# Double tapping a transformer



## MotoGP1199 (Aug 11, 2014)

Yep, done all the time


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## wiz1997 (Mar 30, 2021)

If the secondary terminal has a single lug, meaning only one place to put a wire, I would replace the lugs with lugs made for two.
Things not mentioned;
Distance from power source to transformer and size of breaker.
Distance from transformer to panels.
Do the panels have a main breaker?
Temporary or permanent.


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## Southeast Power (Jan 18, 2009)

Jimmyspark said:


> Hello everyone, I have been out of the game for awhile, but was recently asked to do a little job for a buddy in a pinch.
> So I have a 480v MDP and need to feed a step down transformer that will handle two 200 amp MB 208/120v 42 circuit panels. Panels will only be filled with single 20a breakers feeding small cord reels. My question is can I double lug the secondary side to feed each panel?


I wouldn't do it that way, I would buy a feed through panelboard. Double lugs are for parallel feeders, not so much for A and B connections.


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## micromind (Aug 11, 2007)

If the panels are main breaker (as stated) then double lugging the transformer is ok and it's done often.


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## Southeast Power (Jan 18, 2009)

micromind said:


> If the panels are main breaker (as stated) then double lugging the transformer is ok and it's done often.


Ive done it but, if I were setting up two identical panels, I would set it up as a feed-through.


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## HertzHound (Jan 22, 2019)

The panels have to have main overcurrent protection, and they do as stated by the OP. The OCP at each panel is said to be 200 amps. What we don't know is the KVA of the transformer?

My question is what size wire would feed each panel? If the transformer is sized at 400A, 208V, Then it would be a 145 KVA transformer? So the taps would have to be sized for the 400A to each panel? The 200 A main breaker only protects the panelboard. If the wire left the transformer as two 200 amp taps, and one became shorted, the primary OCP would not trip before the wire burned up? If thats the case, the wire to each 200A panel would have to be parallel 250s or single 600s? Maybe I'm off a bit depending on the 10 or 25' tap rule? 

I think I've only double taped a transformer on one job a long time ago. There were several 75 KVA transformers along a wall with (2) 100A enclosed breakers on the secondaries of each. I do remember fighting the wire in the small enclosures. For this reason I would go with a 400A feed through panel. That seems to be the normal approach that I have seen.

In this case though, the 200A main breaker panels were probably already provided? It's also a possibility that the transformer is only a 75 KVA?


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## MotoGP1199 (Aug 11, 2014)

HertzHound said:


> My question is what size wire would feed each panel? If the transformer is sized at 400A, 208V, Then it would be a 145 KVA transformer? So the taps would have to be sized for the 400A to each panel? The 200 A main breaker only protects the panelboard. If the wire left the transformer as two 200 amp taps, and one became shorted, the primary OCP would not trip before the wire burned up? If thats the case, the wire to each 200A panel would have to be parallel 250s or single 600s? Maybe I'm off a bit depending on the 10 or 25' tap rule?


Each set of conductors would be sized only to handle the load for each panel. Step up rules don't apply so the conductors would have to be sized at 100% of the OCPD. Per 240.21 (C) 2. And you are allowed up to 6 overcurrent devices grouped together Per table 430.3(A) NOTE: 2. This also states that the total of both OCPD on the secondary cannot not exceed what a single OCPD would be. I see this as meaning that he would need a transformer large enough for 400 amps, otherwise he would have to set up the panels as a feed through setup on a transformer sized for 200 amps.



> 430.3(A) NOTE: 2
> 2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to
> consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings
> shall not exceed that allowed for fuses





> 240.21 (C) 2
> (2) Transformer Secondary Conductors Not over 3 m (10 ft)
> Long. If the length of secondary conductor does not exceed 3 m
> (10 ft) and complies with all of the following:
> ...


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## oldsparky52 (Feb 25, 2020)

Southeast Power said:


> Double lugs are for parallel feeders, not so much for A and B connections.


I didn't know that. Do you know why this is so? Would you share?


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## HertzHound (Jan 22, 2019)

> Each set of conductors would be sized only to handle the load for each panel. Step up rules don't apply so the conductors would have to be sized at 100% of the OCPD. Per 240.21 (C) 2. And you are allowed up to 6 overcurrent devices grouped together Per table 430.3(A) NOTE: 2. This also states that the total of both OCPD on the secondary cannot not exceed what a single OCPD would be. I see this as meaning that he would need a transformer large enough for 400 amps, otherwise he would have to set up the panels as a feed through setup on a transformer sized for 200 amps.


I spent way to long reviewing tap rules last night. I could not find one example in all the books I have with this scenario. I have some questions from your post above. You were quoting (C)2 which is the ten foot tap rule. I never even look at the ten foot tap rule, I always go to the 25’ tap rules. 

In the OP’s situation how would you measure the tap? Wouldn’t you have to add both panel lengths together and be under 10’? 

If you are reducing the wire size from what’s available from the transformer, isn’t that considered taping a tap? Wouldn’t that be the same as coming off the transformer with 400A wire and tapping off (2) 200A taps?

I agree that you can have up to six disconnects, I’m just not clear on the wire size, and it looks like it may be different depending on the 10 and 25’ tap rules?


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## MotoGP1199 (Aug 11, 2014)

HertzHound said:


> In the OP’s situation how would you measure the tap? Wouldn’t you have to add both panel lengths together and be under 10’?


If using the 10' tap rule, they would each have to be 10' or less. Not added together. They are each their own set. Each set would go individually back to the transformer so its not tapping a tap.. Where I have seen this scenario is in an environment where they have multiple panels off one transformer where they don't want to spend the money for 400 amp panels/switch gear and wire. They could have two 200 amp panels feeding different things with cheaper 200 amp main breakers to isolate each set of breakers. The other scenario is in industrial where they have a few machines being fed by a single transformer with cheap disconnects(and smaller wire sized for that equipment) instead of expensive switchgear being fed by large wire/conduit etc. 

I personally have used this rule a few times where someone has an oversized transformer that is feeding a single machine(probably changed at some point) and they bought another machine that uses the same voltage. I just added a second disconnect to the secondary of the transformer and made sure that both OCPD added together complies with NOTE 2. I then didn't have to touch anything for the first machine and the second smaller machine just gets a small disconnect, conduit, and wire sized for its load.

The 25' rule makes it hard to comply with the 1/3 ampacity rule for multiple pieces of equipment and also comply with NOTE 2 of table 430.3A that says all the OCPD can't add up to more than the OCPD rating if their was only one OCPD device. If you have two panels this wouldn't be an issue but if you have 3(maybe) or more disconnects for equipment being fed directly from the transformer its not possible to comply with both of those rules. 


feeder conductors


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## mofos be cray (Nov 14, 2016)

Why not out of the xformer to a trough and then feed the panels from the trough? Then you just have one set to fit into the transformer and one set to each of those panels.


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## HertzHound (Jan 22, 2019)

You could use a trough and run separate sets of 200A wire back to the transformer, but if you were to tap the two 200A wires to a 400A wire in the trough, and then run the 400A wire to the transformer, that would be tapping a tap which is not allowed.

Moto was right in the above post. I read it in the books last night, but I just couldn’t get it that running two 200A amp wires back to the transformer wasn’t the same as taping a tap. Or that every wire/conduit that left the transformer wasn’t additive to the max of 10’. It just seemed to me that they wouldn’t want 60’ (6 x10’ taps) outside the transformer. It does seem impossible to have six disconnects all within ten feet. Ten feet is pretty short.

Thanks Moto.
















And thanks McGraw Hill.


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