NEC 517.30(B)(4) compliance question

[Jon Lee]

Hello all, long time reader, first time posting.

I have a surgery center that I am inspecting, it is in the process of being expanded. It has two transfer switches, each one has a mixed/essential systems load. Due to the expansion the load on the essential systems exceeds the 150kVA allowed for a single transfer switch. My interpretation is that this does not comply with 517.30(B)(4), however the project engineer interprets it as complying, due to the fact that both transfer switches have a load NOT exceeding 150kVA. My position is that the entire essential systems load cannot exceed 150kVA, not just the load on a single transfer switch.

Thoughts?

Thank you for your time, this is a great forum.

 

[iwire]

Wow, the more I read this the less I understand it. :huh:

One transfer switch shall be permitted to serve one or more
branches or systems in a facility with a maximum demand
on the essential electrical system of 150 kVA.

 

[GoldDigger]

Hello all, long time reader, first time posting.
My position is that the entire essential systems load cannot exceed 150kVA, not just the load on a single transfer switch.

Thoughts?

Thank you for your time, this is a great forum.

in a facility with a maximum demand
on the essential electrical system of 150 kVA.

I agree with you on that.

 

[LIM]

Hello all, long time reader, first time posting.

I have a surgery center that I am inspecting, it is in the process of being expanded. It has two transfer switches, each one has a mixed/essential systems load. Due to the expansion the load on the essential systems exceeds the 150kVA allowed for a single transfer switch. My interpretation is that this does not comply with 517.30(B)(4), however the project engineer interprets it as complying, due to the fact that both transfer switches have a load NOT exceeding 150kVA. My position is that the entire essential systems load cannot exceed 150kVA, not just the load on a single transfer switch.

Thoughts?

Thank you for your time, this is a great forum.

You are correct. Obviously this designer has never done any healthcare work to believe his own statement.

 

[MasterTheNEC]

Hello all, long time reader, first time posting.

I have a surgery center that I am inspecting, it is in the process of being expanded. It has two transfer switches, each one has a mixed/essential systems load. Due to the expansion the load on the essential systems exceeds the 150kVA allowed for a single transfer switch. My interpretation is that this does not comply with 517.30(B)(4), however the project engineer interprets it as complying, due to the fact that both transfer switches have a load NOT exceeding 150kVA. My position is that the entire essential systems load cannot exceed 150kVA, not just the load on a single transfer switch.

Thoughts?

Thank you for your time, this is a great forum.

The ability to have 150 KVA or less applies to the use of a single transfer switch with regards to Essential Electrical Systems....over 150 KVA then multiple transformers are required.

 

[steve66]

I'm not sure exactly what loads are "mixed", but "optional" loads do not fall under the exception. If there are "optional loads", they would still require a separate transfer switch no matter what the essential load demand is.