Sizing Cables for a Xfmr 1

[Redskinsdb21]

Hello,

I have a 13.8 Kv-480/277 V, 1000 KVA Xfmr.

Using S = V*I*sqrt(3) I got:

I available on primary side = 41.84 Amps

I available on secondary side (Using 277 Volts) = 2,084 Amps

My questions:

1) Do I simply take 41.84 Amps (I on primary side) and multiply by 1.25% = 52.3 Amps and choose a cable with at least this Amp rating to provide a safety factor? Then do same for secondary side?

2) Is it best to use 480 V or 277 Volts on secondary side when calculating Xfmrs rated amps capability on that side? Above I calculated 2,084 Amps but 480 would only provide 1,202 Amps. Which is best to use? Would'nt you want to provide the ability to use either voltage level?

 

[wcaseyharman]

I don't think that jives with the NEC.
Disclaimer: I am no means an NEC expert. I'm sure there are others on this forum that can speak to this topic much better than I can.

240.4 states:
"Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.14, unless otherwise permitted or required in 240.4(A) through (G)."

Overcurrent is defined in Article 100 as:

"Any current in excess of the rated current of the equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault."

From my reading, the NEC requires the conductors, unless specifically exempted in the subsequent parts of 240.4, to be protected for both overload and short circuit current, and per 240.21, generally the protection is done at the point of supply. The sizing for feeders and branch circuits in 215.2 and 210.19 is the minimum size and may have to be upsized due to other constraints including overcurrent protection. There's a list of exceptions to this requirement in table 240.4(G), the major one for my work being conductor sizing of motors, which is covered in article 430. I can't find a section where transformer primary conductors are exempted from the general requirements of 240.4 (unless they fall under the "tap" exemption in 240.4(E)) and thus would need to be upsized if the overcurrent protection setting of the transformer were increased.
Often on larger equipment I'll see molded case circuit breakers with adjustable instantaneous trip units, or draw-out breakers with LSI (long-short-instantaneous) trip settings, so just the instantaneous portion of the trip curve can be increased to ride through the inrush current, which would not require an increase in conductor size as the thermal or "overload" portion of the trip curve remains the same.

One quick comment - you probably saw this but the tables in 450 are maximums.

Just my $0.02

 

[waross]

As with many things, the definition of over-current has changed over the years.
But, cable ampacity, 125% of rated current.
The protection setting, by code, depends on a number of factors:
Transformer voltage,
Transformer %impedance.
Fuses, breakers, dual element fuses,
Overload protection on the secondary (not over-current) is allowed under the Canadian Code.
Over-current (short circuit protection) must still be provided for the Primary conductors.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Redskinsdb21]

As with many things, the definition of over-current has changed over the years.
But, cable ampacity, 125% of rated current.
The protection setting, by code, depends on a number of factors:
Transformer voltage,
Transformer %impedance.
Fuses, breakers, dual element fuses,
Overload protection on the secondary (not over-current) is allowed under the Canadian Code.
Over-current (short circuit protection) must still be provided for the Primary conductors.

Appreciate feedback. I now see where I was getting confused. I was thinking the percentages in NEC Tables 450.3(A) & (B) (2000 NEC) were minimums. I guess I was thinking they were minimums to keep the protective devices from tripping when inrush occurs.