Multiple single phase taps from 3-Phase transformer secondary.

[philly]

Is there anything in the code that prevents tapping secondary of a 3-phase transformer to feed (3) different L-L single phase circuits to (3) separate downstream panelboards. I'm not aware of anything in code that prohibits that but wanted to see if others were aware.

I know that it is permissible to tap multiple 3-phase circuits to different locations as long as each secondary conductor is adequately protected according to 240.21 however i've never seen separate single phase circuits tapped directly at transformer secondary. I would mostly see the single phase circuits derived 3-phase panelboard fed from transformer secondary.

 

[jaggedben]

Pretty sure there's nothing that prohibits that. I understand it's done with some frequency with services on apartment buildings where each dwelling get's a single phase panel. Don't see what's different about a transformer, or know of any code section that talks about it if you don't see anything in 240.21(C).

 

[LarryFine]

Sounds permissible, as long as the primary is protected and tap rules are followed.

 

[Elect117]

You should be installing separate secondary runs at the transformer's secondary and not tapping the secondary conductors. Tapping the secondary conductors is tapping a tap. The panelboard must also be protected by 408.36(B).

If you are using a delta 120/240V 3ph secondary, then you can not just use any of the legs with the neutral. The legs that you are allowed to use are split by the neutral. Typically known as the high leg or stinger leg will measure 208V to ground. Do not use the high leg with line + neutral or line+line+neutral loads.

But yes, you can pull a L-L off the secondary bushings and protect that with a fused disconnect or a breaker. The secondary conductors, overload protection should be based on both 240.21 and 450.

 

[augie47]

. Tapping the secondary conductors is tapping a tap. T

I respectfully disagree. By definition of a tap the transformer secondary conductors are not a tap.
I agree that it would be best to bring separate conductors from the transformer to each panel but do not see it as a violation if one chooses to bring one properly sized secondary to a point and tap to that.

 

[LarryFine]

 

[wwhitney]

I agree that it would be best to bring separate conductors from the transformer to each panel but do not see it as a violation if one chooses to bring one properly sized secondary to a point and tap to that.

That "properly sized secondary" would need to comply with one of the sections of 240.21(C). My reading is that 240.21(C)(1) "Protection by Primary Overcurrent Device" would be the only option, and only then if the transformer topology is delta-delta.

Cheers, Wayne

 

[Elect117]

240.21 ..."Conductors supplied under 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4."

Secondary conductors are protected under section 240.21(C), therefore the secondary conductors, while being a separately derived system, is by definition a tap with respect to ground fault interruption and potentially overload. The point of supply is the primary side of the transformer (at least in this case).

This is why you will find the rules around protecting the secondary conductors of a transformer in 240.21(C) and not in 240.21(B) where protection of Feeder taps are located.

The handbook (2020) has a description just under the definition of tap conductor, "Section 240.21 allows for the installation of branch-circuit and feeder conductors that are protected against overcurrent using an OCPD downstream of the point of supply. Short-circuit and ground-fault protection is provided upstream of the tap."

If you installed a tap on the secondary conductors, your upstream protective device is the protection on the primary side of the transformer and not the secondary conductors you just tapped. You could not use section 240.21(C) twice because of the last sentence of 240.21 saying "Conductors supplied under 240.21(A) through (H) shall not supply another conductor".

The handbook (2020) further elaborates just under 240.21(C), "The secondary terminals of a transformer are permitted to supply one or more than one set of secondary conductors. For example, the secondary terminals could supply two separate sets of secondary conductors that feed two panelboards. One set of conductors could be installed using the 25-foot secondary conductor rule of 240.21(C)(6), while the other set of conductors could be installed using the 10-foot secondary conductor rule of 240.21(C)(2). Each set is treated individually in applying the applicable secondary conductor requirement."

Section 240.21(C)(2) specifies in (2) "The secondary conductors are enclosed in a raceway, which shall extend from the transformer to the enclosure of an enclosed switchboard, switchgear, a panelboard, or control devices or to the back of an open switchboard."

