National Electrical Installation Standards

Standards as High as Your Own

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Tuesday, April 24, 2018

Question:

For a system can be supplied either strictly from a utility source or strictly from a generator, the 2014 NEC code specifically addresses where to ground the generator neutral based on using either a 3 pole transfer switch or a 4 pole transfer switch. However, each of these scenarios appear to assume that there are neutral loads and therefore, a neutral will be ran from both the utility service entrance equipment to the transfer switch and from the generator to the transfer switch. If a 3 pole transfer switch is used, the generator neutral is not grounded at the generator but is ran to the transfer switch where it is solidly connected to the neutral from the utility service entrance equipment (i.e a non-separately derived system). If a 4 pole transfer switch is used, the generator neutral is grounded at the generator and both the generator neutral and the utility service neutral are connected to the 4th pole of the transfer switch where they are switched (i.e. a separately derived system). However, where in the code does it address what to do when there are no neutral loads and therefore, there is no need for the utility neutral to continue beyond the service entrance equipment or for the generator neutral to extend beyond the generator? The code seems to imply that there must be a neutral wire between the generator and the utility service entrance equipment, but I can't find where it specifically states this. If you have a 4 pole transfer switch and you ground the generator neutral at the generator, this appears to be the definition of a separately derived system. However, in this situation, if you do not run a neutral from the utility service to the 4 pole transfer switch or from the generator to the 4 pole transfer switch, is this a code violation? If this is not a code violation, would it become a code violation if you replaced the 4 pole transfer switch with a 3 pole transfer switch under this situation (i.e. no neutral loads - therefore, no neutral from the generator or the utility service entrance equipment)? In each of these situations, please assume ground fault protection/indication has been addressed when operating solely off the utility and solely off generator.

Thanks, Chris D.

A

Answer:

Hey Chris thanks for your question. The definition of Separately Derived System in Article 100 states that to be one there is not a direct connection to circuit conductors of any other electrical source - ignoring grounding and bonding connections. As you mention for three phase systems if a 4-pole transfer switch is used between a generator set and service conductors the generator is installed as a separately derived system and 250.30 applies. System grounding requirements are in 250.20 and 250.26. If a 3-pole transfer switch is used and if there is a grounded (typically neutral) conductor that is directly connected to a service grounded conductor then the system (generator set) is not a separately derived system. In that case 250.30 does not apply and 250.24(A)(5) prohibits re-grounding grounded conductors on the load side of the service. The generator supplied system is grounded by being directly connected to the service grounded conductor and reconnecting it to ground (and all of the other normally non-current carrying metal parts) would create paths for objectionable current in violation of 250.6.

There are advantages and disadvantages for both non-separately derived systems and separately derived systems. Some have strong feelings on both sides and the NEC does not mandate one over the other but it provides rules for both.

For a grounded service a grounded conductor must be installed from the utility transformer to the service equipment as stated in 250.24(C). This provides an effective ground fault current path - whether or not there is any neutral current. If a system supplied from a generator is grounded but there is no grounded (neutral) conductor installed a 3-pole transfer switch can be used and that system is separately derived because it is not directly connected to a service grounded conductor. That is not a NEC violation if effective ground fault current paths are provided by equipment grounding conductors or supply side bonding jumpers.

It is confusing and hope this helps, if not send in a follow up question.

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