Conductor ampacity

Therealcrt said:
in the end it’s not illegal to use the 25a breaker and then charge the customer to install a larger breaker if it keeps tripping
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Not illegal perhaps, but uninformed to install the 25A breaker in the first place, and questionable to charge the customer for the oversight.

Cheers, Wayne
 
wwhitney said:
Have you run into inverters for which any of the following apply?

1) Terminations that are listed and marked for use of an ampacity Table other than 310.16.
2) Terminations that are listed for 90C.
3) Inverters that are "listed for continuous operation at 100 percent of its rating", so that you don't need to apply a 125% continuous factor per 690.8(B)(1) Exception, at least on the DC side where there is no OCPD required.

Cheers, Wayne
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1) No
2) No
3) No

OCPD on the DC side is sometimes necessary but it is to keep faulted strings from damaging other strings.
 
wwhitney said:
3) Inverters that are "listed for continuous operation at 100 percent of its rating", so that you don't need to apply a 125% continuous factor per 690.8(B)(1)
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It's not the inverters that 690.8(B)(1) is talking about; it's everything else in the circuit.
 
ggunn said:
It's not the inverters that 690.8(B)(1) is talking about; it's everything else in the circuit.
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If you have a DC circuit consisting of PV panels in series going to an inverter connection, then in order to skip the 125% continuous use factor in selecting the conductor ampacity for that circuit, seems like both ends of the circuit would need to be "listed for continuous operation at 100 percent of its rating". So the PV panel MC4 connectors (if that's the correct term) and the inverter terminations.

Cheers, Wayne
 
wwhitney said:
If you have a DC circuit consisting of PV panels in series going to an inverter connection, then in order to skip the 125% continuous use factor in selecting the conductor ampacity for that circuit, seems like both ends of the circuit would need to be "listed for continuous operation at 100 percent of its rating". So the PV panel MC4 connectors (if that's the correct term) and the inverter terminations.

Cheers, Wayne
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Once we have adjusted the DC current for excess irradiance (except when there are optimizers at the modules) we calculate DC conductor sizing the same as AC. Incidentally, we almost always use #10 wiring for individual strings which usually has significant excess ampacity.
 
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ggunn said:
Once we have adjusted the DC current for excess irradiance we calculate DC conductor sizing the same as AC.
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Right, but it's a continuous current? So you have a 125% continuous use factor for the termination ampacity checks, unless both ends of the circuit are rated for continuous operation at 100 percent of their rating. Which I gather never happens in practice, but 690.8(B)(1) Exception at least provides the necessary permission to allow it in theory.

Cheers, Wayne
 
wwhitney said:
Right, but it's a continuous current? So you have a 125% continuous use factor for the termination ampacity checks, unless both ends of the circuit are rated for continuous operation at 100 percent of their rating. Which I gather never happens in practice, but 690.8(B)(1) Exception at least provides the necessary permission to allow it in theory.
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Yes, in theory, but I have never encountered it in the real world. As I said, we typically use #10 for DC strings, which can have twice the ampacity we need, so 100% or 90 degree rated connectors would be of little or no use to us.
 
I don't think anybody directly said this yet. Ampacity tables are primarily based on protecting conductor insulation. There are methods to adjust the ampacity for conditions that vary from the conditions mentioned at the top of the table - mostly adjustments for ambient temperature and number of current carrying conductors in raceway or cable. 310.16 is the main table that applies to a lot of common applications for up to 2000 volts. The other tables that follow have specific conditions that only allow use of a table if those conditions apply. If you transition to other conditions you either need to splice onto another conductor to meet the new conditions or if unspliced into both conditions which ever conductor is needed for the lesser conditons must be utilized which can lead to having larger conductor in a portion of the run than what the minimum for those conditions may allow.

A bare conductor can carry more current than same size with insulation on it as it won't effect any insulation. Bare conductors are kind of rare in under 1000 volts other than grounding/grounded conductors. Bare conductors at any voltage typically have to be run in free air and supported by insulators, they need separation since they have no insulating covering.
 
Therealcrt said:
Can somebody stop reciting every code article and just make a very simple response? You size your wire based on the over current protection device. There’s no need to get into the wires ampacity capability in 100 different situations. I have never looked at a piece of equipment and seen the amp rating but the looked at the lug rating to determine the wire amp rating to use. Quit over killing it if it’s 20a use #12 if it’s 30a use #10 if it’s 40a use #8 ect..
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The reason we do this is because some are planning on taking their state journeymans test and want to know exactly what to answer. There are many codes within different sections and exemptions
 
jim dungar said:
Not only possible, it is extremely likely.
A standalone lug body is normally rated 90°C, however the termination is much more than just the lug body. A termination includes the bus bar the lug is bolted to as well as the enclosure it is mounted in.
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Therefore as mikeholt said, we do not base terminations on bolt rating alone but equipment rating
 
Therealcrt said:
It’s very simple buddy 25 amp breaker 10 gauge wire the NEC “ allows“ and NEC “ minimum”

Again, you’re going way too far down the rabbit hole just to find the bare minimum take it at face value obviously size the breaker 125% go up to the next standard size a.k.a. 25 A and just be safe and run number 10 have a good day
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That’s a very thumb rule type comment . Using a #10 conductor at its 60c ampacity is not any safer than using a #12 conductor at its 75c ampacity for a 25 amp load . Just because you never feel the need to use anything other than the 60 c ampacity of a conductor does not mean it’s the only or best way. There’s a reason terminal temperature is in direct relation to the ampacity of our conductors at their termination point
 
tortuga said:
Are you sure thats how UL 486A-B does it?
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Ugh now you made me go look at 486A-B. That seems to cover both lugs and other terminations that a piece of equipment might use, and wire connectors like wire nuts that would be used in the middle of a circuit, rather that at equipment terminations.

