Separate 480 and 120 sources

[Rson]

I have a customer who wants me to make a control panel for a single motor (480VAC) but they are supplying the start/stop signal from a remote drive. They want a push-button station with a safety disconnect (within sight of motor) and will send a separate 120V source for the controls.

My question is for best practice on the 120VAC. I obviously will fuse the hot side – but should the neutral be fused? Should I have some kind of load switch or disconnect on the inside of the panel for the 120VAC? It seems goofy to me to have two disconnect switches on the outside of the panel – but maybe that is the way to go?

I’m also assuming I should ground/bond both the 480VAC and the 120V neutral like usual – even though I’m getting them from two different sources?

Thanks for any/all input.

EDIT:

So I was just looking and I think what I plan to do is use a rotary disconnect with two AUX contacts to run the 120VAC source through. This way a single disconnect will remove both sources for opening the panel.

 

[Aabeck]

You can not put a fuse on the neutral.

You can use a 2 pole circuit breaker and open the neutral with the hot together when the breaker trips.

I have separate 120V going into panels sometimes and all I put in is a circuit breaker on the incoming line with a warning notice of multiple power sources feed this panel.

And, tie all grounds together on a single common terminal.

 

[parky]

Use a four pole or 6 pole disconnect
And the neutral should not be fused.

 

[Rson]

See attached drawing.

View attachment E011.pdf

 

[dogleg43]

Use a four pole or 6 pole disconnect
And the neutral should not be fused.

Use a 480/120 control xfmr in your panel. It would be cheaper than a 6-pole disconnect.

 

[Rson]

Use a 480/120 control xfmr in your panel. It would be cheaper than a 6-pole disconnect.

Not allowed.

Customer is only sending 480 to the motor (from a VFD, another panel, I don't know?) and a separate 120VAC feed for control.

 

[parky]

That drawing is the way I would do it. as far as the disconnect

 

[jraef]

No need to switch the neutral though in a standard 120V scheme in North America. You CAN do it, so long as it is absolutely switched at the same time as the hot, but there is no requirement that it be switched at all. The NEC and NFPA79 only require that all UNgrounded circuits be disconnected.

People sometimes will switch it if the neutral is “floating“ though, meaning it is coming from a Separately Derived System in which the neutral is NOT bonded to ground somewhere. That is highly unusual in land based systems (more common on ships, especially fighting ships in the Navy).

 

[rootboy]

My question is for best practice on the 120VAC. I obviously will fuse the hot side – but should the neutral be fused? Should I have some kind of load switch or disconnect on the inside of the panel for the 120VAC? It seems goofy to me to have two disconnect switches on the outside of the panel – but maybe that is the way to go?

I’m also assuming I should ground/bond both the 480VAC and the 120V neutral like usual – even though I’m getting them from two different sources?

Don't fuse the neutral. The times when this is appropriate are incredibly rare (and this isn't one of them).

As for using the Aux contacts to interrupt the 120VAC, I would suggest that you series the contacts if you have more than one Aux contact, and use that signal to drive a relay to enable the 120VAC. It's going to be cheaper to replace a relay than it would be to replace the Aux contacts. But be careful, I have seen plenty of disconnect Aux contacts fail (cough-GE-cough). So if you have two available, use both of them.

Don't tie either of the neutrals to ground in your panel. It's not where your sources are derived. And does your 480 even need a neutral? But in any case, don't tie the 120VAC neutral to ground.