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so i have a 30A disconnect available for use at a customer site for running 2 5HP motors ( at 480V) the only question i have though is if the line is sized for a 10HP motor would both the 5HP motors need additional fuse protection?

Or could i get away without having addtional fuses protecting the motors ?

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  • \$\begingroup\$ What you can "get away with" is determined by the requirements of the authority having jurisdiction. In some jurisdictions familiar to me, there are circumstances under which the described details would be acceptable, but all aspects would need to be reviewed to determine if that is acceptable. In some jurisdictions, your need for asking that question here would disqualify you from doing the work. In any event, you have not described the installation in sufficient detail. Even if you did, the question would be too broad. Voting to close the question. \$\endgroup\$ – Charles Cowie Jun 20 at 12:11
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It depends on a lot of "ifs, "and" or "buts"...

Assuming you are in the US by virtue of your use of "HP" and 480V...

If the motor's are for continuous duty (will operate for periods of 3 hours or more) and have internal thermal protection (rare on 5HP 3 phase motors) or you have something else protecting them, such as a motor starter of some sort that has overload protection, then the up-stream fuse can be looked at as just providing Short Circuit protection only.

But article 430, table 430.52 of the NEC requires that the maximum fuse size ahead of a motor be no more than 175% of the motor FLC (from the tables in the NEC) if using a time delay fuse. So that would potentially preclude having two 5HP motors since a fuse sized for 175% of a 5HP motor would be only 87.5% of a 10HP load and would likely blow if both motors were fully loaded.

But if you use non-time delay fuses, you can go to 300% of the FLA, so that might work.

But if both motors turn on at the same time, chances are you will be replacing those fuses a lot.

If the motors are classified as "intermittent duty" and you don't have separate OL protection for each motor (and the motors don't have internal thermal protection) and you were planning on using the fuses AS the OL protection, and the conductors are sized for the combined FLC x 1.25 then again, it can only work using non-time delay fuses sized at 300% max. because the 175% rule would preclude allowing them both to run at the same time.

But if you are never going to run both motors at the same time, it changes the possibilities...

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  • \$\begingroup\$ If you interlock them so only one motor will be on at a time then you can size the system for a 5hp motor ? \$\endgroup\$ – Guitarman11 Jun 21 at 22:33
  • \$\begingroup\$ I know if you size it for 10hp then both motors need extra fuse protection but as long as they are interlocked then by sizing the system for one then it should be ok \$\endgroup\$ – Guitarman11 Jun 21 at 22:34
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    \$\begingroup\$ Yes, if you cannot operate both at the same time, then you can size the circuit for only the one motor that will be operating. A simple way to do that is with a "double throw safety switch", aka a "manual transfer switch". One source, two loads, but only one or the other. \$\endgroup\$ – JRaef Jun 22 at 0:01

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