Voltage Drop Calculation for Conductors larger than 310.16

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NorthwestPV

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I need to do a voltage drop calculation on a 800 Amp continuous load. The actual load is 760A with the 1.25 calculated. I will be paralleling 4 sets of 250 THHN In 2 conduits. The conductors will be derated to 80%. It is 3-phase 480V WYE. Do I take the 760A and divide that by 4 then use the resistance of the 250 MCM?
 
NorthwestPV said:
I need to do a voltage drop calculation on a 800 Amp continuous load. The actual load is 760A with the 1.25 calculated. I will be paralleling 4 sets of 250 THHN In 2 conduits. The conductors will be derated to 80%. It is 3-phase 480V WYE. Do I take the 760A and divide that by 4 then use the resistance of the 250 MCM?
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Voltage drop will depend on actual load drawn and resistance of conductors so forget the 125% or any other ampacity adjustments - they have nothing to do with voltage drop.

I believe you want to take your total load and assume you have one conductor with (250 x 4) MCM or 1000 (which I guess should still give you same result)

Note this parallel conductor with a net of 1000 MCM has more current carrying capacity than an actual 1000 MCM single conductor has.
 
You don't need to added 25 % to the load for voltage drop calculations. Usually you divide the total amperage by 4 but your way would work.
Use the calculator posted.
 
Ok, so the actual load is 608A. I was just curious about conductors outside the range. Maybe I should have used a 2000A load question. :)

I'll take a look at that calculator. I have one on my smartphone but it will not calculate this problem. It just says conductor exceed range for sizing. Thanks guys.
 
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kwired said:
Voltage drop will depend on actual load drawn and resistance of conductors so forget the 125% or any other ampacity adjustments - they have nothing to do with voltage drop.
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But the ampacity adjustment change the size of the conductor and so the voltage drop associated with it.
 
T.M.Haja Sahib said:
But the ampacity adjustment change the size of the conductor and so the voltage drop associated with it.
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True because size determination is based on load current, conditions of use, and meeting all design criteria. Ultimately, the chosen conductors must meet or exceed ampacity, voltage drop, and all other design considerations.

Nonetheless, I believe the point is to use the correct values in the determination process.
 
T.M.Haja Sahib said:
But the ampacity adjustment change the size of the conductor and so the voltage drop associated with it.
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Ampacity adjustments are for protecting the insulation of the conductor from overheating. You can have a conductor loaded well past its 310.15(B)(16) value and still have an acceptable level of voltage drop for your application, but you may end up damaging the insulation of your conductor.

Ampacity and voltage drop are not directly related - application of one can effect the other, but they are two different concerns when selecting which conductor to use. Length effects voltage drop but not ampacity, heat effects ampacity (which is really protection of insulation, not the conductor) but not voltage drop.
 
kwired said:
Ampacity adjustments are for protecting the insulation of the conductor from overheating. You can have a conductor loaded well past its 310.15(B)(16) value and still have an acceptable level of voltage drop for your application, but you may end up damaging the insulation of your conductor.

Ampacity and voltage drop are not directly related - application of one can effect the other, but they are two different concerns when selecting which conductor to use. Length effects voltage drop but not ampacity, heat effects ampacity (which is really protection of insulation, not the conductor) but not voltage drop.
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If you give an example or examples substantiating your contention (s), it would be more illuminating. Thanks.
 
Smart $ said:
Ummm... heat affects voltage drop.
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Yes. Another flaw in his argument is he argued from the standpoint of checking up of voltage drop before checking up of current carrying capacity for a conductor.


kwired said:
You can have a conductor loaded well past its 310.15(B)(16) value and still have an acceptable level of voltage drop for your application, but you may end up damaging the insulation of your conductor.
Click to expand...
 
T.M.Haja Sahib said:
Yes. Another flaw in his argument is he argued from the standpoint of checking up of voltage drop before checking up of current carrying capacity for a conductor.
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Let's not make this more complicated than it has to be. :happyyes:

We have one conductor with [not less than] two design criteria. Because the minimum conductor size determined for each criterion may be different [and one conductor cannot be two different sizes], the conductor has to be sized to the larger of the two to satisfy both. Simple logic ;)
 
T.M.Haja Sahib said:
Yes. Another flaw in his argument is he argued from the standpoint of checking up of voltage drop before checking up of current carrying capacity for a conductor.
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His argument is sound. A conductor can safely carry a lot more current than the NEC tables allow. What happens to the insulation, while a valid one, is another concern.
 
ActionDave said:
His argument is sound. A conductor can safely carry a lot more current than the NEC tables allow. What happens to the insulation, while a valid one, is another concern.
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Safely? A subjective term that, if you are going to disregard NEC purpose and intent, needs defining parameters. Your statement would be better served with 'safely' omitted, IMO.
 
ActionDave said:
His argument is sound. A conductor can safely carry a lot more current than the NEC tables allow. What happens to the insulation, while a valid one, is another concern.
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The ampacity tables in the NEC are all about the insulation, not the metal of the conductors. If the insulation melts, it isn't safe.
 
T.M.Haja Sahib said:
If you give an example or examples substantiating your contention (s), it would be more illuminating. Thanks.
Click to expand...

Very well. Lets say you select a conductor based on voltage drop alone and not ampacity based on NEC tables. For long runs you often will come up with larger conductor than ampacity tables but not for short runs.

Lets say we have a 10 foot feeder, with a total current of 200 amps @ 480 volts three phase, and we want a voltage drop of no more than 5%. I come up with 16AWG being able to meet the objective. I am guessing the insulation will melt down but voltage drop should be less than 5%.

My calculation is probably off some because of the excessive heating will likely raise the resistance and therefore the amount of voltage drop, so just shooting from the hip I may use 10AWG instead, probably will have acceptable voltage drop there.

Sound like a good choice of conductor for a 200 amp load?
 
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