Xfrmr Question

[Alwayslearningelec]

Ok back to my xfrmr temp setup. Engineer called me and was not sure if we are code complaint.

3P 70A 277/480v breaker feeding 45kva xfrm. 45 xfrm feeds (3) 120/208v disconnects. These disconnects feed (3) single phase trailer panels.

The disconnect sizes are 125A and (2) 60A.( BTW do they need to be fused). The transformer is located within 25’ of the disconnects but not within sight of each other.

The engineer is referring me to 450.3(B) and note #2 and not sure how to interpret the following last part of note #2.

the total of all the device ratings shall not exceed the allowed value of a single overcurrent device

Any thoughts? Thanks.

The first part of note #2 does say where secondary overcurrent protection is required. Not sure if required in my case so the note #2 wouldn't apply.

 

[Barney B]

Ok back to my xfrmr temp setup. Engineer called me and was not sure if we are code complaint.

3P 70A 277/480v breaker feeding 45kva xfrm. 45 xfrm feeds (3) 120/208v disconnects. These disconnects feed (3) single phase trailer panels.

The disconnect sizes are 125A and (2) 60A.( BTW do they need to be fused). The transformer is located within 25’ of the disconnects but not within sight of each other.

The engineer is referring me to 450.3(B) and note #2 and not sure how to interpret the following last part of note #2.

the total of all the device ratings shall not exceed the allowed value of a single overcurrent device

Any thoughts? Thanks.

The first part of note #2 does say where secondary overcurrent protection is required. Not sure if required in my case so the note #2 wouldn't apply.

1. Per Table 450.3(B), secondary overcurrent protection is not required in this case and thus Note 2 does not apply.
2. Per § 240.21(C)(6), overcurrent protection is required at the three disconnects.
3. Note that § 240.21(C) prohibits "rounding up" to the next higher standard rating of OCPD, so be sure you conductor ampacity is not less than OCPD rating.

 

[jap]

The 70a primary protection seems fine at 125% of the primary current.
The tap conductors on the load side of the transformer need to be protected from overcurrent per the tap conductor rules.
The transformer does not need to be located within sight and within 25' of the disconnects seeing as how a transformer is not a disconnecting means.
That disconnecting means rule applies to disconnects that are installed to disconnect power for servicing motors or pieces of equipment.

As far as the total device ratings not exceeding a single overcurrent protection device, I would have to dig a little deeper into that.

Just my 2 cents worth.

JAP>

 

[Alwayslearningelec]

So your saying I need OCPD between the secondary side of xfrm and the disconnects? Another disconnect in between them?? Weird. I am using FUSED disconnect switches.

 

[jap]

You had asked the question in your 1st post if your disconnects needed to be fused.

"quote" ( BTW do they need to be fused).

A fuse is one means of OCP if one chooses to use that method.

JAP>

 

[Alwayslearningelec]

1. Per Table 450.3(B), secondary overcurrent protection is not required in this case and thus Note 2 does not apply.
2. Per § 240.21(C)(6), overcurrent protection is required at the three disconnects.
3. Note that § 240.21(C) prohibits "rounding up" to the next higher standard rating of OCPD, so be sure you conductor ampacity is not less than OCPD rating.

Barney isn't #1 & 2 a contradiction??

 

[jim dungar]

Barney isn't #1 & 2 a contradiction??

No.

Transformer secondary conductors always need to be protected per 240.21(C).
450.3(B) never requires secondary side conductor protection. This section is about allowing the primary side protection to be in excess of 125% if appropriate secondary side protection is provided.

 

[don_resqcapt19]

Barney isn't #1 & 2 a contradiction??

You have to remember that there are two totally separate sets of rules. The rules in Article 450 apply to the protection of the transformer windings only. The conductors must protected per the rules in 240, specifically 240.21(C) for the secondary conductors. These rules really have nothing to do with each other, but in cases where Article 450 requires transformer secondary protection, the protection required in 240.21(C) will likely provide protection for both the conductors and the secondary winding.

 

[Charged]

I have trouble with 240.21C. Don’t the disconnects need to be within 10 ft of the transformer ?

 

[augie47]

I have trouble with 240.21C. Don’t the disconnects need to be within 10 ft of the transformer ?

