Secondary of transformer design

[PhaseShift]

I have a 25kVA 480-240/120 transformer. I am feeding the secondary of this transformer into an 100A 120/240V panel. The transformer and panel are located outdoors, and the panel will be about 10ft from transformer secondary. Per 240.21 (B) and (C) I planned on running a #2 from the secondary of the transformer to a 100A main breaker in the panel. Will this 100A breaker be adequate protection for the transformer secondary protection?

Also I will need to ground the transformer since it is a seperately derived system. I'm not sure what size GEC from table 250.66 to use for this application. I'm guessing that since my secondary condutors are #2 then I would use a #8 copper GEC from the neutral poing of the transformer to the grounding electrode.

What section specifies what size grounding electrode I need to use?

As far as sizing the neutral what section specifies the size neutral needed for this secondary? Will this neutral serve as the EGC between the transformer and panel as well?

 

[raider1]

Will this 100A breaker be adequate protection for the transformer secondary protection?

The 100 amp breaker will satisfy the secondary conductor protection requirements in 240.21(C). As far as the secondary of the transformer, provided that the primary overcurrent device is not larger than 125% of the primary current then the secondary winding don't require specific protection.

Also I will need to ground the transformer since it is a seperately derived system. I'm not sure what size GEC from table 250.66 to use for this application. I'm guessing that since my secondary condutors are #2 then I would use a #8 copper GEC from the neutral poing of the transformer to the grounding electrode.

What section specifies what size grounding electrode I need to use?

Yes, a #8 would be adequate for the grounding electrode conductor. Take a look at 250.30 for separately derived system grounding.

As far as sizing the neutral what section specifies the size neutral needed for this secondary? Will this neutral serve as the EGC between the transformer and panel as well?

Take a look at 250.30(A)(8)

Chris

 

[StephenSDH]

Looks goods.

Since the secondary transformer is center tapped then you are required to have secondary protection.

I don't think there is a 10' limit since you are outside.

Good luck, Steve

 

[PhaseShift]

Yes, a #8 would be adequate for the grounding electrode conductor. Take a look at 250.30 for separately derived system grounding.

In 250.30(A)(3) I am confused with some of the terminology. The Grounding electrode conductor is what connects the grounding electrode to the grounded conductor. What is the grounded conductor they are refering to? Is this the neutral connection?

It also states that the GEC must be connected to the grounded conductor at the same point that the bonding jumper is connected to the grounded conducotr. Which bonding jumper are they refering too? The equipment bonding jumper, or system bonding jumper?

I am correct is saying that I indeed need to install a grounding electrod and GEC since this is classified as a seperately derived system?

Is 250.52(A)(5) the only requirement for the actual grounding electord if using a rod?

 

[Smart $]

In 250.30(A)(3) I am confused with some of the terminology. The Grounding electrode conductor is what connects the grounding electrode to the grounded conductor. What is the grounded conductor they are refering to? Is this the neutral connection?

Yes.

It also states that the GEC must be connected to the grounded conductor at the same point that the bonding jumper is connected to the grounded conducotr. Which bonding jumper are they refering too? The equipment bonding jumper, or system bonding jumper?

The system bonding jumper (SBJ). 250.30(A)(3) Exception No. 1 permits the GEC to be connected at the other end of the SBJ.

I am correct is saying that I indeed need to install a grounding electrod and GEC since this is classified as a seperately derived system?

Assuming there is no grounding electrode specified in 250.30(A)(7), then yes you will have to install one (or two).

Is 250.52(A)(5) the only requirement for the actual grounding electord if using a rod?

Not quite sure I getting the gist of your question here. If no grounding electrode specified in 250.30(A)(7) is present, any of the electrodes listed in 250.52(A) shall be used. If installation of an electrode is necessary, then a ground rod or pipe is generally the easiest and most economical way to be code compliant. However in using a rod or pipe electrode it must meet the 25 ohm resistance to ground requirement of 250.56. If it does not, which is quite likely, you will have install and connect to a second electrode at least 6' away.

 

[wasasparky]

Since the secondary transformer is center tapped then you are required to have secondary protection.

What if the secondary was not center tapped?
(straight 120 or 240, for example)

 

[don_resqcapt19]

What if the secondary was not center tapped?
(straight 120 or 240, for example)

See 240.21(C)(1).

 

[PhaseShift]

From looking at 250.30 it appears that I have an option of where to connect the GEC to. It can be connected to the grounded conductor (neutral) at either the transformer itself or the panel.

If it is connected at the transformer, then i need to run a conductor from the grounded conducotr at the transformer to the neutral bar at the panel. I would also need to run a system bonding jumper between the grounded conductor at the transformer secondary and the equipment ground bar at the panel.

If the GEC is connected directly to the grounded conductor (neutral) at the panel then the system bonding jumper can be installed between the neutral block and ground block internally in the panel. The only other wire I would need between the transformer and panel would be for the neutral wire. It looks like I would need an equpment bonding jumper connected betwen the transformer enclosure and panel enclosure.

What section covers the sizing of the neutral wire between the transformer and panel? All other jumpers look like their covered in 250.66.

If a panel only has a neutral block and not a seperate equipment ground block, can all the neutrals and equipment ground conductors be connected to this same block?

