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Scott:
Could you please explain the methodology for a MOCP of 1200 amp, on three (3) sets of 500KCmil, with 75 degree terminations. 500 KCmil Cu is 380 amps, which is compliant for 400 amp OCP. 2 sets would be compliant for 800 amp OCP, but 3 sets only total 1140 amps, and being >800 amps, a lower OCP is required, right?
Refer to Table 450.3(B), see note #1.
300 KVA Xformer - Secondary FLA (nom.) = 830 Amps.
Secondary Feeder Protection: 125% of FLA
Max. OCPD - Secondary Feeders: 1038 Amps.
Note #1 allows next higher OCPD rating, or 1200 Amps.
If the designed load from this Transformer was only 800 Amps, we could have used 3 sets of 400 MCM THHN CU. (335 Amps x 0.8 = 268 Amps; x 3 Sets = 804 Amps Max.), and still use the 1200 Amp OCPD for Secondary Feeder Protection.
I would not choose this design parameter, but it is compliant.
In fact, I would only choose the (3x) 500 MCM with 1200 Amp OCPD if the design LCL load was >80% of the Secondary FLA rating, and there were Motors > 5 HP started across the line from this Transformer (208V 3 Phase Motor > 16.7 FLA - 100.2 LRA).
Under normal design conditions, I would opt to use a 1000 Amp OCPD for the (3x) 500 MCM Secondary Feeders' Protection.
With the exception of one job, with one transformer, in my life as an EC, all transformer installs had OCP on primary and secondary. It was a matter of design and choice, mot NEC compliance.
I have designed + installed Transformers with OCPD for Feeders on both sides (Primary & Secondary), and for Primary Feeder Protection only - where the Secondary (SDS) is only One Voltage.
i.e. 480V x 240V 3Ø 3 Wire Delta...
When using Primary Feeder Protection only, the Primary Feeder's OCPD have been no more than 125% the rated Primary Current - as the only Transformers I have dealt with (design-wise or installed) were rated 9 Amps or larger.
If there is an advantage to run the Primary side "high", I will go with Primary + Secondary Protection - so the Primary Feeders may use an OCPD sized 250% the FLA.
Kind of like a Motor Circuit...
The RFI response sounds like the designers investigated your question and found that they fouled up the design...and they're grasping at straws to avoid a big change order.
I could imagine the environment around that EE Firm, when they reviewed the 1 Line -vs- the RFI!!!
After the initial "We can't be wrong" CYA statements ended, there would be a long series of "D'OHHH!!!" statements!!!
They (the EE Firm) should be greatful that the OP (matt b) caught this design issue and issued an RFI / RFC prior to performing the install per revised plan!
Currently there is one C/O (Change Order) for the original revisions,
then an addendum C/O for the revised-revisions
The C/O list could have been really painful! (for the EEs, not the EC!)
This post serves as a great example scenario to anyone in the field + Project Management & Designers.
In-depth review of ANY Electrical Design criteria should be done, so as to reduce Change Orders and impacting the Project Schedule.
This benefits the Client, which also benefits the Construction & Development teams (Electrical Contractor is part of C & D teams).
How much "fun" is it to submit a request for Change Order, and get approval to proceed within a few days!
Then to do so without impacting the critical path (Project's Schedule), due to the fact that the C/O issue likely will delay another trade by days / weeks / months!!!
Scott
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