690.64(B)(2) as applied to a feeder tap

[eventhorizons]

Customer's existing setup consists of a 200A 3-phase Service meter with a 200A fused disconnect immediately adjacent to it. From the disconnect, it penetrates the building and goes to a 200A panel with a main breaker. 4/0 wire for all.

We are installing a solar array on the roof that will supply a peak of 42A. The original intention was to use the "10-foot Feeder tap rule" and tap between the outside fuses and the inside breaker, installing a 100A disconnect with 60A fuses. All panels are immediately next to each other.


Meter <----> 200A Fused Disconnect <----Tap----> 200A MB panel

Tap <--------> 60A-fused source


The county plan-checker rejected that idea, saying that 690.64(B)(2) applied here. This required changing the plan such that we would instead add a 200A panel and run 4/0 wire, making it a "splice" instead of a "tap" and also downsizing the main meter source to 175A. The customer does not need the full 200A service, so this is not a problem for him.

Question: Is the checker applying the code correctly? Based on my detail reading, especially the articles by John Wiles, I feel that he is. But this is one of the situations that begs for an exception, since the INTENT of the rule is to prevent overload of the conductors/bus due to multiple sources. In this case, there is no way that the current in the conductors can exceed 200A.

Thanks in advance for your feedback.

Ken

 

[Dennis Alwon]

I am not sure what the difference is between a splice and a tap as you stated.

 

[jaggedben]

The county plan-checker rejected that idea, saying that 690.64(B)(2) applied here.

The plan-checker is correct that the code section applies, but why he thought that your plan violated that section is not clear to me.

In your original plan, the conductor you are tapping needs to be rated for 206A/1.2, or 216A. If it's 4/0 copper rated 75C or greater with no further derating then it's good for that according to Art 310. If for whatever reason it has a lower ampacity, then downsizing the main fuses to 175A should solve the issue by itself with no other change to the plan.

Like Dennis I don't understand the difference between your original plan and your new one.

 

[eventhorizons]

The original intention was to use the "10-foot Feeder tap rule" and tap between the outside fuses and the inside breaker, installing a 100A disconnect with 60A fuses. All panels are immediately next to each other.

Meter <----> 200A Fused Disconnect <----Tap----> 200A MB panel

Tap <--------> 60A-fused source

My apologies for neglecting to mention the size of the feeder-tap wires that were in the original plan.

The lead from the tap to the 100A panel was 4AWG, while the original service wires were all 4/0. The ampacity of the tap was 72A, after de-rating. This complied with the 10-foot feeder tap rule, but not with the "multiple source" rule.

By my understanding, a "tap" is when you take off a lead with a lower ampacity, while a "splice" has the same ampacity.

The new plan increases the rating of the solar source panel from 100A to 200A and installs 60A fuses in it, with 4/0 wires leading to it. It also reduces the the main service fuses to 175A.

Now, per (B)(2), (175A + 60A) < (200A x 1.2)

Thanks for the confirmations.

 

[jaggedben]

My apologies for neglecting to mention the size of the feeder-tap wires that were in the original plan.

The lead from the tap to the 100A panel was 4AWG, while the original service wires were all 4/0. The ampacity of the tap was 72A, after de-rating. This complied with the 10-foot feeder tap rule, but not with the "multiple source" rule.

By my understanding, a "tap" is when you take off a lead with a lower ampacity, while a "splice" has the same ampacity.

The new plan increases the rating of the solar source panel from 100A to 200A and installs 60A fuses in it, with 4/0 wires leading to it. It also reduces the the main service fuses to 175A.

Now, per (B)(2), (175A + 60A) < (200A x 1.2)

Thanks for the confirmations.

Your descriptions are still confusing. (Is the 'splice' for the solar leading to a panel or a disconnect? If a panel, is it a main breaker or MLO panel?)

It's a valid interpretation that the 'tap' conductors need to be the same ampacity as the feeder they are tapped to in order to meet the 120% rule, thus turning them into a 'splice' in your terminology. The same would apply to an MLO panel.

However if those conductors terminate in a fused disconnect or a main breaker for a solar combiner panel, then that piece of equipment shouldn't need to be rated 200A. It can be rated 60A if it's a fused disco. It should only need to be rated 100A if it's a panel that has a 60A breaker from the utility and 60A of breaker(s) from the inverter(s).