Lastly, the fundamental change from240.21 (B) to (C) is that the term taps is replaced by secondary conductor. So when you read secondary conductor, you should see Tap. So saying you can tap the secondary conductor is like saying you can tap the tap.

Let me know what you think. If there are some resources out there that say secondary conductors can have feeder taps then I don't get the point of saying, "Conductors supplied under 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4."

 

[augie47]

Hopefully Larry didn't choke on his popcorn
I was too fixated on the definition of a "tap" but in review I see the same basic restrictions on transformer secondary conductors.
My apologies.... a separate set of conductors to each load would be the compliant method.

 

[Jpflex]

I respectfully disagree. By definition of a tap the transformer secondary conductors are not a tap.
I agree that it would be best to bring separate conductors from the transformer to each panel but do not see it as a violation if one chooses to bring one properly sized secondary to a point and tap to that.

Isn’t a tap just simply splicing a smaller conductor to a larger conductor feed without providing OCPD at the larger feed wire upstream of smaller “tap” wire?

 

[Jpflex]

Is there anything in the code that prevents tapping secondary of a 3-phase transformer to feed (3) different L-L single phase circuits to (3) separate downstream panelboards. I'm not aware of anything in code that prohibits that but wanted to see if others were aware.

I know that it is permissible to tap multiple 3-phase circuits to different locations as long as each secondary conductor is adequately protected according to 240.21 however i've never seen separate single phase circuits tapped directly at transformer secondary. I would mostly see the single phase circuits derived 3-phase panelboard fed from transformer secondary.

In the first paragraph did you mean splicing feeders at transformer secondary to feed a

1 phase panel from phase A&B
2nd single phase from B&C
3 rd single phase from A&C
Each terminating at the proper sized panel main breaker

I thought this was standard and not a tap sine the feeders terminate to a breaker and all panels can be balanced properly?

 

[Jpflex]

Is there anything in the code that prevents tapping secondary of a 3-phase transformer to feed (3) different L-L single phase circuits to (3) separate downstream panelboards. I'm not aware of anything in code that prohibits that but wanted to see if others were aware.

I know that it is permissible to tap multiple 3-phase circuits to different locations as long as each secondary conductor is adequately protected according to 240.21 however i've never seen separate single phase circuits tapped directly at transformer secondary. I would mostly see the single phase circuits derived 3-phase panelboard fed from transformer secondary.

The other way is feeding all three phase from 3 phase secondary transformer to a single 3 phase transformer with a derived neutral. Each phase and the neutral feeds 1 phase loads?

 

[jaggedben]

Isn’t a tap just simply splicing a smaller conductor to a larger conductor feed without providing OCPD at the larger feed wire upstream of smaller “tap” wire?

In 240.2, a tap is just any wire that is too small for the overcurrent device on the supply side of it. It doesn't have to be spliced to a bigger wire that isn't too small.

For example the rules are exactly the same for both of these:
- two 100A circuits tapped off 200A conductors protected by a 200A breaker
- two 100A circuits connected directly to the 200A breaker (with appropriate lugs)

They are both taps governed by 240.21(B).

 

[Jpflex]

- two 100A circuits connected directly to the 200A breaker (with appropriate lugs)

This sounds like a parallel connection or two 100 ampere conductors for a 200 ampere breaker

 

[Jpflex]

In 240.2, a tap is just any wire that is too small for the overcurrent device on the supply side of it. It doesn't have to be spliced to a bigger wire that isn't too small.

For example the rules are exactly the same for both of these:
- two 100A circuits tapped off 200A conductors protected by a 200A breaker
- two 100A circuits connected directly to the 200A breaker (with appropriate lugs)

They are both taps governed by 240.21(B).

Sounds like what I was saying: a tap being from a larger supply wire (feeder) protected by an OCPD at its ampacity where it receives its source but terminates to a smaller (tapped) wire without having an OCPD sized for the smaller wire where it receives its power

 

[LarryFine]

This sounds like a parallel connection or two 100 ampere conductors for a 200 ampere breaker

The 200a breaker is the source, with two sets of 100a load conductors, load ends not tied together.