10 ft is probably the most common but you might note there are (6) possible conditions and allowable conditions with variable lengths.
(actually if you look at (C)(1) under certain conditions there is no limitation)

 

[Alwayslearningelec]

I conveyed what you guys mentioned to the engineer and this is what he responded.

I’m having a problem with 240.21 (C)(6)….are the secondary conductors less than 25’ long? -please explain that the three conditions are satisfied for this scope of work.

Should it be 240.21 (C)(4) for your scope – with four conditions satisfied?

 

[don_resqcapt19]

I conveyed what you guys mentioned to the engineer and this is what he responded.

I’m having a problem with 240.21 (C)(6)….are the secondary conductors less than 25’ long? -please explain that the three conditions are satisfied for this scope of work.

Should it be 240.21 (C)(4) for your scope – with four conditions satisfied?

(C)(6) is correct as long as the actual wire length does not exceed 25' and the OCPD at the load end of the wire has a rating that does not exceed the ampacity of the conductors. The "round up" provision in 240.4(B) cannot be used for transformer secondary conductors.
Given the OCPD sizes of 125 and 60 amps, the requirements of (C)(6)(1) will be satisfied, the individual fusible disconnects will satisfy (C)(6)(2), and a raceway, or in some cases a cable, will satisfy (C)(6)(3).

(C)(4) can only be used where the transformer is outside of the building, and the OCPD is outside of the building or inside "nearest the point of entrance of the conductors"

 

[Alwayslearningelec]

(C)(6) is correct as long as the actual wire length does not exceed 25' and the OCPD at the load end of the wire has a rating that does not exceed the ampacity of the conductors. The "round up" provision in 240.4(B) cannot be used for transformer secondary conductors.
Given the OCPD sizes of 125 and 60 amps, the requirements of (C)(6)(1) will be satisfied, the individual fusible disconnects will satisfy (C)(6)(2), and a raceway, or in some cases a cable, will satisfy (C)(6)(3).

(C)(4) can only be used where the transformer is outside of the building, and the OCPD is outside of the building or inside "nearest the point of entrance of the conductors"

So the ampacity of the conductors from the secondary of the xfrmr to the fused disconnects can't be LESS than the disconnect fuse size? Can it be equal?

 

[augie47]

Ampacity must be equal to or greater than then fuse size.

 

[Engser18]

Ok back to my xfrmr temp setup. Engineer called me and was not sure if we are code complaint.

3P 70A 277/480v breaker feeding 45kva xfrm. 45 xfrm feeds (3) 120/208v disconnects. These disconnects feed (3) single phase trailer panels.

The disconnect sizes are 125A and (2) 60A.( BTW do they need to be fused). The transformer is located within 25’ of the disconnects but not within sight of each other.

The engineer is referring me to 450.3(B) and note #2 and not sure how to interpret the following last part of note #2.

the total of all the device ratings shall not exceed the allowed value of a single overcurrent device

Any thoughts? Thanks.

The first part of note #2 does say where secondary overcurrent protection is required. Not sure if required in my case so the note #2 wouldn't apply.

Definitely, you are not code compliance:
1. Secondary conductors need to be protected IAW 240.21. These disconnected need to be fused.
2. 450.3 (B), combine/equivalent single OCP, it would be 185A. the 45KVA Xformer secondary FLA/Rated is 125A*1.25=~156A, next standard size is 175A but I wouldn't use 175A. I would use 150A.

 

[Tulsa Electrician]

Definitely, you are not code compliance:
1. Secondary conductors need to be protected IAW 240.21. These disconnected need to be fused.
2. 450.3 (B), combine/equivalent single OCP, it would be 185A. the 45KVA Xformer secondary FLA/Rated is 125A*1.25=~156A, next standard size is 175A but I wouldn't use 175A. I would use 150A.

I'm not following can you elaborate.

 

[Engser18]

I'm not following can you elaborate.

His loads are single phase 3W (120/208V) assume he connected his load as follows to balance 125A (A-B), 1st 60A ( B-C), 2nd 60A (C-A). Total on A and B is 185A which is exceed the rating. I would fix that with 100A fuse DS/ECB, and 2- 50A fuse DS/ECB. If his load required more, then it would need to upgrade Xformer. He can use 2-60A fuse DS/ECB instead of 2-50A, it would still meet the code. But I wouldn’t do that.

 

[Tulsa Electrician]

Thank you for the explanation. I'm following you now.