Originally Posted By: Bob Badger
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OK I just noticed the question was Why service panels do not need isolated neutral bars.
I answered why sub panels need isolated neutrals.
If you read above you may have figured it out already.
If we did not connect the neutrals to the grounds at the service panel we would not have what is called an adequate fault clearing path.
Lets say the neutrals and grounds where kept separate at the main panel and somewhere in the house a 15 amp 120 volt plug circuit has a fault from the hot conductor to the grounding conductor.
The fault current would flow through the ground wire and out the ground rod "trying" to get back to the POCO transformer.
And it will do that BUT the combined resistance of the ground rod at the house, the earth itself and the POCO ground will be to high to trip the breaker.
It is hard to say what the total resistance would be it changes depending on how wet the soil is.
But lets use 25 ohms as that is what the NEC would like for a ground rod (25 ohms or less)
120 volts / 25 ohms = 4.8 amps of current flow.
This will not even warm a 15 amp breaker never mind trip it.
By attaching the neutrals to the grounds at the service panel you create a low resistance path back to the transformer.
Ground rods are for two things High voltage utility company faults and lightning.
They have nothing to do with the day to day operation of a the house wiring.
If one of the POCO high voltage lines breaks and falls across the overhead service drop to the house it may try to energize the bare neutral ground conductor on that overhead feed.
Your house wiring will not like 13,800 volts, if this happens the ground rod will help.
13,800 volts / 25 ohms = 552 amps which will be about 50 times more current then the POCOs high voltage fuse, this will cause the fuse to open ASAP hopefully before the romex bursts into flame. ![icon_eek.gif](upload://yuxgmvDDEGIQPAyP9sRnK0D0CCY.gif)
Bob (AKA iwire)
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