75 KVA primary

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I am considering substituting a 125 amp 480 volt breaker for a 75 KVA primary that has been on order for 5 months and was recently updated with a ship date in August with a 150 amp that is available now ,I already pulled the #1 cu primary and based on 450.3b and 240.4b I feel this should be permissible. This breaker will be fed from a 10 foot tap with 1/0 cu and the transformer secondary will be protected by a 225 amp breaker, Am I overlooking anything? I never fed a 75 KVA with anything over 125 amp
 
In my mind I know that's "right" but it is a bit concerning in this case where you don't have a fixed load.
The conductor needs to be sized to carry the load which in this case could exceed the 75kva transformer rated design input,
 
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augie47 said:
In my mind I know that's "right" but it is a bit concerning in this case where you don't have a fixed load.
The conductor needs to be sized to carry the load which in this case could exceed the 75kva transformer rated design input,
Click to expand...
Not really any different than using 500mcm for a 400A feeder.

The load is fixed by whatever is connected to the panelboard, so in this case, the load on the primary is limited to 97A.
 
Assuming your secondary is 208V the 225 A breaker better protects the transformer than a 125A primary. As david luchini
said the load is fixed by restricting the output. Converting the secondary 225 A to primary would give 97.5 A which multiplied by 1.25 gives 122 A.
# 1 AWG at 75 °C has 130 A rating. I my opinion you are good.

These days energy efficient transformer inrush current often forces contractors and/or manufacturers to oversize primary breaker to handle the inrush on energization. Limiting current on secondary (like you did) is the best way to avoid oversizing the primary feeders while maintaining full circuit protection.

You may find these apps from https://mc-group.ca useful for this type of calculations.

⤓ Electrical_Calculator-R1121.zip 2.3MB Freeware (Rev. Nov 2021)

⤓ Conductors_Ampacity_NEC-R1121.zip 2.1MB Freeware (Rev. Nov 2021)
 
Is'nt the next size up already taken with the primary being 125% of 90 amps at 112.5 amps rounded up to 125a OCPD for the primary?

The 150 seems awful heavy.

Jap>
 
jap said:
Is'nt the next size up already taken with the primary being 125% of 90 amps at 112.5 amps rounded up to 125a OCPD for the primary?

The 150 seems awful heavy.

Jap>
Click to expand...
Different next size up rule. 240.4(B) permits a conductor with an ampacity of 130 to be protected by the next standard size up OCPD...150A.

(The primary ocpd is permitted to be up to 225A by 450.3(B)).
 
david luchini said:
Different next size up rule. 240.4(B) permits a conductor with an ampacity of 130 to be protected by the next standard size up OCPD...150A.

(The primary ocpd is permitted to be up to 225A by 450.3(B)).
Click to expand...
Interesting thanks,

JAP>
 
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