What is the difference between the Main bonding jumper and the system bonding jumper that is referenced in these sections?

 

[PhaseShift]

I have decided to connect the GEC to the neutral bar in the panel. This way I can have the system bonding jumper between the panel neutral and panel ground bar inside the panel. I then will need to run an grounded conductor between the transformer neutral and the neutral bar in the panel. I figure a #8 AWG for this grounded conductor based on 250.66.

I have a question regarding the grounded conductor being subject to 220.61 as a result of unbalanced current. How do you determine the maximum unbalanced current as specified in 220.61(A) for adjusting this conductor size?

I have attached the connections between the transformer secondary and panel for review.

 

[Smart $]

How do you determine the maximum unbalanced current as specified in 220.61(A) for adjusting this conductor size?

Split up your Line-to-Neutral loads into per Line loads. Add them up per Line and use the Line with the highest demand to size the neutral (grounded conductor). However, it cannot be smaller than required for the GEC per Table 250.66.

In your diagram, spaces 1 and 2 are connected to the Line 2 or B (right-side) bus. TTBOMK, these are generally connected to the Line 1 (left-side) bus for top fed panelboards. It also appears that spaces 9 and 10 break the pattern L2-L1-L2-L1... (which generally is L1-L2-L1-L2...). This would only be the case for tandem breakers occupying spaces 7 and 8 (and I would number the tandems' circuits 7a, 7b, 8a, and 8b).

PS: Don't forget to bond the grounded conductor to the transformer frame/case

 

[PhaseShift]

When installing a new seperately derived source as listed above is it absolutely required by code to achieve a ground resistance via the grounding electrode of less than 25ohms? Does this have to be verified by a fall of potential or some other method?

 

[raider1]

You must connect the separately derived system to the buildings grounding electrode system. If that system happens to be a single rod pipe or plate electrode then you must have a resistance of 25 ohms or less. If the grounding electrode system is any of the other electrodes listed in 250.52 then there is no maximum resistance requirement for the grounding electrode system.

Chris

 

[LarryFine]

In other words, if the existing electrode system is adequate, then you just need to connect to it.

 

[raider1]

In other words, if the existing electrode system is adequate, then you just need to connect to it.

Well said.

Chris

 

[Smart $]

Well said.

Chris

Don't open the champagne just yet... :roll:

I'm seeing too many IFs in this scenario, not knowing all the details... so I can't in good conscience let it go at a very subjective "adequate"

First off, way back in the OP, it was said both the xfmr and panel are outdoors... with no indication of a building being present nearby or otherwise. What if this were an isolated installation 400 ft from the nearest building? Do you still connect to the GES of that building? I think not.

Another "what if" is say the install is adjacent to and serves, in part or whole, a building. Say the existing GES has only one rod, pipe, or plate electrode. Would one not have to prove that one electrode meets the 25 ohm requirement or forego that and add a second?

 

[LarryFine]

First off, way back in the OP, it was said both the xfmr and panel are outdoors... with no indication of a building being present nearby or otherwise. What if this were an isolated installation 400 ft from the nearest building? Do you still connect to the GES of that building? I think not.

No, you would connect to the 'local' GES, whatever it may be.

Another "what if" is say the install is adjacent to and serves, in part or whole, a building. Say the existing GES has only one rod, pipe, or plate electrode. Would one not have to prove that one electrode meets the 25 ohm requirement or forego that and add a second?

Well, I did say "adequate," which means "compliant."



Can we pop the cork now?

 

[Smart $]

No, you would connect to the 'local' GES, whatever it may be.

And where none currently exist, one has to be created... and we're back to the original question...

Well, I did say "adequate," which means "compliant."

...and we could respond to a gazillion code questions asked here on the forum in a similar manner. What is this forum coming to

Can we pop the cork now?

You tell me :-?

 

[david luchini]

Per 240.21 (B) and (C) I planned on running a #2 from the secondary of the transformer to a 100A main breaker in the panel. Will this 100A breaker be adequate protection for the transformer secondary protection?

Your planned use of #2 from the secondary of the transformer to a 100A main breaker in the panel MAY or MAY NOT be correct. Per NEC 110.14(C)(1)(a), equipment rated 100 Amps or less shall be used with conductors rated 60degC or conductors with higher temperature ratings provided the ampacity of such conductors is based on the 60degC temperature rating. The 60degC rating of #2 AWG is only 95 Amps.

Per NEC 240.21(C) (Location in circuit of overcurrent protection for transformer secondary conductors

You could either follow part (2) "transformer secondary conductors no over 10Ft long" which says that the ampacity of the secondary conductors is NOT LESS than the rating of the overcurrent-protective device at the termination of the secondary conductors.

Or you could follow part (6) "transformer secondary conductors not over 25Ft long" which says that the secondary condctors will terminate in a single circuit breaker that limits the load current to not more than the conductor ampacity that is permitted by 310.15.

​

If the ampacity of #2 AWG is 95 Amps, then you would not meet the requirements listed above, and would have to install #1 AWG instead.

The exception to this would be NEC 110.14(C)(1)(a)(3), which says you can use conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors. If BOTH the circuit breaker lugs and the transformer terminals were listed for use with 75degC conductors, then #2 AWG (or even #3 AWG) would be acceptable.