10 foot or 25 foot

[mannyb]

one of our jobs has 3 transfomers to be installed in ceiling under 50 kva. per drawing they are all required to be installed with 10' tap rule. The area is small and the xfmrs will need to be installed further than 10'. If I cant install the xfmrs within 10' using 10' foot tap rule can I use he 25' tap rule. The transformer will be 480v 3 phase to 120/240v 3 phase. Should the engineer be notified concerning the 10' rule but either way and can i use the 25' tap rule for transformer install ? Or does the 10' and 25' only apply if i dont install a OPCD on secondary side of xfmr?

10-Foot Secondary Tap Rule [240.21(C)(2)]
Secondary conductors can be run up to 10 ft without overcurrent protection, but they must be installed in accordance with the following requirements: Figure 4
(1) The ampacity of the tap conductor is:
Not less than the computed load in accordance with Article 220, and
Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors.
(2) The secondary conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.
(3) The secondary conductors are enclosed in a raceway.

25-Foot Secondary Tap Rule [240.21(C)(6)]
Secondary conductors can be run not over 25 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 6
(1) The secondary conductors have an ampacity that (when multiplied by the ratio of the secondary-to-primary voltage) is at least 1/3 of the rating of the overcurrent device protecting the primary of the transformer.
(2) The secondary conductors terminate in a single circuit breaker or set of fuses that have a rating not greater than the conductor ampacity.
(3) The secondary conductors are protected from physical damage.

 

[LarryFine]

This is an architectural/engineering question. You may use whatever falls within the requirements you posted.

The designer who specified that you follow the 10' rule is the one with whom you need to have this discussion.

Unless I misunderstand your question.

 

[mannyb]

This is an architectural/engineering question. You may use whatever falls within the requirements you posted.

The designer who specified that you follow the 10' rule is the one with whom you need to have this discussion.

Unless I misunderstand your question.

I agree with you. BTW is there a rule on how many xfmrs can be installed in ceiling? I was told that you cant install more than 3 xfmrs in ceiling tile. I asked them for code reference but they couldnt provide. I tried looking but didnt find anything. I was thinking if true its probably a city ordnance or something like that instead of NEC.

 

[Carultch]

one of our jobs has 3 transfomers to be installed in ceiling under 50 kva. per drawing they are all required to be installed with 10' tap rule. The area is small and the xfmrs will need to be installed further than 10'. If I cant install the xfmrs within 10' using 10' foot tap rule can I use he 25' tap rule. The transformer will be 480v 3 phase to 120/240v 3 phase. Should the engineer be notified concerning the 10' rule but either way and can i use the 25' tap rule for transformer install ? Or does the 10' and 25' only apply if i dont install a OPCD on secondary side of xfmr?

10-Foot Secondary Tap Rule [240.21(C)(2)]
Secondary conductors can be run up to 10 ft without overcurrent protection, but they must be installed in accordance with the following requirements: Figure 4
(1) The ampacity of the tap conductor is:
Not less than the computed load in accordance with Article 220, and
Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors.
(2) The secondary conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.
(3) The secondary conductors are enclosed in a raceway.

25-Foot Secondary Tap Rule [240.21(C)(6)]
Secondary conductors can be run not over 25 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 6
(1) The secondary conductors have an ampacity that (when multiplied by the ratio of the secondary-to-primary voltage) is at least 1/3 of the rating of the overcurrent device protecting the primary of the transformer.
(2) The secondary conductors terminate in a single circuit breaker or set of fuses that have a rating not greater than the conductor ampacity.
(3) The secondary conductors are protected from physical damage.

Taps are governed by 240.21(B). Conductors governed by 240.21(C) are no longer called taps, they are called transformer secondary conductors. The rules are very similar to rules for taps, but the NEC no longer calls them tap conductors. A tap is a conductor with protection in excess of its ampacity, that eventually terminates in an overcurrent device protecting it at its ampacity. A transformer secondary conductor connects from the secondary busbar of the transformer, through a raceway, to the first OCPD on the secondary side (if there is one).

The length limits in sections of 240.21(C) apply any time a secondary OCPD *is* required. In the specific situations where it isn't required, no length limit applies. Those are situations based on transformer topology (single phase 2-wire to 2-wire, and delta-to-delta 3-wire), such that the primary OCPD already does enough to protect the secondary conductors (as I like to say, "by proxy"). The reason those topologies allow the primary OCPD alone to suffice, is that fault currents line up directly across the winding, and don't get intermingled the way they do in systems with centertapped windings and wye systems.

You have your choice of either using the 10 ft rule, or the 25 ft rule, as long as you meet the sizing and context rules in 240.21(C) accordingly. The engineer might have another reason to specify a 10 ft length limit, so I would recommend informing the engineering team of the issue to get approval to use the 25 ft rule instead of the 10 ft rule.

 

[AtTheKeyboard]

I haven't heard of that 3-transformer rule you just mentioned either.

If time allows, it may make sense to send up a RFI to the engineer, that way you're covered either way.

 

[victor.cherkashi]

if you connect only one panel or one disconnect switch to secondary transformer, it is very easy to comply with the 240.21(C)(6)

Sent from my ONEPLUS A6013 using Tapatalk

 

[charlie b]

I agree with those who have said the 25 foot rule could be applied. I further agree with AtTheKeyboard, that you may need to submit an RFI. It depends mostly on the wording of your contract. Do you have the authority to alter the approved design, so long as you comply with code? I am inclined to suspect that you do not have that authority. Let the engineer into this conversation, explain the circumstances, and ask whether he or she agrees with your proposed path forward.

 

[charlie b]

I was told that you cant install more than 3 xfmrs in ceiling tile.

Told by whom (i.e., what degree of authority does the person have)? Told how (i.e., note on a drawing, paragraph in a specification, or simple word of mouth)?