240.21(C)(6)(1)

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chris kennedy

chris kennedy

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60 yr old tool twisting electrician
I have a tranny, 208V primary and 480V secondary protected by a 400A primary OCPD. To calculate the secondary conductors ampacity I would;

208÷480=.43
400×.43=172A

Correct?
Thanks

Note: Not a test question, real world.
 
So you are basing the trany size on the overcurrent protective device? I would like to know the wattage of the trany-- the overcurrent protective device could be undersized or possibly oversized
 
Dennis Alwon said:
So you are basing the trany size on the overcurrent protective device? I would like to know the wattage of the trany-- the overcurrent protective device could be undersized or possibly oversized
Click to expand...

Its a 112.5. Whats that got to do with 240.21(C)(6)(1)?
 
chris kennedy said:
I have a tranny, 208V primary and 480V secondary protected by a 400A primary OCPD. To calculate the secondary conductors ampacity I would;

208÷480=.43
400×.43=172A

Correct?
Thanks

Note: Not a test question, real world.
Click to expand...


480/208 = 2.31

Primary to secondary

Edit

Second one to miss read your post!! Sorry
 
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chris kennedy said:
I have a tranny, 208V primary and 480V secondary protected by a 400A primary OCPD. To calculate the secondary conductors ampacity I would;

208÷480=.43
400×.43=172A

Correct?
Thanks

Note: Not a test question, real world.
Click to expand...
You're figuring this backward from what is stated in 240.12(C)(1)...

Maximum Primary OCPD ≤ (Secondary Conductor Ampacity) × (Secondary:primary Voltage)
400A OPCD ≤ (?) × (208:480)
400A OCPD ÷ (208:480) ≤ 923A minimum secondary conductor ampacity
 
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chris kennedy said:
Its a 112.5. Whats that got to do with 240.21(C)(6)(1)?
Click to expand...
The OCPD is required to comply with 450.3. Primary rated current of a 480V - 208Y120V 3Ø transformer is...

112,500kVA ÷ 480V ÷ sqrt(3) = 135A

The max OCPD rating you can have with primary-only protection is 125% per Table 450.3(B).

135A × 125% = 169A

...rounded up to a 175A standard OCPD rating maximum per Table 450.3(B) Note 1.
 
From there, using a 175A OCPD...

Maximum Primary OCPD ≤ (Secondary Conductor Ampacity) × (Secondary:primary Voltage)
175A OPCD ≤ (?) × (208:480)
175A OCPD ÷ (208:480) ≤ 404A minimum secondary conductor ampacity
 
I have a 208 primary and a 480 secondary.

400A primary OCPD
175A secondary OCPD

Trying to figure secondary conductor ampacity for over 10' and under 25'.
 
Smart $ said:
The OCPD is required to comply with 450.3. Primary rated current of a 480V - 208Y120V 3Ø transformer is...

112,500kVA ÷ 480V ÷ sqrt(3) = 135A

The max OCPD rating you can have with primary-only protection is 125% per Table 450.3(B).

135A × 125% = 169A

...rounded up to a 175A standard OCPD rating maximum per Table 450.3(B) Note 1.
Click to expand...

Sir, the OP states that 208V is primary. I miss read it also.
 
chris kennedy said:
I have a 208 primary and a 480 secondary.

400A primary OCPD
175A secondary OCPD

Trying to figure secondary conductor ampacity for over 10' and under 25'.
Click to expand...
Ahh... I missed the 208V PRI - 480V SEC. :ashamed1:

Maximum Primary OCPD ≤ (Secondary Conductor Ampacity) × (Secondary:primary Voltage)
400A OPCD ≤ (?) × (480:208)
400A OCPD ÷ (480:208) ≤ 173A minimum secondary conductor ampacity

...using primary-only protection. There is no length restriction in 240.21(C)(1) cases, as the secondary conductors are not treated as taps.
 
Smart $ said:
...using primary-only protection. There is no length restriction in 240.21(C)(1) cases, as the secondary conductors are not treated as taps.
Click to expand...

The paragraph in the title was 240.21(C)(6)(1), not 240.21(C)(1). So its a 25' limit. :)
 
chris kennedy said:
208÷480=.43
400×.43=172A
Correct?
Click to expand...
Isn't there supposed to be a factor of 1/3 in the equation? Or do I need to finish my morning coffee before responding to anything on this forum? Shouldn't the entire formula be (208/480) times (1/3) times (400), or about 58 amps?

(Edited to add that I should learn to type faster. :happyyes:)

 
charlie b said:
Isn't there supposed to be a factor of 1/3 in the equation? Or do I need to finish my morning coffee before responding to anything on this forum? Shouldn't the entire formula be (208/480) times (1/3) times (400), or about 58 amps?

(Edited to add that I should learn to type faster. :happyyes:)
Click to expand...

Great minds think alike :lol:
 
charlie b said:
Isn't there supposed to be a factor of 1/3 in the equation? Or do I need to finish my morning coffee before responding to anything on this forum? Shouldn't the entire formula be (208/480) times (1/3) times (400), or about 58 amps?

(Edited to add that I should learn to type faster. :happyyes:)
Click to expand...
I was going to bring that up as well.

240.21(C)(6)(1) is minimum size tap conductor allowed, if the load is higher then 58 amps then that minimum increases with the load, but yes if the load were only say 25 amps, the tap conductor would still need to be at least 58 amps.
 
Max OCPD and min. conductor size

Max OCPD and min. conductor size

I think we have consensus for the minimum secondary conductor ampacity for the OP.
Do we further agree that (assuming 75 degree C) that the maximum OCPD would be 60 amp and the minimum conductor would be #6?
 
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