Sub Panel & Conductor Sizing

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bbehringer

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I'm new to the MH Forums, I have a plan checker in So Cal that issued a PV plan correction for the following: Provide Calculations for the Load Center and the conductors between the Load Center and the Main Service Panel per 690.64(B)(2).

My main panel is 225a and main breaker is 200a. I am backfeeding the PV with a 45a breaker. For the Sub Panel (load center) I have a 100a loadcenter, with 1 - 20a 2p breaker and 1 - 25a 2 pole breaker. He has indicated that based on 690.64(B)(2) I have to calculate for the sub panel as well as the conductors between the sub panel and the main service panel

So my calcs are Sub Panel: 100a 20+25+45x1.2=108a.
Conductors: 20+25+45/1.2=75a.

So the 100a panel is adequate based on calc. The conductors will need to be #4 (82.65 allowable ampacity) from my chart based on 113f. I guess my question is, is, I have never calc'd for the conductors in this manner (having to add the breakers from the service panel and subpanel then dividing by the 1.20 rule. Is my sub panel and conductor calcs correct?
 
Welcome... Sounds like you have a solar aggregation subpanel and some AHJ's don't see it that way...

I think it is like this...100x120%=120amps so the max backfeed is 20amp PV breaker. Same for the conductor feeding the subpanel...must be rated 120amps.


shortcircuit
 
bbehringer said:
I'm new to the MH Forums, I have a plan checker in So Cal that issued a PV plan correction for the following: Provide Calculations for the Load Center and the conductors between the Load Center and the Main Service Panel per 690.64(B)(2).

My main panel is 225a and main breaker is 200a. I am backfeeding the PV with a 45a breaker. For the Sub Panel (load center) I have a 100a loadcenter, with 1 - 20a 2p breaker and 1 - 25a 2 pole breaker. He has indicated that based on 690.64(B)(2) I have to calculate for the sub panel as well as the conductors between the sub panel and the main service panel

So my calcs are Sub Panel: 100a 20+25+45x1.2=108a.
Conductors: 20+25+45/1.2=75a.

So the 100a panel is adequate based on calc. The conductors will need to be #4 (82.65 allowable ampacity) from my chart based on 113f. I guess my question is, is, I have never calc'd for the conductors in this manner (having to add the breakers from the service panel and subpanel then dividing by the 1.20 rule. Is my sub panel and conductor calcs correct?
Click to expand...

The conductors between the main panel and the subpanel form an interesting area for discussion when PV backfeed is involved. Some inspectors assume that some day someone might tap a load to the feeder somewhere between the main and the sub, thus creating a situation where the feeder could be fed from both ends and overloaded. This, in their minds, justifies applying the 120% rule to the feeder conductors.

Others argue (with logic but not necessarily literal code support) that if there are no taps on the feeder now you can calculate on the basis that there will never be a load in the middle of the feeder. So the current in the feeder cannot exceed the larger of the PV output current or the feeder breaker current.

The same issue comes up when the sub-panel is currently just a combiner panel for the PV output and is not intended to have load breakers installed. It is more reasonable (if there are spaces left in the subpanel) to apply the 120% rule to the subpanel bus. If the PV backfeed occupies all available slots in the subpanel, one has a good argument that the 120% rule does not apply, but....
If the subpanel already has load circuits, then the 120% rule clearly applies there.

Note that what size feeder breaker you backfeed at the main panel, you can treat the PV input to the main panel bus as limited to the sum of the two PV breakers in the subpanel (which in your case is the same 45A value, but might be different for others).
 
shortcircuit2 said:
Welcome... Sounds like you have a solar aggregation subpanel and some AHJ's don't see it that way...

I think it is like this...100x120%=120amps so the max backfeed is 20amp PV breaker. Same for the conductor feeding the subpanel...must be rated 120amps.


shortcircuit
Click to expand...
Your 20A backfeed limit for the 100A subpanel is only accurate if it is MLO or has a 100A main breaker AND the feeder breaker at the main panel is also 100A.
Since the OP is feeding the sub from a 45A breaker, his calculation is correct.

As I discussed (not posted before your remark), the feeder conductor is fed from the 45A feeder breaker at one end and 45A total of PV at the other, and there is no reason to size it for 120A.
Many inspectors will accept that it does not even have to be sized for more than the normal for a 45A feeder circuit.
 
GoldDigger said:
Your 20A backfeed limit for the 100A subpanel is only accurate if it is MLO or has a 100A main breaker AND the feeder breaker at the main panel is also 100A.
Since the OP is feeding the sub from a 45A breaker, his calculation is correct.

