Conductor sizing

Status
Not open for further replies.

encoreman

Member
Location
Austin, TX
I just got comments back from the electrical inspector:
SHOW COMPLIANCE WITH NEC 2008 ARTICLE 240.21 YOUR SECONDARY TAP CONDUCTOR OF
500KCMIL IF FUSED AT 400AMP AS 240.21 REQUIRES YOUR SECONDARY CONDUCTORS TO
MEET OR EXCEED THE SECONDARY OVERCURRENT DEVICE.

I have a 480-120/240 v 75 kVA transformer with 500 kcmil conductors. I have 200A fuses on the disconnect on the primary side. There are 400A fuses in the disconnect between the transformer and the panel.The loading on the panel is 236 amps at 240 volts.

I'm using Mike's "Commercial and industrial Wiring and Raceway Chart". What do I have to do to get in compliance with this inspector?

Jack
 

raider1

Senior Member
Staff member
Location
Logan, Utah
240.21(C) does not permit the next size up rule for the secondary conductor overcurrent protective device.

So 500 Kcmil copper secondary conductors can't terminate in a 400 amp breaker they must terminate in a breaker that does not exceed the ampacity of the conductors.

500 Kcmil copper conductors have a ampacity of 380 amps at 75 degrees C therefore you can only terminate them on a 350 amp breaker or up size the secondary conductors to a conductor that has an ampacity of at least 400 amps.

Chris
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
240.21(C) references 240.4(B) which is for the next standard size up rule.
240.21(C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
240.4 Protection of Conductors.
(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
What are the distances from the transformer to the disconnect and from the disconnect to the panel? You could save a bit of money of the first number is much smaller than the second. The requirement that the transformer secondary conductors be protected at (and not above) their ampacity ends at the fused disconnect. So you could use 600 MCM from the transformer to the disconnect and 500 MCM from the disconnect to the panel (i.e., you don't have to run 600 MCM all the way).
 
The normal 480VAC feeder size to a 75 KVA transformer would be 90A, the primary side of a 75 KVA transformer feeder should not be larger than 110A @ 480VAC(125%). The secondary breaker or fuses should be sized for 2 poles so you would use the 240 VAC in your ampacity calculation and then divide by the square root of 3. So at 240VAC you would get a 125% maximum of 225A @ 240 VAC.
 

iwire

Moderator
Staff member
Location
Massachusetts
To the OP, are you sure you have a 75 KVA here?

The normal 480VAC feeder size to a 75 KVA transformer would be 90A,
No, that is the transformers rating not the feeder size.


the primary side of a 75 KVA transformer feeder should not be larger than 110A @ 480VAC(125%)
.

No, the primary breaker could be up to 225 amps for this transformer and the condutors would have to match or exceed the breaker rating

See table 450.3(B)

The secondary breaker or fuses should be sized for 2 poles so you would use the 240 VAC in your ampacity calculation and then divide by the square root of 3. So at 240VAC you would get a 125% maximum of 225A @ 240 VAC.
I agree that the maximum size OCPD on the secondary side of this 75 KVA trans would be a 225.
 
Status
Not open for further replies.
Top