Tri rated cable short time ratings 2

[panelman]

the standard charts say that 50mm tri-rated can do 200A presumably all day for 20 years

Does anyone have short time ratings? We need to run a 280A pump for a day. Pump is on softstart and runs for 90 secs about once every 4 hours.

Cables would be about 5m long.

Gut feel says it will be ok but would value opinions of others

 

[waross]

What does your code say?

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

 

[panelman]

Code (or regs over here) says bigger, on a permanent job we'd certainly go bigger, the rest of the panel is 95mm but that's designed to last 20 years when running 24/7.

The temporary job just needs to run the pump for 4 90 seconds runs spread over 12 hours, my thinking is that whilst the current is over the long term rating the short running time is not going to let the cable get anywhere near hot enough to cause problems

 

[7anoter4]

First of all 200 A it is the ampacity of this cable in free air 45oC ambient 85oC conductor maximum temperature.
The cable will be cooled by the equipment connected at both ends.
Cable is PVC insulated, copper conductor [flexible] 90oC- maximum permissible.
According to:
Siemens-Power Cables and Their Applications by Lothar Heinhold

http://books.google.co.il/books/abo...cations_Thir.html?id=u1sjSgAACAAJ&redir_esc=y

ch.18.6.1 up to ch.18.6.5.
DT= (TCr-Ta)*(1-exp(-t/tau) eq. 18.113
Where:
DT=conductor temperature rise [oC]; TCr=90oC max.permissible conductor temperature;Ta=air temperature. tau=time constant[SEC]
tau=T*C 18.114 or finally tau=B*(q/Ir)^2
From Table 18.38 for copper conductor 90oC B=9.09*10^15 [A^2*sec/m^4]
q=50/10^6 m^2 conductor cross section area. Ir=280 A
tau=9.09*10^3*(50/280)^2=290 sec
DT= (90-45)*(1-exp (-90/290)) =12 oC
The conductor temperature will be= 45+12= 57 oC
If the second start will be after 4 hours the conductor will be at ambient temperature.
According to:
EPRI POWER PLANT ELECTRICAL REFERENCE SERIES VOL.4 WIRE AND CABLES
Tc(t)=Tc(0)+[Tc(f)-Tc(0)]*(1-exp(-t/K) Eq.4-23
Tc(f)=(Ic/IA)^2*(Tc-TA)+TA
Ic=280 A; IA=200 A ;TA=45;Tc=85;Tc(0)=TA[starting from ambient]; K=tau=290 sec
Tc(f)=(280/200)^2*(85-45)+45=123.4 o C.
Tc(90 sec)=45+(123.4-45)*(1-exp(-90/290))=65.9 oC
No problem at all.

 

[panelman]

Cable is 50mm2, 5m long and carries 280A for 90 seconds

my own workings from first principles and assuming no heat loss from the cable…..…

Resistance of 50mm2 cable = 0.386 ohms/km

Resistance of 5m of 50mm2 = 0.386/200 = 0.00193 ohms

Total energy into cable = I2.R.t = 280 x 280 x 0.00193 x 90 = 13.6 kJ

Density of copper = 8940 kg/m3

Volume of 5m of 50mm2 cable = 0.00025 m3

Mass of 5m of 50mm2 cable = 8940 x 0.00025 = 2.235 kg

Specific heat capacity of copper 0.39 kj/kg k

Temp rise of 2.235 kg of copper caused by adding 13.6kJ of energy = 13.6/(2.235 x 0.39) = 15 degrees

This is about half what I would have expected intuitively but isn't far away from 7anoter4 offering.

Feel free to pick holes in the above

 

[ScottyUK]

7anoter4,

Many components such as circuit breakers and contactors rely on the cable or busbar as a heatsink. You can't assume that the cable ends are cooler.

Panelman, there should be a cable damage curve available from the manufacturer. Normally these are used when selecting fuses or setting protection relays.. You need to stay below the curve. Regardless of that, I'm not sure where you would stand legally.

 

[7anoter4]

Thank you, Scotty, for your remarks.
First of all the temperature rise calculation did not take into consideration cable ends cooling.
Second, if the supplied pump starts only for 1.5 minute all 240 minutes the connected equipment will be at ambient temperature, for sure.
The thermal conduction could be elevated and was not taken into consideration. The 200 A rated was stated for continuous heat dissipation to
still air, by convection and radiation only.
The damage curve could be an idea. But, usually, it is presented for short-circuit currents-short time, large currents, adiabatic phenomena].
In our case-intermittent load- the maximum permissible current-up to rated conductor temperature [90oC] will be:
IAB=Iz*fAB see: Siemens-Power Cables and Their Applications by Lothar Heinhold ch. 18.6.6-here attached.
fAB=SQRT((1-e^(-ts/tau)/(1-e^(-tb/tau)) where: tau=290 sec;ts=14400 sec ;tb=90 sec
fAB=SQRT((1-EXP(-14400/290))/(1-EXP(-90/290)))=1.936
IAB=1.936*200= 387 A
By the way in the above formula of DT [eq.113] TCr it is not 90oC[ the desired temperature]but also 123.4 as at EPRI way –it is the maximum temperature reached
when the current will be 280 A, continuously. Then the final conductor temperature will be the same: 65.9 oC [I forgot it, sorry].

 

[panelman]

Scotty, I did the adiabatic one from the regs and that came up with 700s to raise the temp from 90 to 250, assuming that's linear and ignoring temperature related resistance changes that gives about a 20 degree increase in 90s.

I did try niceic (don't know...try the cable manufacturer) Draka (don't make trirated in the uk so have no info), Clynder (no answer), Permanoid (don't know try Basec) Basec (don't know, try ERA), ERA (the guy you need is out of the office)

 

[rhatcher]

7anoter4 has done a great (lps) job calculating the cable temperature rise at this level of overload to show that the cable will not be overheated or damaged.

To approach this from a different angle, note that a NEMA Class 10 (submersible) motor overload will allow a 140% overload to persist for almost 150 seconds before tripping and a Class 20 (standard) motor overload will allow a little over 350 seconds.

The overloads are designed to protect the motor and the interconnecting motor lead cables from being damaged by the overload. In addition, the presumption is that the motor and cables are already operating continuously at or near full load and are therefore, at or near rated temperature.

When you consider a cold start (0.625% duty cycle) with a 140% overload for 90 seconds, this is well within the limits of most industrial protective devices such as overloads, fuses, and circuit breakers. The implication is that no damage will result from this level of overload for this amount of time.

However, a safety inspector or other local "authority" may still object if you do not have supporting ducumentation from the manufacturer.

If there is a flaw to this logic, I welcome the feedback as a chance to learn from my mistakes.

 

[ScottyUK]

rhatcher,

I agree with your reasoning. I think the biggest problem will be complying with regulations which were not really written with this type of application in mind. It clearly is possible to use 'undersized' cable because it is done all the time inside engineered products which are then tested either by the OEM or an accredited independent lab.