere other than the main disconnect (exceptions exist), then you cannot mix them.

Notice how the grounded, and grounding bus bars are connected in the main service panel. This means that; electrically speaking, they can be considered a single bus bar. Which means that both grounded (neutral), and equipment grounding conductors can be terminated on either bus bar.

In the subpanel, the bus bars are kept separate. So grounded (neutral), and equipment grounding conductors cannot be mixed.

First I would check the electrical connections to the compressor and the breaker in the panel, see that they are tight and not corroded. Next I would measure the starting current of the compressor with a clamp on ammeter. I had a problem like this on a friends AC, only his problem was tripping the breaker at times. His compressor is about 10 years old, and is harder to start now. When I measured the starting current it measured 175 Amps! Which on this compressor is the locked rotor current! We replaced the starting capacitor and added a "Hard start kit". Now the starting current is a respectable 50 Amps.

Some tech info: The induction type motors used in these compressors have an extra motor winding. This is called a "start" winding and is used with the "start capacitor" it is there only to get the motor to initially turn and a mechanical switch disengages it at a low speed. What happens over time is that the motor load is increased due to wear, and the start capacitance becomes weaker. So why not just build a compressor with a large start up capacitance to begin with? Some higher end units do just that, but most don't to save $$$. Contrary to popular belief, the start capacitor size does not "Give a bigger jolt" to the motor, but delays the 60 cycle feed to the second winding fooling the motor to think it is driven by a poly phase feed. The larger the capacitor value, the bigger the delay - hence the lower the starting current. Without a start winding, the motor will sit there humming until it trips the breaker. Three phase motors (poly phase) do not have start windings because they have three windings.