400a 3ph MB with 300a trip unit - wire size?

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James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
I installed a 3ph 400a service - outside CT cabinet, CTs, meter can, inside MB panel. Existing conduit is 3" PVC. Considering pipe size, I fed it with (4) 500kcmil aluminum (ATHHN). However, the wire is not rated at 400 amps, so I had the trip unit changed to 300 amps. The breaker itself is marked 400 amp, but the trip unit is clearly marked 300 amp. Total connected load will be about 160 amps. My issue is that the inspector declared it to be a 400 amp service because the labels show that main breaker and buss are rated at 400 amps. He insists that I change he cable to 500kcmil copper, a pretty expensive upgrade. I tried to explain that it is akin to using a 60 amp disconnect with 40 amp fuses in it, and feeding it with 8 gauge wire. Yet he would not budge that I installed a 400 amp service. Is there any issue with having a 400 amp label on the breaker casing, while though the trip unit is marked 300 amp?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
IMO if the CB complies with 240.82 it should be compliant.

240.82 Nontamperable. A circuit breaker shall be of such
design that any alteration of its trip point (calibration) or
the time required for its operation requires dismantling of
the device or breaking of a seal for other than intended
adjustments.
 

iwire

Moderator
Staff member
Location
Massachusetts
Your not expressing your opinions very clearly this morning. No coffee yet?:)

I was thinking about the tens (hundreds?) of thousands of installations this inspector would fail due to frame size vs rating plug size.

Maybe he just did have his coffee yet. :p
 

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
IMO if the CB complies with 240.82 it should be compliant.


Thanks for the code reference. I did tell the inspector that it's not as if somebody could simply "plug in" a 400 amp trip unit, that the breaker has to be dismantled in order to change it, but I was not aware of that being specifically addressed in the NEC.
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
With that Code reference hopefully you can convince the inspector or his (her) supervisor that you are compliant.
POCO equipment ripe with similar situations. Do they size their lines based on the largest fuse that can be installed ?
 

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
One reason I asked is that LA members have reported that LADWP goes by the bus rating regardless of what size breaker is installed.

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Yikes !! Good food for thought.
I'm not aware of that being the case here (Independence, Mo). But POCOs can require whatever they want, even if they don't comply with it in their installs.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Yikes !! Good food for thought.
I'm not aware of that being the case here (Independence, Mo). But POCOs can require whatever they want, even if they don't comply with it in their installs.

It is particularly frustrating when you want to install a higher amp panel with a small main breaker to satisfy the 120% rule for solar backfeed and DWP then insists on the service being upgraded. :(


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James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
Another failed inspection

Another failed inspection

I have an update about this service I've been trying to get passed for weeks

As a recap, I have a 400 amp main breaker, which has a 300 amp trip unit installed in it. Fed with 500 kcmil AL

I worked through the 400 amp rating of the panel buses, so that's good
I did not have a total connected load as calculated by an engineer. I had my own load calculation (160 amps), which was not good enough.
Had to go back to the architect to have his engineer do a load calc - 156 amps. Now that's good

Now the inspector has cited Article 230.42, that the amperage of the conductor must not be smaller than the amperage rating of the service disconnect. His interpretation is that even though the unit trips at 300 amps, the disconnect is still rated at 400 amps

I asked him if I could pose a goofy hypothetical scenario for a frame of reference:
suppose I install a 600 amp fusible disconnect, and install 100 amp fuses in it?
His answer is that I would have to feed a 600 amp disconnect with a cable large enough for 600 amps, regardless of fuse size

Has anyone ever interpreted Article 230.42 this way?

Maybe even as important - Is there any code which states explicitly that the disconnect rating is determined by the overcurrent rating?
 
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augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I have know of one TN inspector that required the conductor match the maximum fuse capability of a service disconnect.
I have never seen such a NEC requirement.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Has anyone ever interpreted Article 230.42 this way?

Maybe even as important - Is there any code which states explicitly that the disconnect rating is determined by the overcurrent rating?

230.42(A) says the conductors are sized based on the load (not the equipment).
230.42(B) says says underground conductors need to be sized based on the equipment, for specific installations outlined in 230.79.

240.6(C) says an adjustable breaker is based on its actual settings if access is restricted by something as simple as a cover that is sealed.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I'm surprised at your patience, I would have already gone over his head to his boss.
 

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
I'm surprised at your patience, I would have already gone over his head to his boss.

I'm not a fan of going over an inspector's head. I really prefer to reason things out, because if his perspective is changed it can be to everyone's benefit in the future

But I had to do it after this issue. Next higher up says he sees no problem with it, but it is not technically his call to make. His higher up is out of the office this week, should be resolved on Monday.

I really appreciate the input I received from everyone.
 
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