disconnect rating at the point of entrance to a building

[jonmathison]

An emergency generator enters a building through a disconnect switch rated at 300 Amps. Technicians have measured a base load of 30 Amps with spikes to 271 Amps at this disconnect switch (the spiking is due to two 30 HP elevator motors). Does the NEC impose an 80% safety factor on disconnect switches serving as the entrance point to an emergency generator (i.e. the switch should not be subject to more than 240 Amps)? I know this is good design practice but haven't been able to locate it in the code. Thank you.

 

[bdarnell]

Re: disconnect rating at the point of entrance to a building

Originally posted by jonmathison:
An emergency generator enters a building through a disconnect switch rated at 300 Amps.

A 400 amp switch with 300 amp fuses, maybe?

I'd be more worried about the size of the generator starting those (2) 30 HP motors. My handy dandy generator sizing program calls for a 100 kW genset just to start (1) 30 HP motor across the line.

Make sure you have enough generator.

 

[iwire]

Re: disconnect rating at the point of entrance to a building

Originally posted by jonmathison:
Technicians have measured a base load of 30 Amps with spikes to 271 Amps at this disconnect switch (the spiking is due to two 30 HP elevator motors). Does the NEC impose an 80% safety factor on disconnect switches serving as the entrance point to an emergency generator (i.e. the switch should not be subject to more than 240 Amps)?

I have never heard of sizing the equipment suppling a motor to the LRA of the motor.

If that was required services would be huge.

 

[frenchelectrican]

Re: disconnect rating at the point of entrance to a building

I hate to crash in here but the question what you saying about spike or peak current during start up on elevator motor but genrally if my theory is correct and i did reserch [ looking ] about motor surge the question is what type of motor if class A or class B motor other wise if super high effeicy engry saving motor it can draw starting current like nuts for a sec or two.

and most generators are rated for full step load with 30% voltage drop for few sec [ which it should handle motor surge ]

the other option is have " oversized " generator to handle surge of motor starting.

the other question is that when the generator go to emegercy mode will both elevator motor start the same time ??

i came up with figures to start up this kind of load you have to know the KW rating on running amp on elevator motor and lighting loads combined
and i take a peek with my figures you need at least 175 KW genny to handle both motor but for lighing load i dont know how much load you have there and keep in your mind you may have to over size it for future demand.

belive me it will add up fast


Merci , Marc

 

[mhulbert]

Re: disconnect rating at the point of entrance to a building

Equipment can't be continuously loaded over 80%...your motor starting spikes should not be continuous.