2023 NEC applies
Installing a new 500 kVA transformer (480-208Y/120V) to feed two (2) stand alone 2000A, 208V switchboards
The trip plugs are rated 2000A and the manufacturer (Siemens) no longer makes the plug.
A load study was conducted per NEC 220.87 on the 208V switchboards and concluded that the peak combined load was ~150kVA. In discussions with the owner, we agreed 500 kVA was an appropriate size to accommodate existing loads plus significant future loads (if any).
Photo is attached of the system.
The primary side is protected by an 800A breaker
Per table 450.3(B), the primary side is protected by max of 125% (601A primary x 125% = 752A, using next size up is 800A per Note 1 in that table)
Using the primary only protection method, this table indicates we don't need any secondary protection.
However, going to 240.21(C) - Transformer Secondary Conductors, I'm thinking to use (6) - Secondary Conductors Not over 25' which has 3 sub requirements:
1. Satisfies: Secondary Conductors (1680A rated at 75°C) is great than the ratio of primary to secondary voltage x 1/3 OCPD of primary side (800A) =616A
2. Unsure: I'm going to two pieces of equipment from the secondary, so it's not a single set of breakers. But maybe I'm reading too much into it in that each set must terminate to a single breaker. I have 2 sets of cables each going to a dedicated breaker (2000A)
3. Satisfies: Physical Protection required - cables are in RMC conduit
240.21(C) starts by saying you can't use the next size up rule, so would it need to be 2000A+ rated cables to the equipment?
Thoughts/leaning this way: Per 240.21(C)(6), I don't think I even need secondary protection, but since I have the 2000A mains downstream, I need to bump my cable sizes up to 2000A to satisfy all sections of 240.21(C).
Is there any other Article that would supersede or concepts to consider?
Installing a new 500 kVA transformer (480-208Y/120V) to feed two (2) stand alone 2000A, 208V switchboards
The trip plugs are rated 2000A and the manufacturer (Siemens) no longer makes the plug.
A load study was conducted per NEC 220.87 on the 208V switchboards and concluded that the peak combined load was ~150kVA. In discussions with the owner, we agreed 500 kVA was an appropriate size to accommodate existing loads plus significant future loads (if any).
Photo is attached of the system.
The primary side is protected by an 800A breaker
Per table 450.3(B), the primary side is protected by max of 125% (601A primary x 125% = 752A, using next size up is 800A per Note 1 in that table)
Using the primary only protection method, this table indicates we don't need any secondary protection.
However, going to 240.21(C) - Transformer Secondary Conductors, I'm thinking to use (6) - Secondary Conductors Not over 25' which has 3 sub requirements:
1. Satisfies: Secondary Conductors (1680A rated at 75°C) is great than the ratio of primary to secondary voltage x 1/3 OCPD of primary side (800A) =616A
2. Unsure: I'm going to two pieces of equipment from the secondary, so it's not a single set of breakers. But maybe I'm reading too much into it in that each set must terminate to a single breaker. I have 2 sets of cables each going to a dedicated breaker (2000A)
3. Satisfies: Physical Protection required - cables are in RMC conduit
240.21(C) starts by saying you can't use the next size up rule, so would it need to be 2000A+ rated cables to the equipment?
Thoughts/leaning this way: Per 240.21(C)(6), I don't think I even need secondary protection, but since I have the 2000A mains downstream, I need to bump my cable sizes up to 2000A to satisfy all sections of 240.21(C).
Is there any other Article that would supersede or concepts to consider?