Here's a quick overview of the two current tests required. I'll use #4 Cu as an example, which the standard says has an "assigned maximum ampere rating" of 85A, it's 75C ampacity per Table 310.16.

1) Current cycling of 140A (75C rated connectors) or 175A (90C rated connectors) for 500 cycles over 1 hour with 50% duty cycle. The connector is permitted a 125C temperature rise over ambient.

2) A static heating test at 125A, until equilibrium temperature is reached. The connector is permitted a 50C temperature rise over ambient [with a note that higher temperature rises are permitted if the connector is for end use in equipment in which the application permits higher temperature rises]. If the heating follows I2*R, then the 50C temperature limit implies that at 85A (75C base ampacity of #4 Cu), the temperature rise would be at most 23C, while at 95A (90C base ampacity of #4 Cu), the temperature rise would be at most 29C.

[And that for the current cycling current of 140A, the equilibrium temperature rise for 140A would be at most 62C, half the allowable 125C. So the current cycling test is apparently looking for effects that would raise the termination resistance R and thereby increase heating.]

tortuga said:
So is a connector lug rated for a single wire in the range of #4AWG - 250MCM tested per UL 486A-B for each wire size only up to that wire size's value in Table 7 "test Current for connectors intended for a single conductor"?
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I believe it is tested both for #4 AWG and for 250MCM, on the assumption that if those behaviors pass, any intermediate wire size would pass. My skimming of 486A-B was not thorough enough to say that with 100% confidence. Certainly for connectors that are rated for mixed Al and Cu, there is discussion of testing with minimum size Al, minimum size Cu ; maximum size Al, minimum size Cu; etc.

Cheers, Wayne
 
Jaybone812 said:
That’s a very thumb rule type comment . Using a #10 conductor at its 60c ampacity is not any safer than using a #12 conductor at its 75c ampacity for a 25 amp load . Just because you never feel the need to use anything other than the 60 c ampacity of a conductor does not mean it’s the only or best way. There’s a reason terminal temperature is in direct relation to the ampacity of our conductors at their termination point
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Ok so if a terminal is rated for 75c then why not just size wire based on 75c why make it more complicated
 
Therealcrt said:
Ok so if a terminal is rated for 75c then why not just size wire based on 75c why make it more complicated
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That’s the exact point I was making if the terminals are 75 c why use thr 60c ampacity of a conductor . That’s what you said with #10 for 25 amp when #12 is 25 at 75c
 
Jaybone812 said:
That’s the exact point I was making if the terminals are 75 c why use thr 60c ampacity of a conductor . That’s what you said with #10 for 25 amp when #12 is 25 at 75c
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Ok so that’s the base of it then from there you need to do ALL the other factors such as de rating. Ambient temp. Voltage drop. All that. So my point is if there’s a piece of equipment you look at the terminal rating first. Then look at the electrical specs then size the wire based on that then figure any temp corrections then voltage drop
 
Therealcrt said:
Ok so that’s the base of it then from there you need to do ALL the other factors such as de rating. Ambient temp. Voltage drop. All that.
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Actually, no. There are 3 separate checks you mentioned there:

1) The minimum size wire that complies with the termination requirements, using ampacity at the termination temperature from Table 310.16 (unless the termination is listed and marked otherwise) without any adjustment or correction. [A 125% factor for continuous loads applies.] [This also includes the OCPD termination and compliance with 240.4 including possibly 240.4(B).]

2) The minimum size wire that has sufficient ampacity, including adjustment and correction. This has nothing to do with terminations and can use the applicable table and the 90C column if the conductors have 90C rated insulation. [No 125% continuous use factor required.]

3) The minimum size wire that keeps the voltage drop sufficiently low. This depends on the actual expected current and the circuit length, and has nothing to do with ampacity tables.

You do each of those 3 checks separately, and your minimum wire size is the largest of the 3 results. If conditions for ampacity adjustment or correction vary throughout the run, you need to do (2) separately for each set of conditions present.

That of course assumes you are running a single conductor size from end to end. If you have splices (rated at least the conductor insulation temperature rating) in the middle, then segments that don't terminate on any equipment would not be subject to (1) above. They'd just have to comply with (2) and (3).

Cheers, Wayne
 
Therealcrt said:
Ok so that’s the base of it then from there you need to do ALL the other factors such as de rating. Ambient temp. Voltage drop. All that. So my point is if there’s a piece of equipment you look at the terminal rating first. Then look at the electrical specs then size the wire based on that then figure any temp corrections then voltage drop
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Vd is not part of the enforceable code unless applies to 647 and 695 of the nec
 
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