Transformer Secondary Rule

[Rich Elec.]

This is a hypothetical question just for my own knowledge so I am going to omit derating, and continuous load. The question is: is this a tap as specified under the 240.21 (B) tap rules.

A 50 KVA, single phase, 240 volt secondary voltage transformer.
A 50 KVA xfmr can supply 208 amps on the secondary.
If I were to run one set of secondary conductors from the xfmr secondary to a 100 amp panel. Also, run a second set of secondary conductors from the xfmr to another 100 amp panel.
Am I right that both of these runs are "tap" conductors, and the rules under 240.21 (B) need to be followed?

Thanks,
Rich

 

[augie47]

You are correct that each of those is considered a tap.
Note, however, if your transformer is a 2 wire 240volt or 3 wire 240 delta, you might check 240.4(F) as the primary may protect the secondary.
(Not the case in a 3 wire 240/120 or 4 wire 208Y/120 secondary)

 

[Rich Elec.]

Augie,
I was thinking about a 120/240 3 wire.
Thank you for your reply, and your quickness!

Rich

 

[david luchini]

Am I right that both of these runs are "tap" conductors, and the rules under 240.21 (B) need to be followed?

I think you need to be looking at 240.21(C) here (Transformer Secondary Conductors,) not 240.21(B).

 

[augie47]

Good Catch, David. 240.21(C) is correct,

 

[charlie b]

Am I right that both of these runs are "tap" conductors, and the rules under 240.21 (C) need to be followed?

Nope. Not with the information you gave, at least. Tell me one more specific thing about the installation, and then I will agree with the other posters. Clarify this one point: When you say you are running a feeder from the transformer secondary, do you specifically mean that there will not be an overcurrent device located at the transformer, and that the secondary windings are connected directly to the secondary feeder? Remember, you don't have a "tap," if the conductors are protected at the point at which they receive their power.

 

[Rich Elec.]

I want to address two responses here.
First Charlie, the quote that you posted that is noted as "Originally Posted by (me)" is incorrect. I did not reference 240.21 (C), I referenced 240.21 (B). How did you post an original quote that has an error?
On, to your question. I was thinking that this was a "tap" because the two sets of feeder conductors will be taken off of the secondary lugs directly to panelboards. No OCPD between xfmr and panelboards on the secondary.

David and Augie, I am wondering about classifying these specific conductors as "tap" conductors because if they are, then I would be limited by not only tap rules in (B) but transformer secondary rules in (C). Now the rules in the two sections have many similarities, but again this is a question just so I can understand and apply these rules correctly.

I am thinking that this is a tap because the xfrm can supply 208 amps and I would be feeding two 100 amp panelboards with two separate feeders.

 

[david luchini]

I am wondering about classifying these specific conductors as "tap" conductors because if they are, then I would be limited by not only tap rules in (B) but transformer secondary rules in (C). Now the rules in the two sections have many similarities, but again this is a question just so I can understand and apply these rules correctly.

What you have described would not be "tap" conductors, per 240.21(B.) You would need to apply the transformer secondary conductor rules in (C), but not the tap rules in (B).

 

[raider1]

I agree with David, these conductors are transformer secondary conductors and not tap conductors and would need to comply with 240.21(C) not 240.21(B).

Chris

 

[charlie b]

First Charlie, the quote that you posted that is noted as "Originally Posted by (me)" is incorrect. I did not reference 240.21 (C), I referenced 240.21 (B). How did you post an original quote that has an error?

Why I did it and how I did it are separate things. The "why" is that on the basis of posts 4 and 5, I figured that you had simply made a typographical error, and that you had always intended to ask about "C." I now infer that your intent was to ask about "B," so I apologize for "correcting" what was not in need of correction.

The "how" is easy. When you hit the QUOTE button, you get a screen with the entire previous post, preceded by the word "quote" in brackets and followed by "/quote" in brackets. You can edit anything you like within that box. In fact, we encourage people to remove from the quoted text anything extra, anything not related to the comment you are about to make. There is no reason to include a long quote, when you are replying to only one sentence within that quote. Take this comment of mine as an example, and look at the text that is quoted from your previous post. I did not leave your entire post, but only included the three sentences to which I am replying, and I deleted the rest of your post, within the part I am quoting, so as to focus my reply on that particular section.

 

[qcroanoke]

There is no reason to include a long quote, when you are replying to only one sentence within that quote.


Wish more members would do that. Much easier to get your point if I don't have to read the whole post again. Just the part you are responding to.

 

[Rich Elec.]

Thanks for the replies.
I see that these conductors are not "tapped."
They are simply feeders.

Rich

 

[cpal]

Thanks for the replies.
I see that these conductors are not "tapped."
They are simply feeders.

Rich

not sure I would call them feeders (see Def). They are best (IMO) to be evaluated as transformer secondary conductors. This helps direct installation requirements to section 240.21(C) and it's avaliable options.

 

[raider1]

not sure I would call them feeders (see Def). They are best (IMO) to be evaluated as transformer secondary conductors. This helps direct installation requirements to section 240.21(C) and it's avaliable options.

