Are Conductors / Overcurrent Protection Devices sized to the nominal conditions or high conditions?

[BradyJr]

Much of the literature I have seen regarding current and overcurrent protection device sizing has stated to be based off maximum current draw but then the examples appear to be based off a nominal voltage.

To clarify, here is an example,

Design Conditions
Voltage : 480 VAC +/- 5%
Duty : 100 kW @ 480 VAC
Phases : 3

Nominal Conditions (480 VAC)
Duty : 100 kW
Theoretical Current : 120.3 A
x1.25 NEC : 150.4 A

High Voltage Tolerance Conditions (504 VAC)
Duty : 110.3 kW *
Theoretical Current : 126.3 A
x1.25 NEC : 157.9 A

* Note : Assuming resistance is constant and P=V²/R, P_High = [V_High²*P_Nom]/V_Nom²

​

P : Duty, V : Voltage,

Just to clarify, the high voltage condition should be the one used for current component sizing correct?

 

[7anoter4]

The overcurrent protection shall be according to the circuit ampacity as per NEC Art. 240.4
What you intend to know-in my opinion-it is how to state the required ampacity-that means what cable you need to use.
The ampacity required is the maximum load current and voltage of the protected conductor.
NEC Art.220 may assist you for maximum load calculation.
One has to check the short-circuit withstand conditions and voltage drop for the rated conditions.
See also IEEE 141/1993 5.6 Protection requirements- for transient loads.

 

[waross]

Not sure about the NEC but the Canadian code specifies the standard voltages for use in calculations.
120 V, 208 V, 240 V, 277 V, 347 V, 480 V, and 600 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

In general insulated conductor should be sized for;
1) Max. Expected Continuos RMS current to be protected for overload Link
2) Min. Voltage drop/regulation (usually 3% to 5% total).
3) Short circuit protection for excessive heat that could damage the cable insulation (See graph on the link below).
[link [link thread238-456024]Link[/url]](See graphic)[/url]

 

[waross]

Overcurrent protection is a code matter.

Much of the literature I have seen regarding current and overcurrent protection device sizing has stated to be based off maximum current

Would you care to share some of that literature?
I am sure that implied in the literature is the condition; "At rated voltage."
You probably don't want the definition of "Overcurrent" but you need it.
Over current is short circuit protection. Short circuit protection varies depending on the application.
A common over-current setting of a molded case breaker is often adjustable from between 625% to 1250% of the breaker trip rating.
When you are quoting 125% that implies overload protection, not overcurrent protection.
That again is a code matter.
The Canadian Electrical Code specifies that calculations be based on standard voltages.
480 Volts is a standard voltage.
504 Volts is not a standard voltage.
I could never understand the "New Math".
In the "Old Math" that I use 125% is greater than 105% and even greater than 110.5% (105% squared).
125% gives protection while allowing overvoltages within the 5% tolerance.

In your example;

Nominal Conditions (480 VAC)
Duty : 100 kW
Theoretical Current : 120.3 A
x1.25 NEC : 150.4 A

A 150 Amp molded case breaker will be allowed.
In many molded case breakers in that size range the ovecurrent trip may be set at between 5 x 150 Amps and 10 times 150 Amps.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

For motors, use the current ratings in the code book.
These ratings are based on the least efficient common motor and are used to allow later changes to less efficient motors with out overloading the circuits.
The magnitude of an overcurrent is determined by the source impedance and the impedance of the circuit up to the fault.
The original question was to do with using the current at the nominal voltage versus using the current at the high end of the tolerance to calculate protection values.
Use the standard voltage for calculations.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xnuke]

Bill, I don't know how I got motors in my head. I deleted my motor-oriented post. I agree, nominal voltages are used for calculations per the NEC.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

xnuke; Understandable. The difference between the reciprocal of 80% and 125%.
The NEC and the CEC limit the maximum continuous loading on most breakers to 80%.
Both codes demand 125% sizing for conductors for motors.
By inverting 80% breaker loading rule to 125% conductor sizing, confusion with motor circuits is understandable.
An installation such as stated in the OPs original post, 100 kW at 480 Volts DC and a conductor protection of 150 Amps, will be protected by a molded case circuit breaker with an instantaneous trip in the range of 500% to 1000%.
An instantaneous trip at these levels will be within the damage curve of the cable.
Millions, probably billions, of circuits are so protected.
Back to protection and cable damage curves.
Conductor damage curves are important for coordination of various levels with completely adjustable protection relays.
With such relays is is quite possible to implement delayed trips at various current levels that may exceed the damage curve of the cable.
These are complex, engineered, coordination implementations, not simple code based protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BradyJr]

waross :

> Would you care to share some of that literature?

One section I had in mind was NEC 210.19(A)(1) but I may have misinterpreted it. It reads :

"Branch-circuit conductors shall have an ampacity not less than the maximum load to be served."

The equipment I am looking at is a resistance electric heater which will produce a continuous load. My line of thought was that :

[li]The maximum load to be served would occur at the upper range of the voltage tolerance.[/li]

[li]The resistance of the heater should be consistent so the theoretical maximum duty should be proportional to the voltage as noted in the original post[/li]

[li]The theoretical current draw using the upper voltage range power duty and voltage would give the current of the maximum load[/li]

This came up because I was reviewing how we sized our conductors and realized that the voltage tolerance was not being incorporated in the discussion. Customers have requested voltage tolerances as great as 480 VAC +/- 10 % in the past and my gut feeling tells me the possibility of 528 VAC should affect the conductor sizing.

I agree that the code reads like it has "at the rated / nominal voltage" but would you happen to know if that is explicitly stated somewhere ?

Thank you for your input.

 

[xnuke]

I believe it's 220.5(A).

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BradyJr]

xnuke :

Thanks a lot. I don't know how I missed that.


Thank you all very much.

 

[waross]

Canadian Electrical Code:
Section 8 — Circuit loading and demand factors
8-000 Scope
This Section covers
(a) conductor ampacities and equipment ratings required for consumer’s services, feeders, and branch circuits;
and
(b) branch circuit positions required for dwelling units.
General
8-100 Current calculations
When calculating currents that will result from loads, expressed in watts or volt amperes, to be supplied by a
low-voltage ac system, the voltage divisors to be used shall be 120, 208, 240, 277, 347, 416, 480, or 600 as
applicable.

Bill
--------------------
"Why not the best?"
Jimmy Carter