Tap Rule Confusion

[charlie b]

I have a single phase transformer with a 120/240 volt, 3-wire secondary. It supplies a MLO Panel less than 10 feet away. There are 10 breaker positions in use on that panel, and provisions for many more. Does this violate 240.21(C)?

Here?s my problem: Look at 240.21(B)(2)(2). There is an explicit requirement that the tap conductors must land on an overcurrent device. Now look at 240.21(B)(1), and while you are at it, check out 240.21(C)(2). Both cases share the 10 foot limit, and neither appears to require an overcurrent device at the load end of the tap conductors. There is a vague (IMHO) allusion to an overcurrent device in 240.21(B)(1)(1)(a), and another in 240.21(C)(2)(1)(a), but I don?t see an explicit requirement that there be an overcurrent device.

I have always required any panel immediately downstream of a transformer to have a main breaker. Now I am uncertain of a code-related basis for that requirement. Any thoughts?

 

[augie47]

No doubt opinions may vary.
Mine: 240.21(C)(2) would be the applicable Code.
If one accepts the panel as a "device" then your installation meets Code if
the calculated load does not exceed the conductor ampacity and the MLO panel rating is greater than the conductor ampacity.

(assuming less than 10 ft of condcutor)

 

[raberding]

not the Tap Rule, but...

not the Tap Rule, but...

i think that 408.36(B) is gonna require OCP for the panelboard...

 

[raider1]

408.36(B) requires that the overcurrent protection for a panelboard supplied by a transformer be located on the secondary side of the transformer.

408.36(A) requires a panelboard to be protected by an overcurrent device having a rating not greater than that of the panelboard.

So you would need to have a main breaker in the panelboard or an overcurrent device ahead of the panelboard and after the transformer.

Chris

 

[augie47]

Dang It I keep forgetting that
Thanks guys

 

[charlie b]

Actually, that is the part that I had forgotten as well. Many thanks. I will be at the client's office this afternoon, and I wanted to be certain of my facts before I told them they have a problem.

 

[jusme123]

just found what i was looking for

 

[eprice]

I agree with what raberding and raider1 have said. I also think a main breaker is required at the panel for another reason. I think 240.15 requires a single overcurrent device. The tap rules are just a modification to the general requirement in 240.21 that the overcurrent device be located at the supply end of the conductors. They are not an exception to 240.15.

 

[Smart $]

Actually, that is the part that I had forgotten as well. Many thanks. I will be at the client's office this afternoon, and I wanted to be certain of my facts before I told them they have a problem.

Question here... possibly too late for your meeting... but you said ten breaker positions... how many breakers? Is it a service-rated panelboard? I haven't confirmed the notion yet, but I believe a service-rated mlo panelboard is permitted for up to 6 disconnecting means in such an installation.

 

[raider1]

Question here... possibly too late for your meeting... but you said ten breaker positions... how many breakers? Is it a service-rated panelboard? I haven't confirmed the notion yet, but I believe a service-rated mlo panelboard is permitted for up to 6 disconnecting means in such an installation.

Exception #1 to 408.36 only applys to panelboards used as service equipment.

Chris

 

[Smart $]

Yep... I was thinking of, in part, 240.92(C)(2)(2), which is for supervised industrial locations.

 

[cadpoint]

So Charlie has two problems ?

 

[charlie b]

Question here... possibly too late for your meeting...

It was, but thanks anyway.

. . . you said ten breaker positions... how many breakers?

Nine breakers (one being a 2-pole).

Is it a service-rated panelboard?

I don?t know, and indeed I don?t know how to find out. Not that it will matter in this instance, but where would look for that kind of rating? The panel cover is going to be removed next week, so that voltage readings can be taken. Will there be a statement imprinted somewhere on the enclosure, or would I have to look up the make and model on the manufacturer?s web site?

 

[charlie b]

Yep... I was thinking of, in part, 240.92(C)(2)(2), which is for supervised industrial locations.

Supervised? Yes. Industrial? Not sure what that means. All maintenance is performed by qualified persons using documented procedures and controls. But this is a university, not a manufacturing facility. What qualifies as "industrial"?

 

[pete m.]

Will there be a statement imprinted somewhere on the enclosure, or would I have to look up the make and model on the manufacturer?s web site?

Typically, what I see is on the panelboard or disconnect itself. It usually states something along the lines of "Suitable for use as service equipment if..."

Pete

 

[Smart $]

... What qualifies as "industrial"?

240.90 General. ... The provisions of Part VIII shall be
permitted only to apply to those portions of the electrical
system in the supervised industrial installation used exclusively
for manufacturing or process control activities.

 

[LarryFine]

Nine breakers (one being a 2-pole)

There you go: replace the eight 1-p's with four 2-p's, and you'll have five handles.

 

[yired29]

I have a single phase transformer with a 120/240 volt, 3-wire secondary. SIZE]

240.4 (F) would require a multi voltage secondary to have OCPD for the secondary in compliance with 240.21 (C) 2-6.

Question here... possibly too late for your meeting... but you said ten breaker positions... how many breakers? Is it a service-rated panelboard? I haven't confirmed the notion yet, but I believe a service-rated mlo panelboard is permitted for up to 6 disconnecting means in such an installation.

Table 450.3 (B) note 2 only allows six breaker rule.

 

[david luchini]

look at 240.21(B)(1), and while you are at it, check out 240.21(C)(2). Both cases share the 10 foot limit, and neither appears to require an overcurrent device at the load end of the tap conductors. There is a vague (IMHO) allusion to an overcurrent device in 240.21(B)(1)(1)(a), and another in 240.21(C)(2)(1)(a), but I don?t see an explicit requirement that there be an overcurrent device.

I've always had in my mind that transformer secondary conductors must terminate at an OCPD, but as Charlie notes under 240.21(C), the secondary conductors must have an ampacity of at least the rating of the OCPD or DEVICE at which they terminate.

So what are the DEVICEs that the code is talking about. For instance, if I had a 480 Delta service, but wanted to install an L22-20, 20A, 480/277, 4pole, 5wire, grounding receptacle could I install a 15kVA, 480-480/277 transformer with primary only protection, running:

3#10+#10G from a 25A c/b on the primary, and 4#14+#8G from the transformer (10' or less) to the receptacle, with no circuit breaker on the secondary side?

 

[yired29]

I've always had in my mind that transformer secondary conductors must terminate at an OCPD, but as Charlie notes under 240.21(C), the secondary conductors must have an ampacity of at least the rating of the OCPD or DEVICE at which they terminate.

So what are the DEVICEs that the code is talking about. For instance, if I had a 480 Delta service, but wanted to install an L22-20, 20A, 480/277, 4pole, 5wire, grounding receptacle could I install a 15kVA, 480-480/277 transformer with primary only protection, running:

3#10+#10G from a 25A c/b on the primary, and 4#14+#8G from the transformer (10' or less) to the receptacle, with no circuit breaker on the secondary side?

240.4 (F) and 240.21 (C) 1 will not allow primary only protection on a multi voltage secondary.