I agree, a feeder is a defined term in the NEC from Article 100. It is best to refer to them as secondary conductors.

Chris

 

[scott thompson]

How about this Schematic... does this comply to NEC 240.20 / 240.21 and Article 450?

Scott

 

[scott thompson]

1 Phase 3 Wire version of the above Panelboard Feeders option schematic:

Drawing 1PH3W-4: 480V x 120/240V 1 Phase 3 Wire System - Center Tap Grounded Neutral
(Max. Voltage To Ground = 120V, With Grounded Conductor)
Two separate Panelboards fed by Secondary Feeders- each set derived from "X" Terminals.

Location:

http://www.electrical-contractor.ne...ransformers_Feeders_1_Phase_3.html#Post193110

Scott

 

[david luchini]

How about this Schematic... does this comply to NEC 240.20 / 240.21 and Article 450?

Looking at the 3 phase schematic, you seem to be OK with 240.21(C) and 450, but...

On the primary, you are protecting #1/0 with a 300A c/b. That would violate 240.4.

And on the secondary, you show a #6 EGC. I think this would be an "Equipment Bonding Jumper" for the SDS, rather than and EGC, per 250.30(2). It should be sized per 250.102(C) and T250.66, which would require a #4 for the #2/0cu secondary conductors.

 

[scott thompson]

David;

Thanks for the response.

I have addressed your comments below:

On the primary, you are protecting #1/0 with a 300A c/b. That would violate 240.4.

This approach is similar to the Max. setting for Short Circuit & Ground Fault Protection of a Motor Circuit: 250% of the FLA for Branch Circuit OCPD Maximum rating; Conductors sized per FLA. (Table 430.52).

If there were no Secondary OCPD(s), then the Maximum OCPD for the Primary Feeder would be 125% of the Primary FLA (Full Load Amperes).

Personally, if this drawing was part of a Project I was compiling, I would size the Primary feeder's OCPD at maximum 175%, which would be 225 Amp max., while still using the 1/0 THHN Cu. Feeders.
The Drawing is pointing out the maximum parameters only.

Side Note: Could you please denote which part of 240.4 you are referring to in the quoted text.
240.4(B) would be applicable to the Secondary Feeders, but not the Primary Feeders.

_________________________________________________________________________
_________________________________________________________________________

And on the secondary, you show a #6 EGC. I think this would be an "Equipment Bonding Jumper" for the SDS, rather than and EGC, per 250.30(2). It should be sized per 250.102(C) and T250.66, which would require a #4 for the #2/0cu secondary conductors.

The #6 Cu. in reference is the Equipment Grounding Conductor, sized per the OCPD - as described in Table 250.122:
OCPD Not Exceeding 200 Amps; 6 Cu

The GEC would be sized per Table 250.66 as described below:
Largest Ungrounded Conductor = (2) 2/0 Cu.
Equivalent size: 133,100 cm x 2 = 266,200 cm. Figured as 300 MCM.
Per Table 250.66; > 3/0, through 350 MCM = #2 Cu. or 1/0 Al.

Grounding Electrode Conductor (GEC) would be #2 Cu. for this SDS.
System Bonding Jumper would also be #2 Cu.

___________________________________________________________________
___________________________________________________________________

Thanks once again for the reply.

Scott

 

[don_resqcapt19]

Scott,
I agree with David. There is no code provision, other than 240.21(B)(3), that permits you to protect the primary feeder conductors with an OCPD that has a rating higher than the ampacity of the feeder conductors. I also agree with him that the conductor on the secondary shown as a EGC is a really an equipment bonding jumper and is to be sized based on the size of the secondary conductors and T250.66. 250.30(A)(2)

 

[scott thompson]

Don,

I have not found anything in the NEC which Prohibits applying this technique.
240.21(3) would actually agree with the Drawings if this was a Tap situation.

Please explain what Table 450.3(B) would be getting at, by allowing the Primary OCPD to be 250% max. of the Primary FLA.
Reference: T450.3(B); Pri & Sec. Protection - Amperes 9A and higher:
Max. OCPD Settings:
Primary = 250% rated Primary FLA,
Secondary = 125% of the Secondary FLA.

The Transformer is limited in KVA by the total of the Secondary OCPDs.

I see it to be similar to starting an Induction Motor, which is why I want to be absolutely sure on this possible issue.
Starting a Transformer may easily trip the Primary OCPD, due to Inrush, so this approach would allow for Primary side Inrush.

As to the EGC, please look closely on the Drawings.
The SBJ is bonded to the Grounded Conductor, and the Green Grounding Bus. There is no size callout for the SBJ.
Along with the SBJ, there is a bond to the Enclosure on the same Grounding Bus.

The Green Wires run with the Secondary Feeders are Equipment Grounding Conductors for each of the Secondary Fed Panelboard Feeders.
Each Panelboard has an EGC, as well as Ungrounded + Grounded Conductors.


Thanks for the reply.

Scott