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Carultch

Senior Member
Location
Massachusetts
Huh? 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?

My example assumes 75C terminations.

If you read the 10 ft tap rule in its entirety, you will see that the one tenth part only applies on the condition that the conductors leave the enclosure. It doesn't directly say that it doesn't apply when they don't leave the enclosure, but it is a logical consequence of what the rule does say.
 

hhsting

Senior Member
My example assumes 75C terminations.

If you read the 10 ft tap rule in its entirety, you will see that the one tenth part only applies on the condition that the conductors leave the enclosure. It doesn't directly say that it doesn't apply when they don't leave the enclosure, but it is a logical consequence of what the rule does say.

First their is NO 30a fuse load side of the 400 feeder breaker to the 400A panelboard.

When I read that section I dont see anything that remotely says if they leave enclosure part applies?

Logic dont count as plan reviewers have to go by code language
 

ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
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"
Excellent point. Thank you for making this discussion more interesting.

Lets also consider 310.15(C),
With engineering supervision #10 does 40A at 90°C, or even 50A per Table 310.17 in knob & tube or aerial configurations.
 

ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
If engineering supervision per 310.15(C) can ignore NEC 240.4(D)7, #10 cu works as a 40A 10% Tap Rule with Table 310.16, if raceway is buried near the frost line that keeps a temperature of <=59°F year round, per Table 310.15B2(a)
 
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ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
outside unlimited tap rule :sneaky:
After reading Electrofelon's reference to 240.21(5), I'm seeing #10 feeder coming from 400A fuse terminating to 30A Main/branch breaker, rather than main lugs.

No engineering or 40A adjustments needed at all.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
First their is NO 30a fuse load side of the 400 feeder breaker to the 400A panelboard.

When I read that section I dont see anything that remotely says if they leave enclosure part applies?

Logic dont count as plan reviewers have to go by code language

When you ask questions on this forum, you need to be prepared for tangents, and to ferret out the info you need from these tangents.

See post 14 where infinity stated that there is no code rule that would permit a #10 protected by a 400A breaker, at which point we were off on a tangent showing situations where you could have a 400A breaker and a #10.

In the case of your question, the tangent doesn't apply. I believe the smallest conductor that could be used for a general purpose panel protected by 400A breakers is 2 parallel 3/0 conductors with a combined derated ampacity of 360A.

There may be specific purpose applications such as welding or X-ray with duty cycle limits or motors that permit smaller conductors to be used properly protected by 400A breakers. I've not explored these possibilities, but if anyone else wants another tangent....

Jon
 
When you ask questions on this forum, you need to be prepared for tangents, and to ferret out the info you need from these tangents.

See post 14 where infinity stated that there is no code rule that would permit a #10 protected by a 400A breaker, at which point we were off on a tangent showing situations where you could have a 400A breaker and a #10.

In the case of your question, the tangent doesn't apply. I believe the smallest conductor that could be used for a general purpose panel protected by 400A breakers is 2 parallel 3/0 conductors with a combined derated ampacity of 360A.

There may be specific purpose applications such as welding or X-ray with duty cycle limits or motors that permit smaller conductors to be used properly protected by 400A breakers. I've not explored these possibilities, but if anyone else wants another tangent....

Jon
Yes I was mostly joking when I responded to infinity's post. I was making a comical yet true reference to how you really have to be careful using a word like "never" on this forum. We should really try to avoid tangents on stings threads as he really struggles with just the fundamentals.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Yes I was mostly joking when I responded to infinity's post. I was making a comical yet true reference to how you really have to be careful using a word like "never" on this forum. We should really try to avoid tangents on stings threads as he really struggles with just the fundamentals.
I was trying to keep my responses in context with the OP's question. To the OP as a plan reviewer just mark it rejected and send it back to the engineer to correct it whatever code complaint way he sees fit and let him resubmit. He may want to provide 400 amp feeder conductors or apply a feeder tap rule or something else. Plan reviewers aren't supposed to tell an engineer how to design components of an electrical system.
 
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