Transformer Secondary Conductor and Breaker Sizing

[Platinum]

Hey guys,

Think this has been discussed before, but still a little confused. Say i have a 150kVA delta-wye, 480-208V transformer. I am using primary and secondary protection. How would I size my secondary side circuit breaker and conductors? I?m getting confused between articles 240.4(B), 240.21(C), and table 450.3(B).
Based on previous projects others have done, for a 112.5kVA secondary side they use:
Circuit breaker = 350A
Secondary conductors = (4) #500kcmil + (1) #1/0 gnd

Here is my approach:
FLA of a 112.5kVA transformer secondary = 312A
Circuit breaker size is 312 x 125% which = 390A. Table 450 says to round up, so would it be a 400A?
Secondary conductors would be #600kcmil. 600kcmil is rated for 420A. How does article 240.21(C) come into play here?

Thanks for any suggestions.

 

[bsh]

240.21 covers the requirements from the transformer secondary to the OC device and is there for cable protection, not transformer protection. Transformers are in 450. Since the cable technically has no OC protection at the transformer the cable must meet certain requirements for cable size, conduit, etc., 240.21 lists these requirments

 

[jim dungar]

Article 450 has nothing to do with the sizing of your secondary breaker. It only deals with sizing the transformer primary breaker depending on the absence or presence of a breaker on the secondary.

The secondary conductor protection always comes from article 240. 240.21(C) comes into play when there is conductor between the transformer terminals and the overcurrent protection for the secondary conductors.

 

[bob]

Hey guys,
Say i have a 150kVA delta-wye, 480-208V transformer. I am using primary and secondary protection. How would I size my secondary side circuit breaker and conductors? I’m getting confused between articles 240.4(B), 240.21(C), and table 450.3(B).
Based on previous projects others have done, for a 112.5kVA secondary side they use:
Circuit breaker = 350A
Secondary conductors = (4) #500kcmil + (1) #1/0 gnd.

This installation appears to be correct with regard to the NEC. It may not
provide for the maximum available KVA from the transformer. What is the primary OC rating?

Here is my approach:
FLA of a 112.5kVA transformer secondary = 312A
Circuit breaker size is 312 x 125% which = 390A. Table 450 says to round up, so would it be a 400A?
Secondary conductors would be #600kcmil. 600kcmil is rated for 420A. How does article 240.21(C) come into play here?

As previously stated, 240.21(C) covers the overcurrent protection of the secondary conductors only. What you are showing appears to be correct. What is the primary OC rating?

 

[infinity]

Correct me if I'm wrong but isn't the 500 v. 600 kcmil conductors a design issue if the OCPD is properly sized? Couldn't the secondary have #3 THHN conductors with 100 amp protection if so designed?

 

[jim dungar]

Correct me if I'm wrong but isn't the 500 v. 600 kcmil conductors a design issue if the OCPD is properly sized? Couldn't the secondary have #3 THHN conductors with 100 amp protection if so designed?

Yes.

The secondary conductors are protected using article 240, the fact they come from atransfromer just means you can go to 240.21(C).

Article 450 does not have any bearing on sizing the secondary protection. 450.3 come into play once you choose what the secondary protection will be.

 

[bill j]

The transformer secondary is a tap and you can't round up on the circuit breaker. 240.21(C)

 

[infinity]

The transformer secondary is a tap and you can't round up on the circuit breaker. 240.21(C)

You're correct and in the two examples in the OP neither one contained a rounded up OCPD in relation to the conductor size:

Circuit breaker = 350A
Secondary conductors = (4) #500kcmil

Circuit breaker size is 312 x 125% which = 390A. Table 450 says to round up, so would it be a 400A?
Secondary conductors would be #600kcmil.

 

[bill j]

Right, took a second look, sorry!

 

[Platinum]

This installation appears to be correct with regard to the NEC. It may not
provide for the maximum available KVA from the transformer. What is the primary OC rating?


As previously stated, 240.21(C) covers the overcurrent protection of the secondary conductors only. What you are showing appears to be correct. What is the primary OC rating?

The primary OC rating im using for both examples is 175A.
112.5kVA/.831 = FLA of 135A x 125% = 169A.

 

[Platinum]

Article 450 has nothing to do with the sizing of your secondary breaker. It only deals with sizing the transformer primary breaker depending on the absence or presence of a breaker on the secondary.

The secondary conductor protection always comes from article 240. 240.21(C) comes into play when there is conductor between the transformer terminals and the overcurrent protection for the secondary conductors.

I guess I'm getting confused on which row to choose on Table 450.3(B) - Primary only protection or Primary and Secondary protection. I've read that the primary and secondary protection is seldom used? Bear in mind I mostly work on commercial building projects with standard 480-208 delta-wye transformers....so is a secondary breaker typical?

I think I am getting confused with the terms.
99% of the time, I have a breaker before the transformer (fed from the switchboard) and I have a breaker after the transformer, which is the main of the 208V panel. I am thinking this is my secondary protection and I'm using Table 450.3(B) to size this - which I am thinking is wrong now!

 

[jim dungar]

I guess I'm getting confused on which row to choose on Table 450.3(B) - Primary only protection or Primary and Secondary protection. I've read that the primary and secondary protection is seldom used? Bear in mind I mostly work on commercial building projects with standard 480-208 delta-wye transformers....so is a secondary breaker typical?

I think I am getting confused with the terms.
99% of the time, I have a breaker before the transformer (fed from the switchboard) and I have a breaker after the transformer, which is the main of the 208V panel. I am thinking this is my secondary protection and I'm using Table 450.3(B) to size this - which I am thinking is wrong now!

Start at the load and work backwards to the transformer.

Protect the panel per article 408.

Conductors must be protected as allowed/required by 240.21. The panel protective device may be used to protect conductors.

Transformers =<600V must be have primary protection. The primary protection must not exceed 125% of FLA unless a protective device, like the one required by 240.21 is provided on the secondary. Only if this secondary device is not more than 125% of FLA then the primary breaker may be increased in size.

So, yes you are correct in your final result, you just took a different path to get there.

And remember, the panel protective device can be remote from the panel. The conductor protective device can be at the begining or the end of the conductor. If used, the transformer secondary protection may be remote from the transformer

 

[Platinum]

Thanks for the clarification Jim!

So i could have this big 75kVA transformer that feeds a 100A MCB panelboard if I wanted, as long as there is adequate primary protection (125%) for the transformer. The transformer will supply only what is drawn from the 100A load, so it won't blow up the panel.

 

[jim dungar]

The transformer will supply only what is drawn from the 100A load, so it won't blow up the panel.

That is correct, a transfromer does not force an amount of current to flow; it supplies only the amount of current requested by the load.