Transformer OCPD
- Thread starter new_ee
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Re: Transformer OCPD
The 125% is a maximum, not a minimum. You can protect the primary with anything below that value. You can even protect it with a 50 amp breaker, if you wish. The transformer would certainly be protected, but the owner/user might not find that choice to be convenient. I only say this as a reminded that the NEC does not care about convenience, or about whether the owner will be happy with the results. All the NEC cares about is the minimum requirements for a safe installation.
The 125% is a maximum, not a minimum. You can protect the primary with anything below that value. You can even protect it with a 50 amp breaker, if you wish. The transformer would certainly be protected, but the owner/user might not find that choice to be convenient. I only say this as a reminded that the NEC does not care about convenience, or about whether the owner will be happy with the results. All the NEC cares about is the minimum requirements for a safe installation.
Re: Transformer OCPD
According to the first note the maximum ocpd rating would be 250 amps. But Charlie is correct that this is a maximum.
"Notes:
1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted."
I am assuming that this has primary protection only, which is why you used 125%.
[ January 06, 2006, 01:18 PM: Message edited by: spsnyder ]
According to the first note the maximum ocpd rating would be 250 amps. But Charlie is correct that this is a maximum.
"Notes:
1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted."
I am assuming that this has primary protection only, which is why you used 125%.
[ January 06, 2006, 01:18 PM: Message edited by: spsnyder ]
Re: Transformer OCPD
The maximum protection is letting you know that you can not surpass that max. ie. protect the X-Fmr with a 300amp ocpd just because you happened to have one in the shop, that is why you go thru the calculations, the general rule for transformers rated at600V, nominal or less is to protect the primary windings of the XFmer at not more than 125% of the rated primary current of the X-Fmr, what I gather from Charlies posts is I could just as well protect the X-Fmr with a 20Amp ocpd, will the 50 amp or 20 amp not heat up and trip when the X-Fmr is working due to the load? Would you put a 15 amp breaker on an ac condensor nameplate says Max. OCPD 60amps, what is going to happen in peak summer time weather, just like if the said AC condensor keeps tripping the 60amp do you replace it with a 70amp or 80amp.
The maximum protection is letting you know that you can not surpass that max. ie. protect the X-Fmr with a 300amp ocpd just because you happened to have one in the shop, that is why you go thru the calculations, the general rule for transformers rated at600V, nominal or less is to protect the primary windings of the XFmer at not more than 125% of the rated primary current of the X-Fmr, what I gather from Charlies posts is I could just as well protect the X-Fmr with a 20Amp ocpd, will the 50 amp or 20 amp not heat up and trip when the X-Fmr is working due to the load? Would you put a 15 amp breaker on an ac condensor nameplate says Max. OCPD 60amps, what is going to happen in peak summer time weather, just like if the said AC condensor keeps tripping the 60amp do you replace it with a 70amp or 80amp.
Re: Transformer OCPD
charlie b straightened me out. i could use a 225 A OCPD since its technically less than the allowed maximum.
now the question that arises is do you size (OCPD) for the maximum load that the transformer could draw? or do you size for the load that you have calculated? i.e. a fully loaded 150kVA transformer would have a max load of about 180 amps on the primary but youre actually feeding a load of say 125 amps with it.
[ January 06, 2006, 03:51 PM: Message edited by: new_ee ]
charlie b straightened me out. i could use a 225 A OCPD since its technically less than the allowed maximum.
now the question that arises is do you size (OCPD) for the maximum load that the transformer could draw? or do you size for the load that you have calculated? i.e. a fully loaded 150kVA transformer would have a max load of about 180 amps on the primary but youre actually feeding a load of say 125 amps with it.
[ January 06, 2006, 03:51 PM: Message edited by: new_ee ]
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Re: Transformer OCPD
new-ee I do not believe you ever said what you where doing on the secondary side.
Normally I see primary and secondary protection.
For 150 kVA I would expect to see 250 amp primary protection and 400 amp secondary protection.
new-ee I do not believe you ever said what you where doing on the secondary side.
