Lighting wiring conductor size

[Jpflex]

I have 10 lights going up at a job sight will be replacing old cloth type wiring without a ground conductor.

The lamp holders look pretty large such as moguls style. I was thinking of going with #10 wire (may seem over kill Larger than necessary ) but I do not know how much lumens brightness job will want Out of each lamp so I figure after computations and correction factor adjustments I’ll have around 2 amps per receptacle/lamps

Figured math as follows

1.25 percent (continuous load) multiplied by X (number of amperes for 1 light) multiplied by 10 lights = 30 amperes conductor or #10 AWG conductor THHN THHN2 chosen to use (60 degree column of table 310.21)

solving for X unknown usable amperes per light Will result in 2.4 amperes per light Or a total of 24 amperes for entire circuit thus allowing the conductors to be 1.25 % of load amperes. 24i/30i = 80%

However with temperature Correction we lower circuit ampacity 24 amperes again by 0.87% (for copper.net ambient temperature table for 105-113 degrees in my area) Which drops conductors to 20.88 amperes

Finally according to NEC 240.6A table this is not a standard size breaker so you move up to next larger size at 25 amperes

Does this sound correct?

THHM wires are rated for 90 degrees but were referenced at the ampacity of 60 degree column in nec book

 

[retirede]

You can use the 75C ampacity to start with, then apply the temperature correction. Presumes the fixtures are rated 75, which they likely are.

 

[Jpflex]

You can use the 75C ampacity to start with, then apply the temperature correction. Presumes the fixtures are rated 75, which they likely are.

I think the THHN conductors bought by company are rated 90 degrees but maybe it is indeed 75 degrees. Does my calculations look ok?

 

[david luchini]

Does my calculations look ok?

Not quite. You can't put 24 amps worth of continuous load on a 25A c/b.

However with temperature Correction we lower circuit ampacity 24 amperes again by 0.87% (for copper.net ambient temperature table for 105-113 degrees in my area) Which drops conductors to 20.88 amperes

The temperature correction factor doesn't get applied to the circuit load, it gets applied to the conductor allowable ampacity.

For #10 THHN, that would be 40*0.87=34.8

 

[roger]

I think the THHN conductors bought by company are rated 90 degrees but maybe it is indeed 75 degrees. Does my calculations look ok?

As pointed out in your earlier thread, 90 deg conductors can be used for any rating below 90 deg.

 

[Jpflex]

As pointed out in your earlier thread, 90 deg conductors can be used for any rating below 90 deg.

Yes understood but is my math and final ampacity adjustment look correct?

 

[david luchini]

Yes understood but is my math and final ampacity adjustment look correct?

No

 

[Jpflex]

No

Yes thanks I will look it over again.

 

[Jpflex]

Yes I see the table list THHN conductor for 90 degrees At 40 amperes instead of 30 ampere I had used to calculate

So I’m redoing this with temperature correction, final ampacity should be around

1.25(x amperes per lamp) x 10 lamps = 40 amperes listed

= 3.2 amperes max per lamp if total continuous load is to be 80%. Or 3.2i x 10 lamps = 32 amperes

32i/40i = 80% or 40i/32i = 1.25 target

Then there is a temperature correction factor and further reduction of conductor ampacity of 0.87%

32 amperes x 0.87% = 27.84 amperes but this is not a standard breaker so next size up to use is 30 amperes.

This is new calculation based on higher ampacity for THHN wire

 

[Jpflex]

Hopefully this is correct?

 

[Fred B]

It looks like you are mixing your calculations. One calculation should be totally seperate from the other.
You need to start with Article 220 calculations to find your load, then go to your Article 310 to determine your wire size based off your Article 220 load calculations.
So if your Article 220 calculation says you have for example 23A of load your next size up is 25A breaker. Now you go to Article 310 to determine the minimum wire size than can be used on a 25A circuit using appropriate correction factors.

