Transformer question

[JJWalecka]

Hypothetical situation?

Overcurrent protection for the primary conductors is about 125 feet away from a 75 KVA transformer that is to be mounted ten feet AFF. There is no drop ceiling.

The secondary conductors are about ninety feet in length and feed a 225 Amp panel.

It is my assumption that, in this installation, the secondary conductors would need overcurrent protection because they are over ten feet 240.21(C)(6) 240.92(B)(1)(1).

Does the transformer need overcurrent protection additional to the circuit breaker feeding the transformer primary conductors or would a disconnecting means be code compliant?

 

[jim dungar]

Hypothetical situation…

Overcurrent protection for the primary conductors is about 125 feet away from a 75 KVA transformer that is to be mounted ten feet AFF. There is no drop ceiling.

The secondary conductors are about ninety feet in length and feed a 225 Amp panel.

It is my assumption that, in this installation, the secondary conductors would need overcurrent protection because they are over ten feet 240.21(C)(6) 240.92(B)(1)(1).

Does the transformer need overcurrent protection additional to the circuit breaker feeding the transformer primary conductors or would a disconnecting means be code compliant?

Yes, the secondary conductors need overcurrent protection. However there are several 'distance' possibilities other than 10', see all of 240.21(C).

The transformer does not require a primary side disconnect other than the circuit breaker protecting the primary conductors. There is no in-sight rule for transformer.

 

[augie47]

There is no NEC requirement for you to have additional OCP on the primary or to have a primary disconnect at the transformer.

The need for secondary OCP would be based on 450.3, 240.21(C) and, if applicable, 240.92(C)

(sorry Jim, stepped on you)

 

[jim dungar]

The need for secondary OCP would be based on 450.3...

I do not consider 450.3(b) as requiring protection on the secondary of a transformer. IMO, 450.3 is about sizing the primary protection.

 

[augie47]

I agree, but the secondary protection will have an effect on the size of the primary protection. That's the only reason I mentioned it.
It is one reference that must be considered in sizing primary and secondary protection

 

[JJWalecka]

Thank you

So putting an overcurrent device, next to the transformer, to protect the primaries wouldn't eliminate the need for protection of the secondary conductors?

 

[don_resqcapt19]

Thank you

So putting an overcurrent device, next to the transformer, to protect the primaries wouldn't eliminate the need for protection of the secondary conductors?

The location of the primary OCPD has nothing to do with the issue of using the primary OCPD to protect the secondary conductors. The only time the primary OCPD can be used to protect the secondary conductors is when the transformer is a two wire to two wire transformer or when it is a 3 wire delta delta transformer. In all other cases you have to apply the rules in 240.21(C) for the protection of the secondary conductors.

 

[don_resqcapt19]

I do not consider 450.3(b) as requiring protection on the secondary of a transformer. IMO, 450.3 is about sizing the primary protection.

I don't understand. Table 450.3(B) covers the required primary and secondary protection of the transformer itself. Both the rules in 450.3 for the protection of the secondary windings of the transformer and the rules 240.21(C) for the protection of the secondary conductors must be satisfied.

 

[JJWalecka]

don_resqcapt19

"The location of the primary OCPD has nothing to do with the issue of using the primary OCPD to protect the secondary conductors.'

I researched and thought that was the case. I thought I was missing something and figured I would check on the forum. Thanks for clearing that up.

 

[chris kennedy]

There is no in-sight rule for transformer.

Didn't I read this will be required in 2011? Mine hasn't gotten here yet.

 

[jim dungar]

I don't understand. Table 450.3(B) covers the required primary and secondary protection of the transformer itself. Both the rules in 450.3 for the protection of the secondary windings of the transformer and the rules 240.21(C) for the protection of the secondary conductors must be satisfied.

No, table 450.3(B) contains the limits for sizing the primary protective device. If the primary device is not more than 125%, there is nothing in that section that requires secondary protection. But 240.21(C) always requires some type of secondary conductor protection.

 

[don_resqcapt19]

... If the primary device is not more than 125%, there is nothing in that section that requires secondary protection. ...

I don't read it that way. I read it as saying that when the primary OCPD is not more than 125% of the primary current rating that the primary OCPD provides the required overcurrent protection of the secondary windings.

I agree that the rules in 240.21(C) always require overcurrent protection for the secondary conductors, however in two specific cases, that overcurrent protection can be provided by the primary OCPD.

 

[jim dungar]

I don't read it that way. I read it as saying that when the primary OCPD is not more than 125% of the primary current rating that the primary OCPD provides the required overcurrent protection of the secondary windings.

A secondary overcurrent device cannot really protect the secondary windings of a transformer, especially from a fault on the line side of the device. 450.3(B) allow moving the transformer overload (125% rating) protection to the secondary side while maintaining short circuit protection on the primary.

I agree that the rules in 240.21(C) always require overcurrent protection for the secondary conductors, however in two specific cases, that overcurrent protection can be provided by the primary OCPD.

True, 240.21(C) is about sizing the protection for the secondary conductor as well as 'locating' that protection. But it has nothing to do with 450.3.

First you size the conductor to the load. Second, you protect the conductors per 240.21. Third, you see if your conductor protective device allows you to size your 450.3(B) primary protection greater than 125%.

 

[joebell]

Didn't I read this will be required in 2011? Mine hasn't gotten here yet.

Chris,

you are correct 450.14 is going to require a disconnecting means within sight or allowed in a remote location if the disconnecting means is lockable.

I'm still waiting for my 2011 book also. NFPA said it would be shipped in September when I ordered it, that means I should get it in the next 4 days LOL

 

[don_resqcapt19]

Jim,
I still see the rule in 450.3 as requiring protection for the secondary side of the transformer. While it may only be overload protection, I think it is very clear that the rule and table in 450.3 requires it. The fact that the required protection of the secondary conductors will often also provide the required protection of the secondary windings does not change anything.

 

[jim dungar]

I still see the rule in 450.3 as requiring protection for the secondary side of the transformer.

If the primary protection is not more than 125% FLA, what size secondary protection is required by 450.3(B)?

 

[don_resqcapt19]

If the primary protection is not more than 125% FLA, what size secondary protection is required by 450.3(B)?

Yes, in that case the rule says that the primary OCPD provides the required secondary protection.

 

[jim dungar]

Yes, in that case the rule says that the primary OCPD provides the required secondary protection.

But, in all cases the 450.3 transformer protection sizing is not required to offer any secondary conductor protection at all (that is the job of 240.21(C)).

 

[don_resqcapt19]

But, in all cases the 450.3 transformer protection sizing is not required to offer any secondary conductor protection at all (that is the job of 240.21(C)).

Exactly and that is my real point. You look to Article 450 for the rules that cover the required protection of the transformer. The rules in that article have nothing to do with the required protection of either the primay or secondary conductors. You look to Article 240 for the required protection of the conductors and the rules in Article 240 have nothing to do with protecting the transformer itself. However the required protection of the conductors will, in most cases, satisfy the rules in 450.3.