Fusing Main Disconnect

Soooooooo, the main disconnect is providing power to the three supplies mentioned, giving you a worst case phase current of 36+12+3A=51A at 480V three phase (assuming everything is running at full rated load simultaneously, and there are no additional loads anywhere).

Based on this I would provide a three phase circuit breaker rated just higher than 51A for protection. I would not try and use fuses as there is no guarentee that they will all blow and disconnect all the phases to the motors in the event of a fault (bad, obviously). As for protecting the individual transformers and motors downstream, a smaller circuit breaker each (or possibly a single phase circuit breaker or fuse for the single phase transformer).

What's the problem?

Remember that inductive loads - Transformers and motors for example, have a high "in-rush" current. depending on the type and design of the component in question, you can get an in-rush of upto 8 times the normal running current. Generally 5-6 times is usual.

Therefore main fuses/Breakers will need to take that into account and be "slow-Blow" or HRC types to handle the inrush without blowing, but protect the equipment at the normal current levels.

I would also suggest that each item be further individually protected by a smaller breaker. Also, this breaker should be one of those types that when a small (but dangerous to humans) earth fault current is seen, that it removes power to that device immediately. I believe it is called an Earth Leakage Trip Breaker or similar in English.

Andy and Jack both are 100% correct. I just have my own opinion concerning the use of circuit breakers and fuses. The problem is in-rush current. You either have to use fuses which may not disconnect all at the same time, causing dis-symmetry, or a thermo-magnetic circuit breaker that must be high enough to handle the inrush-current and maybe too far from the real FLA. We have solved this problem by using a Thermal circuit breakers combined with a soft-start circuit, a VFD with 1-2 second ramp up/down, in our case, at the main disconnect. But we have different circuits arrangement than yours. In your case you might want to use a separate soft-starter or VFD in each one of your circuits. Not cheap but very effective. and mot of all, very safe.

Wangito

You should Size your breaker so your maximum load (generally at startup) 85% of the breakers potential. Using 51 amps as our load that is 85%of 60. You should fuse the machine at 60amps. You should also take into account your10kva transformer will in itself draw current. Sorry I don't know how to estimate its draw.

Multiply the full load amps of the big XFMR by 1.25, then add each of the other loads. Use a fused disconnect with dual element time delay fuses the next available size.

You simple way is as follows; this meets the National Electrical code requirements. First, the 30 kVa xfmr nor the 10kVa xfmr is the issue, the total individual loads on the xfmr including motor starting characteristics plus addition loads tell the story.

The 30 kVa xfmr, without knowing anything else would have an allowable maximum fuse size of 125% of the transformers primary current. The Primary feed to the transformer would be 125% of 36 amps or 45 amps. The circuit protective devices (fuse/switch or circuit breakers) would also be 45 amps at 480 volts.

Base on the limited info available: 45 amps!

Ok, well most of you got close to the answer.

The actual sizing of the fuse or breaker is as follows:

30kva@480v-3ph=36 amps

10kva@480v-3ph=12 amps

3 amps @480v-3ph= 3 amps

total amp draw is 51 amps

51 amps x1.25 = 63.75 (this is actually 80% of the breaker capacity)

The NEC allows you to go to the next higher breaker

so the answer is a 70a-3p circuit breaker with #4 AWG wires (don't forget the ground wire)

Not exactly; Circuit breakers and fusing are to be sized to protect the wire connected to their load side terminals. Are you suggesting he pulls #4 wire to a 3 Amp motor? In the real world these loads require 3 individual protecting devices. 1 C/B cannot protect the 3 vastly different size loads.

The questioner needs to pick 3 C/B's to protect 3 devices- Specifying a breaker also includes interrupting capacity, and the questioner does not indicate the short circuit capacity of his supply. He would do best by abandoning picking his own breaker, and instead pick a contractor to do the work per local codes.

Additionally, you will want to provide individual ground fault/short circuit/overload protection on each device after the main disconnect.

The 30 KVA, 3 ph. Transformer will require a 50 amp circuit breaker on the primary side.

The 10 KVA, 1 ph. Transformer will require a 30 amp circuit breaker on the primary side.

The 3 amp, 2 hp motor (I'm assuming) will require a 15 amp circuit breaker.

As you can see, the 70 amp main circuit breaker is too large to open in the event of a ground fault/short circuit in any of the 3 devices that are on the load side of the 70 amp breaker.

Do not forget the selectivity of CBs or fuses these installed as a main protection; if of course shorts in the any of three individual circuit +devices can be diconnected by main without indication / selection of the "bad" one. Nobody mentioned the fast disconnecting (magnetoelectr) elements in CBs. Same of them have a short time delay that could help to realize selective disconnection for short-circuit currents.

Be sure to draw a diagram with all protections data and power sources (incl 480Vac system) so the future electricians could do proper / logical t-shooring and necessary repair / retrofits. This is, in fact, the first step before installations. Think - calculate - design/select - do it!

This specific application isn't exactly rocket science. Earth leakage circuit breakers, VFD's to limit inrush currents (which as part of my job I service and test, and have found they actually have quite a large inrush current themselves!). Perhaps it would be best to find out WHERE this site application is before more specific protection schemes are proposed.

Oh, and circuit breakers generally have between 5-14 times rated inrush current rating to handle high inrush applications (depending on the type), as well as motor rated types, and fault ratings up to about 50kA (which makes circuit breakers more than adequate for this type and size of application).