3 Phase Motor Overcurrent Protection

[Heher]

I have a question for the NEC gurus here...

We have an application where we use .5 and 1HP motors. They are controlled by 3-Phase CRYDOM relays. We provide the customer with pigtail connections to the relay where they will hook up their 3-phase, 3-wire power when the unit is installed.

I was asked to clean up our (old) wiring hookup diagrams and I noticed that our diagram only shows fuses on two of the 3-phase supply leads coming from the customer side of the relay. I questioned the tech about this as it seemed to me that each phase should be fused, not just two. His answer was that in the U.S., only two phases need to be fused.

I looked this up in our (1996, unfortunately) NEC book and section 430-36 states that each conductor shall be fused. Can someone clarify this for me?

TIA

 

[DRWeig]

Heher,

You're correct, IF you're using fuses to provide overload protection for the motor or its branch circuit, you gotta have three (one in each current-carrying conductor).

The question might be this though: Are your motors protected from overload by another means? Thermal protectors? Impedance protection?

Give us a little more info -- what is the nameplate FLA of a particular motor, and what size and type of fuse was designed for that motor? Can't tell exactly from your first post what the purpose of the fuses really is.

Best to ya,

Old Dave

 

[Heher]

Good idea. I will have a look at what you wrote and FYI on standard motor we use is a Baldor Catalog # CM7014-50.

Thanks for the help!

 

[DRWeig]

Sure Heher,

But on reflection after I submitted my first post, I can only think of one possible application in which only two fuses would be acceptable. That case is 1) motor overload protection is provided by some other means AND 2) the unfused phase is also the grounded conductor (as in a grounded 3-phase delta supply). If you have three phases coming in, overload is required in each (if fuses). Short-circuit and ground-fault protection is required in ungrounded conductors only.

Let us know what you figure out!

Old Dave

 

[buzzp]

In the old days, only two phases were required to have overloads. This has since changed and overload protection is required in ALL 3 phases per the NEC. The reason is a single phase condition could cause only one leg to increase in current draw by 1.7 (I think - cant recall).

 

[buzzp]

If its a motor overload device, it MUST have one in all 3 legs (unless there is thermal protection built-in, could be on this small motor). If its not for overload protection and the leg is not grounded (as DRweig pointed out), it is still good design to use one in all three regardless of what the code says.

I bet your tech is not young as most youngsters dont know that 2 OLs used to be allowed.