Distribution Fuse Sizing

[wbd]

What is the common practice for sizing fuses on a 4.16/2.4 multi-grounded distribution ckt in terms of connected kVA/load? For example, the connected kVA on a ckt is 145 kVA (60 A) while the minimum SLG fault current is 915 A. The fault current is for a bolted fault, so for coordination I am assuming 67% fault current.

Thanks in advance

 

[redtrumpet]

Maximum fuse size will be in your local electrical code. Minimum size can be what you want it to be, except if the fuse is too small it may nuisance blow or simply overheat. You mention coordination, general approach is to start at the last device and work back toward the primary, without exceeding the maximum ampacity for each device allowed by code. Where do you get using 67% of the fault current for coordination? Primary fuse on a secondary L-G fault through a delta-wye transformer sees only 57.7% of the fault current, and on a secondary L-L fault through a delta-wye transformer sees 115% of the fault current. On solidly-grounded wye systems L-G fault current can be up to 125% of the bolted three-phase fault current. 67% is a new one for me.

In your case you have to consider other variables beside connected load, including cable ampacity and inrush current of your largest load and whether it could be the last load energized. Example: start with largest motor, determine locked rotor current above which your motor circuit protector has to be set. The fuse must lie above and to the right of the motor circuit protector curve up to the maximum fault current available for complete coordination, without exceeding the maximum size allowed by code. However, if the motor is started last, you have to add the inrush to the existing load current and leave more margin for the fuse. Sometimes coordination and/or protection has to be sacrificed, making this more art than science. Check out ANSI/IEEE Std 242 (Buff Book) for more info.

 

[wbd]

Redtrumpet,
Sorry, I guess I didn't explain myself well enough. These are cutout fuse links on an overhead utility distribution system. Therefore the NEC and Buff book aren't that useful.
What I'm trying to find out is what is an acceptable percentage of connected load for fuse sizing? For instance, I know on one side tap on my system, the connected load is 90 A and a 40K fuse link works fine. Obviously, all 90A is not being used. For a 40K link, without taking into account pre-loading, ambient temperature, the melting time for 90A is 100 sec.
The 67% is an attempt to take into account a non-bolted fault, such as a tree limb or downed power line where fault current may look like an overload.

 

[redtrumpet]

Thanks for the clarification - you can probably guess I am not a utility guy. This is outside of my experience so hopefully somebody else can help you.

 

[jnims]

One Electric Utility's practice is as follows:

(1) assume a 60% diversity factor
(2) do not load K link fuses beyond 60% or T links beyond 75% of its rating

for 4 kV this

connected kVA kVA
20K 50 12T 40
25K 60 15T 45
30K 75 20T 60
40K 95 25T 75
50K 120 30T 90
60K 160 40T 120
80K 190 50T 150
100K 235 65T 195

Hope this helps