Need help, having a brain fart 208/120volt changing to 480/277volt

[Dsg319]

On a project where there is a heat trace panel fed temporarily from a genset at 208/120v. They thought it was 277v checked generator and verified it has been powering the 480/277v panel with 208/120v.

We just installed purchase power today and will be re-feeding the panel with the correct voltage (480/277v). All the heat trace circuits are single phase 277v (line-neutral loads)

As of now with the 3ph 4w 208/120v I have around 50amps per phase. This is self regulating heat trace. 10watts per foot at 240volts (manufacture says it’s good from 208-277v)

We have to re-size the feeder and raceway feeing the panel.

My big question that I cannot make sense of today (struggling with a sinus infection my excuse lol) When we change the system voltage from 208/120v to 480/277v the current should increase because of all the loads being strictly resistive right?

Again all loads are single phase (line to neutral) so right now it is 120v but will be changing to 277v line to neutral.

But since it’s from a 3ph 4w system is this math correct.

(208v x sqrt3 )x 50amperes= 18kw

18kw/ (480v x sqrt3)= 21.6amperes.

 

[winnie]

You are confusing a constant power load with a constant resistance load.

Your equation showing the current dropping assumes constant power.

If resistance were constant then current would increase in proportion to voltage and you would have about 115 amps.

But since this is self regulating cable I'd expect a response between the two extremes. With self regulating cable the resistance goes up with temperature. So I'd expect higher current when initially energized (and the system is cold) with the current going down as things warm up.

I expect that at proper voltage the current will be higher than the constant power result, with the cable at higher temperature.

Jon

 

[Dsg319]

You are confusing a constant power load with a constant resistance load.

Your equation showing the current dropping assumes constant power.

If resistance were constant then current would increase in proportion to voltage and you would have about 115 amps.

But since this is self regulating cable I'd expect a response between the two extremes. With self regulating cable the resistance goes up with temperature. So I'd expect higher current when initially energized (and the system is cold) with the current going down as things warm up.

I expect that at proper voltage the current will be higher than the constant power result, with the cable at higher temperature.

Jon

Thanks Jon. You are right I have got the two confused.

We proposed a 100amp breaker as that’s the largest bolt in breaker we can get for the panel it will be fed from. But after thinking on this I think we will need some how to figure the calculated load.

Just wish I knew some way to figure it from the readings already taken on the lower voltages.

I took the known wattage and figured out the actually resistance of all the circuits around 7.2ohms. I know it will change with start up and seasons. Than 480xsqrtt3 / 7.2= 115amperes.

 

[tom baker]

Are you using GFPE breakers for the heat trace? Contractor locally installed 277 heat trace on a waterline under a boardwalk, No GFPE. Caught fire and fire dept had no access to put out. Got disconnected.

 

[Dsg319]

Are you using GFPE breakers for the heat trace? Contractor locally installed 277 heat trace on a waterline under a boardwalk, No GFPE. Caught fire and fire dept had no access to put out. Got disconnected.

Yes sir they are.

 

[synchro]

The resistance of the heat trace will take some time to go up as it heats up, and so the breaker will have to supply this higher initial current.
Can you measure the resistance of the cable when it's cold? You would need to have a meter that is relatively accurate when measuring a few ohms. With multiple heat trace circuits it could be more accurute to measure each of their resistances separately instead of all of them together. Then you can calculate the initial current that will be drawn by that resistance at 277V.

 

[Dsg319]

The resistance of the heat trace will take some time to go up as it heats up, and so the breaker will have to supply this higher initial current.
Can you measure the resistance of the cable when it's cold? You would need to have a meter that is relatively accurate when measuring a few ohms. With multiple heat trace circuits it could be more accurute to measure each of their resistances separately instead of all of them together. Then you can calculate the initial current that will be drawn by that resistance at 277V.

That is a good idea.