Cable Ampacity Question & Number of Cables/Phase (NEC 310)

[milwaukeebob]

Hello,
I have a follow-up question to a thread I started last week titled "Dry-Type Transformer Secondary Cable Sizing". A colleague and I have engaged in another small debate regarding the actual number of cables per phase that need to be installed on the secondary of the transformer.

In short, I have calculated a worst case transformer secondary current of 3551 amperes (occasional, short-time over-load conditions). The transformer has a nominal secondary voltage of 540V. While this would be an ideal application for non-segregated phase bus duct, we have to install DLO 777MCM cabling in cable tray for reasons I will not elaborate on. The calculated derated ampacity for this DLO cable in the enviroment we plan to install is 587 amperes. Using six (6) 777MCM cables/phase results in a total derated ampacity of 3522 amperes which is 29 amperes short of the calculated worst case transformer secondary current. My initial response was that we need to install seven (7) 777MCM cables/phase but one of my colleagues felt there were provisions in NEC to allow for the use of 6. Other then NEC 310.15(C) allowing you to calculate conductor ampacities using a formula (which is not practical in our situation), I have not found any other provisions.

Although I haven't done alot of these type calculations, I've never come this close (29 amperes). That's less then 1% of the total, worst-case current flow. Normally, I wouldn't question installing the 7th cable but we may have some challenges with all these terminations on one side (still being reviewed). I understand a 7th cable/phase means all the cables running at cooler temperatures over the life of the installation which means extending the life and performance of the cable. From a technical standpoint though, I also know there are some margins built in to the NEC 310 temperature, raceway derating factors. From that standpoint, it seems like "overkill" to added a seventh conductor.

My question is simply this...given what I've stated above, does NEC dictate a 7th 777MCM cables/phase or is there provision allowing for 6 cables/phase? I want a defensivable position and I don't want to get into a debate with the AHJ.

Thanks in advance for your guidance and help.

 

[dpc]

I'd suggest reviewing Article 240 on Overcurrent Protection, specifically the tap rule covering transformer secondary conductors. The secondary conductors need to be protected by an overcurrent device. The rating of the overcurrent device will determine the conductor size. Above 800 A, the conductor ampacity must be greater than or equal to the overcurrent device rating.

 

[milwaukeebob]

Thanks dpc. I should have made this clear in my original post. This particular transformer feeds a generator excitation system. There is no traditional OCD between the secondary of the transformer and the AC input of the excitation equipment. The primary side is fed from a 1200A, 5kV vacuum circuit breaker with an SEL 50/51 protective relay. From my experiences, this is fairly typical for industrial power plants...GE, Siemens, etc.

The 5kV switchgear, the xfmr which has an occasional, short-time overload rating of 3334kVA (10seconds maximum), and the excitation system are all in very close proximity to one another <20 feet.

 

[dpc]

I understand the setup. I'm assuming there is a main CB for the AC input in the excitation cabinet? Regardless, the conductors must be protected - either from the primary relay settings or by the secondary overcurrent device in the excitation cabinet using one of the tap rules in the NEC. If you are under the NEC, the conductor must be protected.

 

[7anoter4]

In order to preserve the cable ampacity the single core cable running in groups of three phases-in conduit or without in triangle in air-a clearance of 2 group diameters-that means 4 cable diameters-or two conduit diameters, will be kept. It is not a NEC requirement but of IEC 60346-5-52 or DIN/VDE or else.
Also in order to avoid current unbalance- between cables of the same phase- a certain order of cables and phases it is required.

 

[milwaukeebob]

Thanks. Yes there is a CB in the excitation system and of course an OCD on the primary side of the xfmr. Protection will be coordinated accordingly. I've educated myself a little more on this since last week. Also, we do plan the proper phase conductor grouping and have incorporated the proper raceway derating for the installation. Thanks again.