by William Collins

Created on: December 09, 2009

The National Electrical Code (NEC) requires that all sub-panels need to have an isolated neutral conductor. How does this affect your home and safety? Are you in compliance with this code, and does it matter? Read on and find out if your house is in violation of this safety code.

AC IN YOUR HOUSE
Electricity enters your home through a main circuit panel, this is where the "Main Disconnect", the large 200 amp circuit breaker that shuts off all the power to your house, is located. In this panel are also, the circuit breakers that supply all of your lights and appliances.

WHEN DO I NEED TO ISOLATE THE NEUTRAL?
Let's say that you want to do some remodeling, and add a new office in your home. You want to have a nice tall metal cooler drinking fountain, a toaster oven, and a small refrigerator (you want a well stocked office). All of these appliances will need to be connected to circuit breakers.

To accommodate the new appliances, you will need to add another circuit breaker panel, this panel is called a "sub-panel". It gets its power from the "main panel", and must have a isolated neutral.

WHY THE CODE?
The reason NEC 384-20 states that you must have an isolated neutral in sub-panels is one of safety (as most codes are).

AC CIRCUIT WIRES
An alternating current (ac) circuit has 3 wires:
- the black (hot) wire,
- the white (neutral) wire,
- and the green (grounding) wire.

HOW ELECTRICITY TRAVELS
To illustrate the travels of electricity, we'll use your office drinking fountain as an example.

Power travels from the main panel circuit breaker, through the black wire, through the sub-panel circuit breaker, to the fountain compressor motor. Nothing happens until the compressor motor kicks on, then the current does its work through the motor, goes out the white wire and back to the "main panel" ground connection.

Then it reverses and travels the same route only backwards: through the white wire, motor, black wire, sub breaker, black wire, and back to the main circuit breaker. It does this 60 times per second (60 Hz).

REASON FOR ISOLATING THE NEUTRAL
Let's suppose the white wire from the "main panel" to the "sub-panel" was accidentally cut. What would happen?
If the sub-panel is isolated - white wire not connected to the green wire or to the circuit box - then the cooler motor would stop working; it has no return path.

If the sub-panel is not isolated (white and green wires connected), however, the current will travel through the motor, white wire, to the sub-panel, as usual,

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