Conductor ampacity

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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You can do what ever makes you happy, as for others they can do what the code allows and even though it may take them into other code article sections don't think they are somehow wasting time.

I've got one for you and will walk you through it. You have a 3 HP motor



? Table 430.148 gives a 3HP motor an full load current of 17 amps
? 430.22(A) requires conductors be sized at 125% of full load current
? 17 x 125% = 21.25 amps
? Table 310.16, #____ THHN 75 deg C column = ___ amps
? Table 430.52 allows up to 250% for the inverse time breaker
? 17 x 250% = ____ amps
? 240.6 states we can go UP to the next STANDARD size breaker
? you can use a _____ amp breaker and # ____ conductors
 
wwhitney said:
It includes the conductor size terminated on the equipment, and the ampacity we associate to that combination. It does not cover anything about determining the ampacity of a circuit conductor at a location outside the equipment.

Cheers, Wayne
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And this is exactly what i was thinking. So a wire wire in free air bewteen two j boxes terminating into a lug allowing 75 degree c termination can use table 310.17 and higher wire ampacity but another wire from the this point going to equipment termination may have to be larger based on terminatiom of 60 deg c and table 310.16 if the equipment is not listed otherwise?
 
roger said:
You can do what ever makes you happy, as for others they can do what the code allows and even though it may take them into other code article sections don't think they are somehow wasting time.

I've got one for you and will walk you through it. You have a 3 HP motor



? Table 430.148 gives a 3HP motor an full load current of 17 amps
? 430.22(A) requires conductors be sized at 125% of full load current
? 17 x 125% = 21.25 amps
? Table 310.16, #____ THHN 75 deg C column = ___ amps
? Table 430.52 allows up to 250% for the inverse time breaker
? 17 x 250% = ____ amps
? 240.6 states we can go UP to the next STANDARD size breaker
? you can use a _____ amp breaker and # ____ conductors
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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
 
wwhitney said:
It includes the conductor size terminated on the equipment, and the ampacity we associate to that combination. It does not cover anything about determining the ampacity of a circuit conductor at a location outside the equipment.

Cheers, Wayne
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IS it possible that an equipments termination lugs may be enscribed withe different temperature ratings than its equipment?
 
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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Motors are exempt from many of these rules. They have their own section in nec
 
Jpflex said:
IS it possible that an equipments termination lugs may be enscribed withe different temperature ratings than its equipment?
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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.
 
Therealcrt said:
Can be Should be or has to be?
My apologizes honestly.
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Ok, here it is with the blanks filled in.


You can use # 12 conductors

? Table 430.148 gives a 3HP motor an full load current of 17 amps
? 430.22(A) requires conductors be sized at 125% of full load current
? 17 x 125% = 21.25 amps
? Table 310.16, #12 THHN 75 deg C column = 25 amps
? Table 430.52 allows up to 250% for the inverse time breaker
? 17 x 250% = 42.5 amps
? 240.6 states we can go UP to the next STANDARD size breaker
? you can use a 45 amp breaker and # 12 conductors
 
Jpflex said:
And this is exactly what i was thinking. So a wire wire in free air bewteen two j boxes terminating into a lug allowing 75 degree c termination can use table 310.17 and higher wire ampacity
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Sure. And if you are splicing in the j boxes, you can use 90C rated wire connectors, so you could use the 90C column when using a 90C conductor type between j boxes.

Jpflex said:
but another wire from the this point going to equipment termination may have to be larger based on terminatiom of 60 deg c and table 310.16 if the equipment is not listed otherwise?
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Yes, when 110.14(C)(1)(a) applies, 60C is the default, although it is more common that the equipment allows the use of the 75C ampacity per 110.14(C)(1)(a)(3).

Cheers, Wayne
 
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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For PV systems it isn't quite that simple. For an inverter output circuit the minimum OCPD is the next size up from 125% of the maximum inverter current, but since an inverter can run at full output for more than three hours at a time there is also the rule that 125% of the maximum output current cannot exceed the 75 degree ampacity of the conductors unless the terminals are rated at 90 degrees. We run two calculations on minimum conductor size - one based on that rule and one based on OCP, and we use the larger of two as the minimum. Usually (but not always) the calculation using the 75 degree ampacity determines the minimum conductor size even though they are 90 degree wires. The point of this is to ensure, as I said before, that conductors that are connected to 75 degree terminals do not get hotter than 75 degrees.
 
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Therealcrt said:
Can be Should be or has to be?
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wwhitney said:
If you use a 25A breaker in Roger's example, there is significant chance the motor will nuisance trip on startup. That's why the NEC has the allowance for a larger breaker.

Cheers, Wayne
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Yes but isn’t the 25A considered the minimum breaker allowed to use? Per NEC which is mainly based on minimums. It’s not against code to use the 25A breaker
 
wwhitney said:
No, it's not against the NEC use a 25A breaker. It just may not work. : - )

Cheers, Wayne
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Well I think I proved my point then but also I agree with you but 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
 
ggunn said:
For PV systems
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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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