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hhsting

Senior Member
I have 400A panelboard with CALCULATED DEMAND amps of 30amps at this time.

The feeder breaker to the panelboard is 400A while the engineer has provided feeder cable size to panelboard of #10awg copper.

I let the engineer know this is not correct as the cable size has to be 500kcmil copper but engineer argues that demand load right now is 30 amps so #10awg copper is correct.

Is the engineer correct or incorrect?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
If you have a #10 conductor protected by a 400 amp breaker, I would hope you would even need an opinion from here..
 

hhsting

Senior Member
If you have a #10 conductor protected by a 400 amp breaker, I would hope you would even need an opinion from here..

Told the engineer you need 500kcmil feeder cable he kept on saying why when demand load is 30A for 400A panelboard fed from400A FDR BKR.

This is completely incorrect in my opinion but where is the code section?
 

hhsting

Senior Member
Ten foot tap rule, when the conductors don't leave the enclosure, such that the "one tenth" part of the ten foot tap rule doesn't apply. Land in-line fuses on the tap conductors, and then you can travel as far as you need.

What tap rule allows 400A feeder breaker load side has #10awg to terminate in 400A panelboard that has 30A demand load?

Rating of panelboard is 400A not 30A? How is 10 ft tap rule apply?
 

Carultch

Senior Member
Location
Massachusetts
What tap rule allows 400A feeder breaker load side has #10awg to terminate in 400A panelboard that has 30A demand load?

Rating of panelboard is 400A not 30A? How is 10 ft tap rule apply?


From 240.21(B)(1):
"For field installations, if the tap conductors leave the enclosure or vault in which the tap is made, the ampacity of the tap conductors is not less than one-tenth of the rating of the overcurrent device protecting the feeder conductors"

A logical consequence of this rule, is that if the tap conductors do not leave the enclosure, the one tenth part if this rule would not apply. That is, until the tap conductors are load-side of the fuses and get promoted from tap conductors to feeder conductors.

As an example, you have a 400A feeder, and you desire to make a 30A tap. If you terminate your tap conductors on an adjacent 30A fused disconnect, and your tap wires need to be 40A for the wire ampacity (i.e. #8 Cu). By contrast, if you terminate on 30A in-line fuses inside the enclosure where you originate your tap, you can use #10 Cu wire for the tap. You could even use 15A fuses and #14 Cu wire, and it would still comply as long as you don't leave the enclosure until you are load-side of the fuses.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Basic principle is that anywhere on the load side of the service disconnect a tap wire (any wire not protected at its rated ampacity on the upstream side) must terminate on an OCPD that protects it.
Note that Carultch is proposing an inline 30 A fuse. Before the fuse it might be a legal tap if other conditions are met, including the situation he proposes. Below the fuse it is protected at its ampacity.
 

hhsting

Senior Member
From 240.21(B)(1):
"For field installations, if the tap conductors leave the enclosure or vault in which the tap is made, the ampacity of the tap conductors is not less than one-tenth of the rating of the overcurrent device protecting the feeder conductors"

A logical consequence of this rule, is that if the tap conductors do not leave the enclosure, the one tenth part if this rule would not apply. That is, until the tap conductors are load-side of the fuses and get promoted from tap conductors to feeder conductors.

As an example, you have a 400A feeder, and you desire to make a 30A tap. If you terminate your tap conductors on an adjacent 30A fused disconnect, and your tap wires need to be 40A for the wire ampacity (i.e. #8 Cu). By contrast, if you terminate on 30A in-line fuses inside the enclosure where you originate your tap, you can use #10 Cu wire for the tap. You could even use 15A fuses and #14 Cu wire, and it would still comply as long as you don't leave the enclosure until you are load-side of the fuses.

Huh? Their is no 30A fuse. Load side of 400A breaker is just #10 awg. Ampacity shall not be less than 1/10 of OCPD.

So lets see 400/10=40A


Is #10 awg cu less than 40A? Where does it say if wires are in enclosure 1/10 does not apply?
 
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