National Electrical Installation Standards

Standards as High as Your Own

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Friday, June 19, 2020

Question:

Comment on Code Question of the Day for May 12, 2020:

Good morning,

This is a follow up on the follow up. I couldn't let his statement go as fact, "Was told to was due to no overcurrent protection outside of the 13,200-volt primary fuse on utility step down transformer, Tom Grabowsky".

There is a misconception among many electricians/inspectors/engineers that the fuse in front of the utility transformer will provide protection for the service entrance conductors, IT WILL NOT! The primary fuse is to protect the primary that feeds the transformer but not the transformer. The intent is to keep the lights on for the neighborhood in case of a fault close to or in the transformer. Most electric utilities fuse their transformers such that it takes 300% or more current to open the time delay fuse. In other words, your service entrance conductors will likely burn open before the utility fuse opens.
 

Take care and stay safe,

Charlie Eldridge
A

Answer:

Good Morning Charlie,

Great to hear from you. Hope you are safe and well during these challenging times. Excellent point about the fuses on the primary side of utility transformers. You explained the purpose of that very well. Continuity of service. Good information for electical workers to be aware of regarding the supply side of the service disconnect and overcurrent protection.

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