13.2kv feeder sizing 2

[ksavoie]

Is sizing a Medium voltage feeder like sizing low voltage?
When sizing a 13.2kv feeder off a 175A fuse, should I multiply the 175A fuse by 125% and figure for the feeder size or should I just compare against the 175A fuse size?

Eg..

1.25 * 175A fuse = 219A and compare aginst 310.77 detail 3 = #350 kcmil

or 175A compared against 310.77 detail 3 = #4/0

 

[davidbeach]

At medium voltage and above, overcurrent devices are not used for protection against overload, only against short circuits. Per the NEC, fuses ratings can be up to 300% of conductor ampacity and relay pickup settings can be up to 600% of conductor ampacity.

 

[rbulsara]

See thread238-106115.

 

[wareagle]

The conductor should be sized for the load and maybe extra for future load growth. It should also be sized to with stand the available fault current. You usually size the fuse for the max under the sub breaker.

 

[DanDel]

davidbeach, I believe your post may be misleading, perhaps by some people's (and the NEC's) different perceptions of the terms 'overload', 'overcurrent', and 'short circuit'.
Though the settings may be above the cable ratings as you mentioned, the NEC requires overcurrent protection of all conductors, including MV, per Art. 240.100.

 

[davidbeach]

DanDel, yes they use the terms perhaps a bit too freely and overcurrent could be overload and it could be short circuit. But the MV rules in the NEC are clear intended to protect against short circuit and not overload. Overload protection becomes a system operation issue rather than a protection issue. 240-100(C) clearly states that conductors are to be protected against short circuits and says nothing about overload. 240-101 gives the setting requirements I mentioned above and these do not protect against overload, but will protect against short circuit except on the softest of systems.

 

[ksavoie]

I sized the fuse for a 13.2KV to 480V, 2500KVA Transformer which has a FLA on the primary of 109A. The fuse mfg recomends 1.5 * FLA of the Transformer so I will use a 175E fuse. Now if I understand you correctly the feeder conductor should be sized for the load, which is 109A. Per Table 310.77 @90 deg C, Detail 6 I could use 3-1/C#1AWG. Is this correct. If so what will keep the wire from burning in two before the fuse blows?

 

[rbulsara]

ksavoie:

davidbeach is correct and also see the thread I posted. MV protection feeder protection is not meant for 'overload' but just short circuits.

Also review characterisitcs of E-rated MV fuses, they are desined to open at 200% or E-rating at 5 min. Under that they may not even open or take very long time.

Overload is protected by "design" or the secondary protection device of the transformer in your case. Also see IEEE Buff bood for more info, which recommends sizing MV tramfomer fuses closer to its FLA rating, mainly to assit in coordination with other protective devices.

A 175E fuse for 2500KVA xfmr is oversized imho, a 125E for 100% rating and 150E for fan-rating (33%more) would be more appropriate. Larger fuse will more likely will not coordinate with upstream protection, you need to verify that.

 

[rbulsara]

But your conductor sizing method is correct. Although it is more important to make sure that #1 conductor is adequately protected by the fuse you select. You need to compare the cable thermal damage curve with the fuse curve and make sure that fuse curve lies to the left or below of the cable damage curve.

Often MV cable size is upsized just for this reason. Again a smaller fuse will permit use of smaller conductor too, as long as it meets load rating.

 

[apowerengr]

What about the additional criteria of sizing for voltage drop and/or economic conductor sizing for loss reduction? Just because you meet the minimum code requirements does not imply that you've designed a good system.

 

[waross]

A 3% voltage drop on a 208 Volt circuit is 6.24 volts.
The same combination of cable impedance and current will give 6.24 Volts drop, regardless of the circuit voltage.
On 13,800 volts, a drop of 6.24 volts is only 0.045%.
Put another way, you can run a circuit (13,800/208=66) times farther at 13,800 volts than at 208 volts for the same percentage voltage drop.
respectfully

 

[ksavoie]

Correct. My feeder length is only around a 100' so voltage drop is not an isue.