As I discussed (not posted before your remark), the feeder conductor is fed from the 45A feeder breaker at one end and 45A total of PV at the other, and there is no reason to size it for 120A.
Many inspectors will accept that it does not even have to be sized for more than the normal for a 45A feeder circuit.
Click to expand...

Yes...I'm abit confused on the where the 45amp breaker is...
 
shortcircuit2 said:
Yes...I'm a bit confused on the where the 45amp breaker is...
Click to expand...
The 45A is the sum of the 20A and 25A backfed breakers in the subpanel, and that is the number you use in calculating the 120% rule in the main panel irrespective of the rating of the backfed breaker in the main which feeds the sub.
 
Maybe a little more detail... the 100 subpanel has no breaker. It's just the 100a busbar Square D HOM612L100RBCP load center. It simply has the breakers for the inverters installed, and will be labeled PV Only. I'm asking if, based on my calcs (and what the BO is asking), that I:

1. Sized the SubPanel correctly (correct calc/math) which includes adding up the subpanel breakers (from inverters) and the main service panel back fed PV breaker (45a) and multiplying by 1.20 ?
2. That the conductors from the subpanel to the main service panel need to be increased based on the calc/math provided, which is requiring adding up the subpanel breakers (from inverters) and the main service panel back fed PV breaker (45a) divided by 1.20?

Any input is appreciated, thanks Brent
 
bbehringer said:
Maybe a little more detail... the 100 subpanel has no breaker. It's just the 100a busbar Square D HOM612L100RBCP load center. It simply has the breakers for the inverters installed, and will be labeled PV Only. I'm asking if, based on my calcs (and what the BO is asking), that I:

1. Sized the SubPanel correctly (correct calc/math) which includes adding up the subpanel breakers (from inverters) and the main service panel back fed PV breaker (45a) and multiplying by 1.20 ?
Click to expand...
No, you divide by 1.2. The sum of the breakers feeding the bus can be up to 120% of its rating.
 
bbehringer said:
Maybe a little more detail... the 100 subpanel has no breaker.
Click to expand...

You use the breaker ahead of it for the calc, regardless of where that breaker is installed. In your case it's your 45A PV breaker in the main panel.

bbehringer said:
Conductors: 20+25+45/1.2=75a.
Click to expand...

You did it correctly here. It is the same calc for both the sub and conductors.

The conductors will need to be #4 (82.65 allowable ampacity) from my chart based on
Click to expand...

Sounds good to me. And your 100A sub is also adequate.
 
bbehringer said:
So for calculating for both the subpanel and the conductors between the subpanel and the main service panel I am dividing by 120%? Ok.
Click to expand...

So the math for above (for both the subpanel and conductors) would be 20+25+45=90/1.2=75a

To determine the allowable amps in my subpanel (which does not have a 100a main breaker, just busbar) I still take the 100a x 1.20 = 120a. and this would be the amps that I can not exceed feeding this panel.
 
bbehringer said:
So the math for above (for both the subpanel and conductors) would be 20+25+45=90/1.2=75a

To determine the allowable amps in my subpanel (which does not have a 100a main breaker, just busbar) I still take the 100a x 1.20 = 120a. and this would be the amps that I can not exceed feeding this panel.
Click to expand...
Correct. Multiply the busbar rating by 1.2 and compare that to the sum of the breakers feeding it, or divide the sum of the breakers by 1.2 and compare it to the busbar rating; it gets you to the same place.
 
ggunn said:
Correct. Multiply the busbar rating by 1.2 and compare that to the sum of the breakers feeding it, or divide the sum of the breakers by 1.2 and compare it to the busbar rating; it gets you to the same place.
Click to expand...

I seem to recall somewhere in the NEC or CEC, a requirement to size a subpanel or load center based on temperature detation of the equipment? I can't recall if/where this is? I recall having a 100a subpanel and with 2x40a breakers, the building official, durning plan review indicated I needed to upsize the panel to a 125a?
 
bbehringer said:
I seem to recall somewhere in the NEC or CEC, a requirement to size a subpanel or load center based on temperature detation of the equipment? I can't recall if/where this is? I recall having a 100a subpanel and with 2x40a breakers, the building official, durning plan review indicated I needed to upsize the panel to a 125a?
Click to expand...
Possibly another reason that some manufacturers already use a 125A rated bus in their 100A panels???
You should not need to upsize the main breaker unless you expect to be pushing the actual single-ended power feed to the breaker limit. And the load calculations already have some safety factors included.
 
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