Panel Calculation for 3phase but only 1 leg submitted

[InspectorPatches]

I have a panel calculation submitted that only included C phase. The engineer is trying to get me to approve this calculation stating that C phase is drawing 70 amps and can handle the added load because the breaker is rated 100amp and since no loads are being added to A and B he didn't include those loads. I'm not inclined to approve this and will still require the whole panel calculation but I wanted to hear from others if this is actually a thing or has anybody else approved a panel calc for a single phase rather than the whole panel.

 

[Dennis Alwon]

Well, I suspect most review boards would want an entire panel calculation but if the "C" phase is only drawing 70 amps and they are only loading it to 100 amps or less then it should be fine. I assume it is only a 120v load they are adding.

 

[InspectorPatches]

Well, I suspect most review boards would want an entire panel calculation but if the "C" phase is only drawing 70 amps and they are only loading it to 100 amps or less then it should be fine. I assume it is only a 120v load they are adding.

Let me ask you this, there is a kiln load on A and B, if the loads on A and B draw more than 30 amps combined would this not exceed the rating of the breaker and panel or does the breaker protect the conductors individually at 100amps each. Would that not then indicate a panel drawing more than its rating when loaded up? My understanding is that breakers trip when the combined load of all phases exceeds its rating. And yes only a 120v load is being added. my concern is the age of the building and a incredible amount of loads have been added previous without permit.

 

[Dennis Alwon]

Let me ask you this, there is a kiln load on A and B, if the loads on A and B draw more than 30 amps combined would this not exceed the rating of the breaker and panel or does the breaker protect the conductors individually at 100amps each. Would that not then indicate a panel drawing more than its rating when loaded up? My understanding is that breakers trip when the combined load of all phases exceeds its rating. And yes only a 120v load is being added. my concern is the age of the building and a incredible amount of loads have been added previous without permit.

So if you calculate the load on a panel and you have a 100 amps on each phase then a 100 amp breaker you can look at it as 100 amps would work assuming loads are not continuous.

 

[petersonra]

Let me ask you this, there is a kiln load on A and B, if the loads on A and B draw more than 30 amps combined would this not exceed the rating of the breaker and panel or does the breaker protect the conductors individually at 100amps each. Would that not then indicate a panel drawing more than its rating when loaded up? My understanding is that breakers trip when the combined load of all phases exceeds its rating. And yes only a 120v load is being added. my concern is the age of the building and a incredible amount of loads have been added previous without permit.

The breaker does not care much what the load is on the three conductors. It will trip if the load on any conductor exceeds the OCPD rating, although it may not trip for a long time, and it can run indefinitely a little above its rating.

The breaker does not care at all if additional loads have been added, permitted or otherwise.

I think it odd that the engineer chose to do this. I don't know what the rules for submitting such load calculations but I doubt they allow for submitting calculations only on a single phase and not the whole panel board.

 

[wwhitney]

My understanding is that breakers trip when the combined load of all phases exceeds its rating.

No, it's per pole. Loads on the A and B legs are not material to whether the C leg gets overloaded.

Cheers, Wayne

 

[petersonra]

It occurs to me that maybe the load calculations would show one or more of the other phases is loaded excessively by NEC load calculation standards.

Suppose the guy submits a full load calculation of the PB. Suppose it shows the phase he is adding loads to is NEC acceptable but one or more of the other phases is not. What basis would you have to disapprove the added load?

 

[InspectorPatches]

The breaker does not care much what the load is on the three conductors. It will trip if the load on any conductor exceeds the OCPD rating, although it may not trip for a long time, and it can run indefinitely a little above its rating.

The breaker does not care at all if additional loads have been added, permitted or otherwise.

I think it odd that the engineer chose to do this. I don't know what the rules for submitting such load calculations but I doubt they allow for submitting calculations only on a single phase and not the whole panel board.

They are doing this because when they do the panel calc for the whole panel board it shows a overload. I haven't seen their calcs yet but they say they cant pass when they do the whole panel.

 

[petersonra]

They are doing this because when they do the panel calc for the whole panel board it shows a overload. I haven't seen their calcs yet but they say they cant pass when they do the whole panel.

It does not show an "overload". It shows the load calculation exceeds the CB rating.

I would check closely what your requirements are for submitted load calculations.

If the load calculation required refers to the whole PB, I think the guy is out of luck. If it only applies to new loads that are added, I'd be inclined to accept it.

In any case, there is no real danger assuming the wires are sized according to the loads and OCPD ratings. The worst that would happen is a breaker trips and protects the circuit conductors, which is what it is supposed to do.

 

[InspectorPatches]

Breaker Ampacity

I understand you absolutely cannot exceed the breaker rating on any single phase, without the breaker tripping. But, isn't one characteristic or advantage of having a 3 phase panel over a 1 phase, besides having 3 phase power for necessary loads, is having an extra 200 amps at your disposal?

forums.mikeholt.com

Here's another thread with some more explanation on the same topic.

I thank you all for your inputs, extremely informative and helpful.