voltage drop

[enireh]

2/0 aluminum thhn is good for 150 amps. I ran it 200' to service a 100 amp panel. At what point is it not good for 150 amps? Its only good for 100 amps at that distance? The voltage at the 100 amp panel is 249 to ground and 125 on each leg to ground.

 

[GoldDigger]

It is good for 150A only when it carries the entire load of a residential occupancy.
The Southwire calculator does not recognize this exception, and so calculates using a minimum size of 3/0 for 150A.
With that conductor size the VD at 300' is 5%, somewhat high. That corresponds to 180' for 3%.

Using 2/0 under the exception does not change the VD associated with that conductor and that rated load.
Fiddling with the calculator (divide current by two, then divide distance by two) gives ~150' for a 3% voltage drop.

 

[Sierrasparky]

If it is service for the entire load of a residence , if not
2/0 AL is good for 150a only if terminations are rated at 90 deg C!
You use that 150a number for derating purposes.

You will need to most likely use the 75 deg column as most breakers and terminals are 75 c.

You check voltage drop in the field under load.

Voltage drop as Goldigger says

 

[kwired]

Although 2/0 is not a 150 amp conductor, outside of the provisions of 310.15(B)(7) it can be protected by a 150 amp overcurrent device if the calculated load connected to it is at or below 135 amps.

Volatage drop is dependent on how much current is flowing. Just because you have a 150 amp overcurrent device doesn't automatically mean you will have a current of 150 amps, plus how much voltage drop is allowed is not regulated by NEC, they do put a suggestion in an informational note though. Otherwise a design spec, certain equipment specs, or sometimes a local code may specify a maximum allowable voltage drop.

 

[DrSparks]

Although 2/0 is not a 150 amp conductor, outside of the provisions of 310.15(B)(7) it can be protected by a 150 amp overcurrent device if the calculated load connected to it is at or below 135 amps.

Volatage drop is dependent on how much current is flowing. Just because you have a 150 amp overcurrent device doesn't automatically mean you will have a current of 150 amps, plus how much voltage drop is allowed is not regulated by NEC, they do put a suggestion in an informational note though. Otherwise a design spec, certain equipment specs, or sometimes a local code may specify a maximum allowable voltage drop.

The entire NEC is an informational note--and it's good information that most localities adopt as law. In our area they require the 3%/5% rule.

 

[kwired]

The entire NEC is an informational note--and it's good information that most localities adopt as law. In our area they require the 3%/5% rule.

Which is too bad, some equipment doesn't really matter if you have 10% VD, you just get a little less heat, light etc. IMO is more design issue then safety issue, and doesn't belong in the NEC. If anything it belongs in energy codes.

 

[DrSparks]

Which is too bad, some equipment doesn't really matter if you have 10% VD, you just get a little less heat, light etc. IMO is more design issue then safety issue, and doesn't belong in the NEC. If anything it belongs in energy codes.

It can be a safety issue when the wire heats up due to current increases as voltage drops.

 

[kwired]

It can be a safety issue when the wire heats up due to current increases as voltage drops.

Yes but we are not talking about 5-10% drop in those instances.

And the current would only increase for inductive loads, everything else it would decrease.

 

[DrSparks]

Yes but we are not talking about 5-10% drop in those instances.

And the current would only increase for inductive loads, everything else it would decrease.

Also include constant-voltage lighting ballasts. And inductive loads are among the largest loads installed in a home, with the exception of heating elements.

 

[ActionDave]

Which is too bad, some equipment doesn't really matter if you have 10% VD, you just get a little less heat, light etc. IMO is more design issue then safety issue, and doesn't belong in the NEC. If anything it belongs in energy codes.

I agree. Specific loads may require designing for it. By the same token, it usually means not worrying about it at all; like you said, most things will work just fine with that 10% VD.

 

[kwired]

Also include constant-voltage lighting ballasts. And inductive loads are among the largest loads installed in a home, with the exception of heating elements.

Maybe a lot of small motors that won't really have much trouble, universal and shaded pole motors use reduced voltage as speed reduction method. The common largest motors in a home are on HVAC equipment and typically are rated 208-240, so a pretty significant drop @ 240 still puts it in it's designed range.

I don't run into much voltage drop problems in a typical dwelling other then if there is significant length to the source - and that is often because the POCO for some reason won't install a transformer closer to the home, and thinks there is unlimited length range on secondary conductors

 

[GoldDigger]

The increase in current as voltage drops is characteristic of constant power loads such as motors and some lighting ballasts.
The inductive nature of the load is not relevant. If you have an inductive load such as a lightly loaded transformer the current will decrease with decreased voltage.

 

[iwire]

The increase in current as voltage drops is characteristic of constant power loads such as motors and some lighting ballasts.

I am sure you know this, just using your post as a jumping off point.

Everyone always point to motors and it is true they can draw more current with reduced voltage but the reality s often they draw less current because they were not delivering their rated HP to start with.

There are even energy saving devices that purposely reduce voltage to motors when they are lightly loaded and restore full voltage when the motor gets loaded up.

For example think of a airport conveyor system, it often runs lightly loaded but at times is heavily loaded.

 

[DrSparks]

The common largest motors in a home are on HVAC equipment and typically are rated 208-240, so a pretty significant drop @ 240 still puts it in it's designed range.

good point.

 

[Tony S]

Maybe a lot of small motors that won't really have much trouble, universal and shaded pole motors use reduced voltage as speed reduction method. The common largest motors in a home are on HVAC equipment and typically are rated 208-240, so a pretty significant drop @ 240 still puts it in it's designed range.

I don't run into much voltage drop problems in a typical dwelling other then if there is significant length to the source - and that is often because the POCO for some reason won't install a transformer closer to the home, and thinks there is unlimited length range on secondary conductors

Universal yes, shaded pole no.

Shaded pole are frequency dependant not voltage, lower the voltage and the current will rise.

 

[kwired]

Universal yes, shaded pole no.

Shaded pole are frequency dependant not voltage, lower the voltage and the current will rise.

Shaded pole is dependent on frequency for determining synchronous speed. Not enough voltage will lower torque and allow more slip resulting in less speed. but yes current will rise some to try to compensate, a solid state control that shaves off some of the wave which will lower both voltage and current is a better method of speed control for those motors then straight voltage reduction only.