Normally I see primary and secondary protection.
For 150 kVA I would expect to see 250 amp primary protection and 400 amp secondary protection.
Re: Transformer OCPD
Again to re-enficize Charlie B....Those are maximum values. You could use OCPD with a smaller value. Only serve to protect the XFMR more.
Also note that the OCPD can be increased to the next available circuit breaker, so if the calculated current times 125% or 250% (depending on whether or nor secondary protection is provided) is 225.5 amps you could use a 250 amp breaker.
[ January 09, 2006, 12:52 PM: Message edited by: spsnyder ]
Again to re-enficize Charlie B....Those are maximum values. You could use OCPD with a smaller value. Only serve to protect the XFMR more.
Also note that the OCPD can be increased to the next available circuit breaker, so if the calculated current times 125% or 250% (depending on whether or nor secondary protection is provided) is 225.5 amps you could use a 250 amp breaker.
[ January 09, 2006, 12:52 PM: Message edited by: spsnyder ]
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Re: Transformer OCPD
We are talking about the transformer protection as spelled out in 450.3 here, but don't forget the transformer secondary conductor protection in 240.21(C).
Don
We are talking about the transformer protection as spelled out in 450.3 here, but don't forget the transformer secondary conductor protection in 240.21(C).
Don
B
bthielen
Guest
Re: Transformer OCPD
There are some advantages and disadvantages for sizing your OCPD for the load vs. the XFRMR. When I design a machine package, I consider the load. Will the secondary be supplying multiple loads? If so, I size the primary OCPD for the XFRMR, which allows for future expansion up to the limits of the transformer without changing wire sizes, OCPD, etc. If the transformer will be used for a dedicated load then it's a preference call but I like to size the OCPD for the load in this situation unless I suspect additional loads will likely be added at a later time.
Bob
There are some advantages and disadvantages for sizing your OCPD for the load vs. the XFRMR. When I design a machine package, I consider the load. Will the secondary be supplying multiple loads? If so, I size the primary OCPD for the XFRMR, which allows for future expansion up to the limits of the transformer without changing wire sizes, OCPD, etc. If the transformer will be used for a dedicated load then it's a preference call but I like to size the OCPD for the load in this situation unless I suspect additional loads will likely be added at a later time.
Bob
Re: Transformer OCPD
If you size the primary protection for the load on the transformer, and not the transformer KVA rating, you run the risk of the transformer blowing the primary OCP on inrush current(i.e. - when you first power up the transformer).
Steve
If you size the primary protection for the load on the transformer, and not the transformer KVA rating, you run the risk of the transformer blowing the primary OCP on inrush current(i.e. - when you first power up the transformer).
Steve
petersonra
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Re: Transformer OCPD
250% is nice for smaller xfmrs but if you have to upsize your conductors on a large xfmr you may not like it all that much.
250% is nice for smaller xfmrs but if you have to upsize your conductors on a large xfmr you may not like it all that much.
B
bthielen
Guest
Re: Transformer OCPD
When sizing for the load one must take into consideration the specifications of the transformer. I assume the transformer has already been appropriately sized to handle the load including any inrush that may be expected.
Bob
When sizing for the load one must take into consideration the specifications of the transformer. I assume the transformer has already been appropriately sized to handle the load including any inrush that may be expected.
Bob
Re: Transformer OCPD
spsnyder:
I don't think it is usually a problem if you have the primary OCP at 250% or even 125% of the primary current.
I was thinking of more extreme cases. Say you only have 30KVA of load on a 100KVA transformer. If you fuse the primary at based on the 30KVA load, there is some chance the fuse may not hold when the power is first turned on.
Steve
spsnyder:
I don't think it is usually a problem if you have the primary OCP at 250% or even 125% of the primary current.
I was thinking of more extreme cases. Say you only have 30KVA of load on a 100KVA transformer. If you fuse the primary at based on the 30KVA load, there is some chance the fuse may not hold when the power is first turned on.
Steve
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