overcurrent for transformer

[mannyb]

What size is primary OCPD is needed for 30kva transformer 480v 3phase? is this right 30kva x 830 = 36.144 x 1.25= 45a so 50a breaker ?

 

[david luchini]

What size is primary OCPD is needed for 30kva transformer 480v 3phase? is this right 30kva x 830 = 36.144 x 1.25= 45a so 50a breaker ?

The primary OCPD needs to be not less than 100% of the noncontinuous load plus 125% of the continuous load. It cannot be larger than 90A, per T450.3(B).

 

[infinity]

What size is primary OCPD is needed for 30kva transformer 480v 3 phase? is this right 30kva x 830 = 36.144 x 1.25= 45a so 50a breaker ?

I think that this is what you meant but your formula should be 30kva/830=36.1amps

 

[mannyb]

I think that this is what you meant but your formula should be 30kva/830=36.1amps

I meant to say MAX OCPD. I got this from a Mike holt post on facebook I am just trying to figure out if the post is wrong. The post shows different formula i just forgot to post the MAX OCPD

 

[david luchini]

I meant to say MAX OCPD. I got this from a Mike holt post on facebook I am just trying to figure out if the post is wrong. The post shows different formula i just forgot to post the MAX OCPD

What does the post say is the Max OCPD for a 30kVA, 480V transformer primary?

 

[topgone]

What size is primary OCPD is needed for 30kva transformer 480v 3phase? is this right 30kva x 830 = 36.144 x 1.25= 45a so 50a breaker ?

Are you sure you are operating at 830V?

 

[FittsJ2]

The primary OCPD needs to be not less than 100% of the noncontinuous load plus 125% of the continuous load. It cannot be larger than 90A, per T450.3(B).

450.3(B) says to size primary over current protection @125% (if 9amps or more) of the transformer-rated current. Where does the NEC say that it’s sized 100% of noncontinuous + 125% of continuous?

(30,000/(480x1.732)) x 125% = 45.107A
220.5(B) says fractions smaller than .5 can be dropped, so that means the MAX overcurrent device is 45amps.
45amp breaker is standard according to Table 240.6(A)

 

[david luchini]

Are you sure you are operating at 830V?

That's 480V , 3 phase.

480×1.732=830 (approximately)

 

[david luchini]

450.3(B) says to size primary over current protection @125% (if 9amps or more) of the transformer-rated current. Where does the NEC say that it’s sized 100% of noncontinuous + 125% of continuous?

The primary to the transformer is a feeder. 215.3 says the feeder OCPD must be sized not less than 100% of noncontinuous + 125% of continuous.

(30,000/(480x1.732)) x 125% = 45.107A
220.5(B) says fractions smaller than .5 can be dropped, so that means the MAX overcurrent device is 45amps.
45amp breaker is standard according to Table 240.6(A)

450.3(B) says the primary ocpd cannot be more than 250% of the primary rated current. That's 90A.

 

[infinity]

I meant to say MAX OCPD. I got this from a Mike holt post on facebook I am just trying to figure out if the post is wrong. The post shows different formula i just forgot to post the MAX OCPD

Then David has your answer. As he asked what did Mike Holt say the answer was?

450.3(B) says the primary ocpd cannot be more than 250% of the primary rated current. That's 90A.

 

[Strombea]

This depends on location per the table 430.3B

but as for it being a Maximum OCPD and it's in a secure location from the public the yes 90 amps (250% is more than 90)

there is no reason to go that high. We simply just do the math based on VA like any other load. Your short circuit rating is more important. 250% plus a 10kaic breaker could make for some bad news.


also I don't know that the feeder rules would apply because a lot of times you will spec out a TX before loads are determined.(tenant units in a large mall, etc) so the Kva rating on the TX is what you are protecting.
but it boils down to which number max or min ocpd is smarter to use. I agree though that max would be 250% but I would use the minimum around 50amps

 

[topgone]

This depends on location per the table 430.3B

but as for it being a Maximum OCPD and it's in a secure location from the public the yes 90 amps (250% is more than 90)

there is no reason to go that high. We simply just do the math based on VA like any other load. Your short circuit rating is more important. 250% plus a 10kaic breaker could make for some bad news.


also I don't know that the feeder rules would apply because a lot of times you will spec out a TX before loads are determined.(tenant units in a large mall, etc) so the Kva rating on the TX is what you are protecting.
but it boils down to which number max or min ocpd is smarter to use. I agree though that max would be 250% but I would use the minimum around 50amps

Yep. You are given a wide window, either 125% up to 250%. 50A may allow your circuit to be adequately supplied if there's no high inrush. But, to do away with the possibility of breakers tripping on start, methinks it's better to have the higher option.

 

[david luchini]

This depends on location per the table 450.3B

but as for it being a Maximum OCPD and it's in a secure location from the public the yes 90 amps (250% is more than 90)

Location? There is nothing in 450.3(B) about location.

 

[Strombea]

Sorry, you are correct. Location rows are in table for over 600v