BONDING JUMPER

[ohmhead]

Happy new year I have a question 500 KVA transformer secondary has 6 runs of 600 MCM ABCN with a grounding conductor that's 500 MCM its feeding a 2000 amp frame 1800 AT breaker I need to bond the secondary neutral what size would be code ? This is not a service this is just a transformer which is fed from a switchboard .

Heres what we did using Art 250 took the 6 X 600MCM = 3600 x 12.5 % 450 MCM So my question is this would be a 500 MCM bonding jumper from the metal case ground of that transformer to the neutral bar ?

 

[infinity]

The system bonding jumper would be 12.5% of the total kcmil value of the secondary conductors. The GEC would be #3/0 and the supply side bonding jumpers would be based on the conductors in each raceway from 250.66.

 

[ohmhead]

The system bonding jumper would be 12.5% of the total kcmil value of the secondary conductors. The GEC would be #3/0 and the supply side bonding jumpers would be based on the conductors in each raceway from 250.66.

to your neutral bar


Thanks for responding infinity so we are correct with the bonding jumper if this was alum wire with 6 runs of 600 MCM a 500 MCM bonding jumper would be accepted ?

The grounding electrode conductor is a 4/0 copper in this case called out by the engineer but yes I agree a 3/0 cu would be fine as you pointed out.

Can we also calculate this as using ampacity of total secondary feeders using 12.5 % say of the total ampacity of secondary conductors and take 12.5 % of that total ampacity and figure your bonding jumper from that size .

 

[infinity]

to your neutral bar


Thanks for responding infinity so we are correct with the bonding jumper if this was alum wire with 6 runs of 600 MCM a 500 MCM bonding jumper would be accepted ? Yes, a 500 kcmil conductor for the system bonding jumper is fine.

The grounding electrode conductor is a 4/0 copper in this case called out by the engineer but yes I agree a 3/0 cu would be fine as you pointed out.

Can we also calculate this as using ampacity of total secondary feeders using 12.5 % say of the total ampacity of secondary conductors and take 12.5 % of that total ampacity and figure your bonding jumper from that size . The system bonding jumper is sized according to 12.5% of the cumulative physical size of the secondary conductors not their ampacity.

You could have used 6 sets of 500 kcmil Al for 1800 amps.

 

[Smart $]

...

Can we also calculate this as using ampacity of total secondary feeders using 12.5 % say of the total ampacity of secondary conductors and take 12.5 % of that total ampacity and figure your bonding jumper from that size .

The system bonding jumper is based entirely on size of the ungrounded conductors (or sum in kcmil thereof). The variation of ampacity does not correlate directly with a variation in size. Also, conductor ampacity is based on insulation temperature rating. So the answer to your question is no.

 

[GoldDigger]

So, if the material of the SBJ is not specified, you could make it out if nichrome or even higher resistivity wire as long as it meets the size requirement?

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[Smart $]

So, if the material of the SBJ is not specified, you could make it out if nichrome or even higher resistivity wire as long as it meets the size requirement?

Sent from my XT1080 using Tapatalk

No...

250.28 specifies material, but other than MBJ's furnished as part of listed service equipment, you end up having to size per Table 250.66, and the only options therein are copper, aluminum, and copper-clad aluminum.

 

[ohmhead]

You could have used 6 sets of 500 kcmil Al for 1800 amps.

Yes we could have and I should have said alu this is a engineered contract drawing which we pull and install whats on the plans per engineer but he doesn't show a bonding jumper on the plan he only shows a GRC to the transformer . So we must figure this out ourselves that's why Iam asking the question to see if were correct using a 500 MCM Alum bonding jumper to the neutral .