Now you maybe overthinking the whole thing.
Are these light now been or previously working (installed) at there current location and wireing? If so what size circuit have they been on? Just go backwards from there for conductor size, as your presumptive Article 220 has been done. (Why reinvent the wheel.)

 

[Fred B]

If you are concerned that the existing installation had been undersized due to visual ques such as burnt or heated wire or evidence of heating at the breaker. Or these will be new replacement fixtures. You will need to know the actual load listing of the fixture or bulbs. Or work from worst case scenario (maximum lighting wattage per unit) and start your Article 220 calculations from there.

 

[Engser18]

Technically, it is base on light fixtures rating not the bulbs, the bulbs shall be rated lesser than or equal fixtures rated. See 220.14 (D)

 

[david luchini]

Then there is a temperature correction factor and further reduction of conductor ampacity of 0.87%

32 amperes x 0.87% = 27.84 amperes but this is not a standard breaker so next size up to use is 30 amperes.

You made the same error as your first calculation. (See post #4)

 

[Jpflex]

I don’t know what the lamps were rated pt

It looks like you are mixing your calculations. One calculation should be totally seperate from the other.
You need to start with Article 220 calculations to find your load, then go to your Article 310 to determine your wire size based off your Article 220 load calculations.
So if your Article 220 calculation says you have for example 23A of load your next size up is 25A breaker. Now you go to Article 310 to determine the minimum wire size than can be used on a 25A circuit using appropriate correction factors.

Now you maybe overthinking the whole thing.
Are these light now been or previously working (installed) at there current location and wireing? If so what size circuit have they been on? Just go backwards from there for conductor size, as your presumptive Article 220 has been done. (Why reinvent the wheel.)

 

[Jpflex]

I don’t know what the previous lamps were rated or wattage used since this is being done to an old ball-mill Building from the 1920-50s with known and tube. (At a mine)

The company just wants lighting to replace the old system. There is no power hooked up and I haven’t measured structure Detentions for general lighting and appliance receptacle load because company so far has told me not to wire receptacles but only lights.

The system will be powered by a separately derived generator but I dint have details on output.

I thought that if I had THHN wire rated at 90 degrees in the Nec book With its ampere rating at this temperature, then after all corrections the ampacity of the conductor would have to be less than or equal to the ampacity of the wire rated at 60 degrees since this circuit is under 100 amps? Atleast this is what mike holt said.

 

[don_resqcapt19]

210.23(B) may prohibit a 30 amp circuit for this load.

 

[Jpflex]

210.23(B) may prohibit a 30 amp circuit for this load.

Thanks I’ll check that code but if I recall the last time I checked code 30 ampere circuit was the limit for lighting loads.

I’m going to read more because increasing wiring size to 120% of load for lighting and yet lowering Wiring size to ambient temperature correction factor, Conductor fill ampere reduction, and lowering wiring ampacity to 60 degree ampere rating for terminations under 100 amperes is a bit confusing

 

[don_resqcapt19]

Thanks I’ll check that code but if I recall the last time I checked code 30 ampere circuit was the limit for lighting loads.

I’m going to read more because increasing wiring size to 120% of load for lighting and yet lowering Wiring size to ambient temperature correction factor, Conductor fill ampere reduction, and lowering wiring ampacity to 60 degree ampere rating for terminations under 100 amperes is a bit confusing

30 amps is the upper limit for lighting circuits, but that only applies where you have you have luminaries with heavy-duty lampholders.
As far as I know, heavy-duty lampholders only exist in incandescent luminaires that have ceramic cased lampholders.

 

[Engser18]

I have 10 lights going up at a job sight will be replacing old cloth type wiring without a ground conductor.

The lamp holders look pretty large such as moguls style. I was thinking of going with #10 wire (may seem over kill Larger than necessary ) but I do not know how much lumens brightness job will want Out of each lamp so I figure after computations and correction factor adjustments I’ll have around 2 amps per receptacle/lamps

Are the lights plug into receptacles? if yes what is the rating for these receptacles?