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Single set of fuses protecting transformer and motor?

Single set of fuses protecting transformer and motor?

Single set of fuses protecting transformer and motor?

(OP)
We are in the middle of a project that involves changing an existing 480V Well motor to a 2300V motor. We already bought the 480V to 2300V step up transformer and placed it on the transformer pad. This transformer is specifically for the motor and no other load. However, we ran across an issue concerning fuses on the secondary of the transformer protecting the motor.

There are 600A fuses protecting the primary (480V) side of the transformer and a 800A main breaker upstream of that. However, we have ran out of room/funding  to install fuses on the secondary of the transformer protecting the motor. I was wondering if it was required by NEC/code to put the secondary side fuses to protect that motor? We will be reusing the starter and the overloads from the 480V motor so this new motor will have overload protection...

I spoke with the electrical representative from the Well company and they told me that it was acceptable to just place the fuses on the primary side and to reuse the overloads. I was a bit skeptical though. The motor is an IEC type rated at 70A 2300V and the transformer is 500KVA. Thanks.

RE: Single set of fuses protecting transformer and motor?

What is the winding configuration of the transformer?  Unless it is delta-delta you are required to have overcurrent protection on the load side of the transformer.

RE: Single set of fuses protecting transformer and motor?

(OP)
I'll have to verify it, but I believe it's a wye to delta configuration. I was wondering where in the NEC does it state this requirement and the exception for delta to delta windings?  

RE: Single set of fuses protecting transformer and motor?

novice EE

Section 240.4(F) gives the requirements for secondary protection of transformers as well as gives the exception that davidbeach listed for a Delta-Delta transformer.

Even if it were a Delta-Delta transformer would the fuses on the primary of this transformer adequately protect the motor from a short circuit.  The overloads will protect the motor from an overload condition however the fuses are relied on for short circuit protection.  Wouldn't the lower magnitude of fault current (due to transformer impedance) cause a short circuit to exist longer at the motor when fused on the primary of the transformer?

RE: Single set of fuses protecting transformer and motor?

(OP)
I checked and it is a delta to delta transformer.

As rockman7892 mentioned, that is one of my concerns. Would the fuses on the primary side of the transformer provide adequate short circuit protection for the motor? If not, are there certain starters out there that have short circuit protection in them?

RE: Single set of fuses protecting transformer and motor?

Look at the impedance of the transformer to estimate how much current may flow in a fault. (Actual fault currents will be reduced by the cable impedance and possibly arcing and will be less than the value indicated by calculations based transformer impedance.)
Consider dual element fuses.  

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

RE: Single set of fuses protecting transformer and motor?

Just a minor point, but one that often leads to confusion.  The rules in 240.5(F) and 240.21(C) do not have anything to do with the protection of the secondary of the transformer.  These rules are for the protection of the secondary conductors.  

The rules in 450.3 cover the protection of the transformer itself.   

The rules in both articles must be complied with.

The confusion normally occurs when someone looks at 450.3 and finds, in somecases, that transformer secondary overcurrent protection is not required.  At this point the person does not even look at 240.21(C) to find that protection of the transformer seconday conductors is required in almost all cases.  

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