Downsizing a feeder or service neutral is feasible using the calculations in the National Electrical Code found here: 220.61 Feeder or Service Neutral Load. (A) Fundamental Calculation The maximum unbalanced load defined by this article should be the feeder or service neutral load. The minimum unbalanced load required by other articles of this code shall not be reduced without first determining that this will not cause overloading of either the primary circuit or any parallel circuits.
In other words, you can't reduce the size of the neutral without also reducing the size of at least one of the hot wires. If this reduces the total current flowing through all three conductors to less than what's permitted by the cable's ampacity rating, then it's okay to do so. However, if doing so would increase the current on any single conductor beyond what's allowed, you need to either replace the cable with one that supports the increased load or move some other load off the now-underloaded wire.
The reason we need to make sure that doesn't happen is because overloads on any conductor could cause damage to your house. The best way to avoid this problem is to follow the feeding pattern specified by the manufacturer and use only those outlets that are supposed to get power from each feeder line. But if you must downsize the neutral, be sure to also reduce the size of at least one of the hot wires to keep current within safe limits on all four conductors.
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The Consequences of Neutral Loss In the case of a neutral failure, the single-phase voltage will climb to the three-phase level, exposing your equipment to > 400V rather than 230V. This results in an overvoltage, which can be fatal to your equipment. The best protection against neutral failures is to use double-neutralized power systems. These are available for any size application from small residential installations to large factories and warehouses.
Double-neutralization means that each phase is carried on a separate neutral. If one neutral gets damaged, it does not affect the other two phases. Double-neutralization also protects you against reverse current problems. If something shorts one of the neutrals to ground, the other neutrals still have enough resistance between them to prevent any significant flow of current.
Double-neutralization reduces your risk of having a neutral failure because there are now two sets of neutrals instead of one. It also keeps your house safe in case of a leak in one of the phases or paths. The remaining neutral can continue to provide power even if one of the phases is bad so you don't need to switch over to another source of electricity.
You should always use double-neutralization unless you have a good reason not to. For example, if you're only using three-wire power distribution panels and aren't planning on adding more circuits later, then you don't need to use double-neutralization.
Here are some tell-tale signals that it's time to downsize:
Dropping a neutral indicates that the grounded conductor of an alternating current system is no longer electrically and physically linked to the system. The cable itself remains intact, but there is now a voltage difference between the two ends of the cable because one end is connected to ground and the other end is not. This could cause damage to equipment that relies on a neutral for a power source.
If you drop a neutral from a house wiring system, have someone else's live circuit contact you. If you are working alone, use caution not to be contacted by another person or device that has a live circuit. Also be sure to call your local electric company immediately after dropping a neutral to make sure that no one has been hurt due to this incident.
If you're working outside you should take the same precautions as if you were inside the home. Always wear protective clothing (gown, gloves, safety glasses), carry protective equipment (utility knife), and follow all safety instructions printed on your utility's handbook or website.
In conclusion, dropping a neutral is never fun, but it can be avoided by careful planning before starting work on any electrical project. Be sure to consider how you will connect all parts of your system, and always use the proper tools for making connections safely.