Industrial Furnace bonding

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Big Boy Stan

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I am a bit confused and hoping for some advice on what grounding and bonding is required for an induction furnace we are installing. The system begins with a 6 phase 575V secondary transformer located outdoors of the plant. I have installed (12) 500MCM cables times 6 in cable tray from the transformer to the furnace power supply where it lands at a circuit breaker. The system is a completely ungrounded separately derived system and includes a ground detection monitor and alarm that will sound if the ground leakage exceeds 100mA. The NEC requires bonding between the equipment to handle any likely fault currents but based on this setup, I do not see how any significant fault current could develop. How do I size this? Do I need a bonding jumper from the power supply to the transformer sized at 12.5% of the (12) 500MCM conductors (Table 250.102(C)(1)? Considering the likely fault current however this seems very excessive. What scenario could lead to higher currents in the bonding jumper?

Thanks for any help or advice.
 
In general, the only difference between that system and a grounded system is that your system will not have a system bonding jumper. Everything else, including the bonding and grounding conductor sizing is the same as you would do for a grounded system.
 
As far as the transformer is concerned I agree with don.
I don't see anything that would preclude you from sizing the EBJ by 250.102(C)

From a past experience:
Is this an Inductotherm system ?
If so, get ready for some interesting grounding challanges. They do things a bit differently and one of their folks is on CMP 665 and well versed in grounding.
Been there,, still have the scars :)
 
It is in fact an Inductotherm Induction Furnace. I have read thru 665 but it does not provide any further info on bonding other then referring back to 250.

So back to the sizing question. 250.102(C) sizes the supply side bonding jumper from the transformer to the power supply as 12.5% of the area of the conductors (in parallel). By the letter this would result in 12.5% of (12) 500MCM = (1) 750MCM jumper. However this is a 6 phase system. The 6 phase transformer is a single body with a single 3 phase primary and 6 secondary bushings all mounted on a single tank. Does this change the calculation at all? The "suggestion" from one of the engineers is that this should require double the jumper size. Thoughts?
 
Thankfully that's one issue I didn't face on my Inductotherm inspections (575v conductors were in PVC :) so my EBJs were sized by conductors in each conduit). My unqualified opinion would be that (1) 750 would suffice but I never took into consideration that both secondary circuits could ground fault at the same time.

Please check your Instant Messages.
 
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I see the secondary as a single circuit and would size the supply side bonding jumper based on 12.5% of the conductors connected to a single secondary bushing.
 
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