Rounding Down OCPD?

[ocoee]

I think this is a simple question. Can someone please direct me to a code reference regarding rounding down if one exists.

I am installing 27 Enphase inverters. Each inverter has a max I of .9 amps 27 x .9 = 24.3 amps max I

Sizing my OCPD at 24.3 x 1.25 = 30.375 A Must I bump my OCPD to 35 amps or is 30 fine given the .375 is less than .5 amps?

Thank you

 

[jumper]

I moved your thread to the Photovoltaic forum.

 

[texie]

I think this is a simple question. Can someone please direct me to a code reference regarding rounding down if one exists.

I am installing 27 Enphase inverters. Each inverter has a max I of .9 amps 27 x .9 = 24.3 amps max I

Sizing my OCPD at 24.3 x 1.25 = 30.375 A Must I bump my OCPD to 35 amps or is 30 fine given the .375 is less than .5 amps?

Thank you

This reminds me of the rounding discussion we had here not long ago. I'll have to find the post.
I think you can round down in this case. Now if you came up with a value of, say, 30.6A I would say that you would go no next standard size.
Interestingly, since we are talking PV, if this were part of the 120% rule for bus bar you would have to use whatever value breaker you end up with for your calcs. as there is no rounding up for this. For example, in the case of a invertor output of 30.6 you must have 125% minimum, so you round to 35 for the breaker. But now you have to use the 35 A value for the 120% bus bar rule calc.

 

[Little Bill]

I think this is a simple question. Can someone please direct me to a code reference regarding rounding down if one exists.

I am installing 27 Enphase inverters. Each inverter has a max I of .9 amps 27 x .9 = 24.3 amps max I

Sizing my OCPD at 24.3 x 1.25 = 30.375 A Must I bump my OCPD to 35 amps or is 30 fine given the .375 is less than .5 amps?

Thank you

220.5(B) is one such instance

 

[ocoee]

your resonnse

your resonnse

Perfect! Thank you for 220.5 b

 

[Smart $]

... Must I bump my OCPD to 35 amps or is 30 fine given the .375 is less than .5 amps?

Thank you

As far as I know, the only place where the Code says you can round calculations to the nearest whole ampere is 200.5(B), i.e. branch-circuit, feeder, and service calculations. Table 220.3 says Article 690 sizing is covered.

 

[texie]

As far as I know, the only place where the Code says you can round calculations to the nearest whole ampere is 200.5(B), i.e. branch-circuit, feeder, and service calculations.

What we are talking about here is OCPD sizing based on inverter rating. From a perspective of technical interpretation, rounding per 220.5(B) does not apply because it is not an Article 220 calculation.

Then we have 690.8(B)(1)(a) which says overcurrent devices to carry not less than 125 percent of the maximum currents calculated in 690.8(A). Even though 690.8(B)(1)(d) says 240.4(B) is permitted, 240.4(B) is based on conductor ampacity, and because conductor ampacity minimum is also required to be equal or greater than the same calculated amount, you cannot round down. That's how I see it at present, but am open to correction.

That's why I mentioned that discussion we had a while back about whether Art. 220.5(B) applies to situations like this. I think the consensus is that it does and John Wiles (the solar guru) agrees that you would round up for this as 690.8(B)(1)(a), as you mention. But as I mentioned in my previous post rounding up would not be allowed for 705.12(D)(2). You would have to use the rounded up value (35 A) for this calculation.

 

[jaggedben]

That's why I mentioned that discussion we had a while back about whether Art. 220.5(B) applies to situations like this. I think the consensus is that it does and John Wiles (the solar guru) agrees that you would round up for this as 690.8(B)(1)(a), as you mention. But as I mentioned in my previous post rounding up would not be allowed for 705.12(D)(2). You would have to use the rounded up value (35 A) for this calculation.

If you are saying that it is allowable to install a 30A breaker, but that the 120% calc would have be use 35A, then I wholeheartedly disagree with you. 705.12(D)(2) refers exactly to the OCPDs installed, not anything else. If a 30A breaker is installed, that's what you use in the calc. If the 30A breaker is a violation, that's a different problem.

(BTW, this will change in 2014.)

 

[ggunn]

I just wanted to point out that OCPD on the AC conductors from an inverter is there to protect them from a fault fed from the service, not to protect them from the inverter. Assuming the system is designed correctly, the inverter is incapable of putting out enough current to endanger the conductors and it cannot feed a short.

The AC OCPD defines a current window for the inverter to operate in and the only downside I can think of of having too small an OCPD on the line is the possibility of nuisance trips when the inverter is maxxed out for long periods of time. It seems a little silly to me to require a 175A breaker if 1.25 times the max inverter current is 150.1A but if it's 149.9A a 150A breaker is OK.

 

[texie]

If you are saying that it is allowable to install a 30A breaker, but that the 120% calc would have be use 35A, then I wholeheartedly disagree with you. 705.12(D)(2) refers exactly to the OCPDs installed, not anything else. If a 30A breaker is installed, that's what you use in the calc. If the 30A breaker is a violation, that's a different problem.

(BTW, this will change in 2014.)

Maybe I didn't articulate it well. That's not what I was saying. I agree with you.
For example, I had a job the other day that the next size up breaker to satisfy the 125% requirement for the inverter output then put the value to high for 705.12(D)(2). They then wanted to use a smaller breaker to comply but that then in turn meant that the breaker was less than 125% of the inverter output, which in my view is a violation. Of course the solution was to install a smaller main breaker to the panel or use a higher rated bus, but costly as it was a fairly large system.

 

[Smart $]

That's why I mentioned that discussion we had a while back about whether Art. 220.5(B) applies to situations like this. I think the consensus is that it does and John Wiles (the solar guru) agrees that you would round up for this as 690.8(B)(1)(a), as you mention. But as I mentioned in my previous post rounding up would not be allowed for 705.12(D)(2). You would have to use the rounded up value (35 A) for this calculation.

If you are saying that it is allowable to install a 30A breaker, but that the 120% calc would have be use 35A, then I wholeheartedly disagree with you. 705.12(D)(2) refers exactly to the OCPDs installed, not anything else. If a 30A breaker is installed, that's what you use in the calc. If the 30A breaker is a violation, that's a different problem.

(BTW, this will change in 2014.)

Note I changed my post after texie quoted it. 220.5(B) applies via Table 220.3 reference to Article 690.

 

[PWDickerson]

The max breaker size for the M215 inverter is 20A, and the max number of M215 inverters on a branch circuit is 17. The OP needs to install the inverters on 2 circuits.

 

[texie]

The max breaker size for the M215 inverter is 20A, and the max number of M215 inverters on a branch circuit is 17. The OP needs to install the inverters on 2 circuits.

Good point. I didn't even notice that as I was so focused on the rounding question.

 

[jaggedben]

The max breaker size for the M215 inverter is 20A, and the max number of M215 inverters on a branch circuit is 17. The OP needs to install the inverters on 2 circuits.

While a totally valid point, the rounding question may still apply to a breaker feeding a combiner panel for the two circuits. On the other hand, if it doesn't, then the OP is stuck with at minimum one 20A breaker and one 15A breaker. This is in fact a rather interesting case...